git/read-cache.c

/*
 * GIT - The information manager from hell
 *
 * Copyright (C) Linus Torvalds, 2005
 */
#define NO_THE_INDEX_COMPATIBILITY_MACROS
#include "cache.h"
#include "config.h"
#include "diff.h"
#include "diffcore.h"
#include "tempfile.h"
#include "lockfile.h"
#include "cache-tree.h"
#include "refs.h"
#include "dir.h"
#include "object-store.h"
#include "tree.h"
#include "commit.h"
#include "blob.h"
#include "resolve-undo.h"
#include "strbuf.h"
#include "varint.h"
#include "split-index.h"
#include "utf8.h"
#include "fsmonitor.h"
#include "thread-utils.h"

/* Mask for the name length in ce_flags in the on-disk index */

#define CE_NAMEMASK  (0x0fff)

/* Index extensions.
 *
 * The first letter should be 'A'..'Z' for extensions that are not
 * necessary for a correct operation (i.e. optimization data).
 * When new extensions are added that _needs_ to be understood in
 * order to correctly interpret the index file, pick character that
 * is outside the range, to cause the reader to abort.
 */

#define CACHE_EXT(s) ( (s[0]<<24)|(s[1]<<16)|(s[2]<<8)|(s[3]) )
#define CACHE_EXT_TREE 0x54524545	/* "TREE" */
#define CACHE_EXT_RESOLVE_UNDO 0x52455543 /* "REUC" */
#define CACHE_EXT_LINK 0x6c696e6b	  /* "link" */
#define CACHE_EXT_UNTRACKED 0x554E5452	  /* "UNTR" */
#define CACHE_EXT_FSMONITOR 0x46534D4E	  /* "FSMN" */
#define CACHE_EXT_ENDOFINDEXENTRIES 0x454F4945	/* "EOIE" */

/* changes that can be kept in $GIT_DIR/index (basically all extensions) */
#define EXTMASK (RESOLVE_UNDO_CHANGED | CACHE_TREE_CHANGED | \
		 CE_ENTRY_ADDED | CE_ENTRY_REMOVED | CE_ENTRY_CHANGED | \
		 SPLIT_INDEX_ORDERED | UNTRACKED_CHANGED | FSMONITOR_CHANGED)


/*
 * This is an estimate of the pathname length in the index.  We use
 * this for V4 index files to guess the un-deltafied size of the index
 * in memory because of pathname deltafication.  This is not required
 * for V2/V3 index formats because their pathnames are not compressed.
 * If the initial amount of memory set aside is not sufficient, the
 * mem pool will allocate extra memory.
 */
#define CACHE_ENTRY_PATH_LENGTH 80

static inline struct cache_entry *mem_pool__ce_alloc(struct mem_pool *mem_pool, size_t len)
{
	struct cache_entry *ce;
	ce = mem_pool_alloc(mem_pool, cache_entry_size(len));
	ce->mem_pool_allocated = 1;
	return ce;
}

static inline struct cache_entry *mem_pool__ce_calloc(struct mem_pool *mem_pool, size_t len)
{
	struct cache_entry * ce;
	ce = mem_pool_calloc(mem_pool, 1, cache_entry_size(len));
	ce->mem_pool_allocated = 1;
	return ce;
}

static struct mem_pool *find_mem_pool(struct index_state *istate)
{
	struct mem_pool **pool_ptr;

	if (istate->split_index && istate->split_index->base)
		pool_ptr = &istate->split_index->base->ce_mem_pool;
	else
		pool_ptr = &istate->ce_mem_pool;

	if (!*pool_ptr)
		mem_pool_init(pool_ptr, 0);

	return *pool_ptr;
}

struct index_state the_index;
static const char *alternate_index_output;

static void set_index_entry(struct index_state *istate, int nr, struct cache_entry *ce)
{
	istate->cache[nr] = ce;
	add_name_hash(istate, ce);
}

static void replace_index_entry(struct index_state *istate, int nr, struct cache_entry *ce)
{
	struct cache_entry *old = istate->cache[nr];

	replace_index_entry_in_base(istate, old, ce);
	remove_name_hash(istate, old);
	discard_cache_entry(old);
	ce->ce_flags &= ~CE_HASHED;
	set_index_entry(istate, nr, ce);
	ce->ce_flags |= CE_UPDATE_IN_BASE;
	mark_fsmonitor_invalid(istate, ce);
	istate->cache_changed |= CE_ENTRY_CHANGED;
}

void rename_index_entry_at(struct index_state *istate, int nr, const char *new_name)
{
	struct cache_entry *old_entry = istate->cache[nr], *new_entry;
	int namelen = strlen(new_name);

	new_entry = make_empty_cache_entry(istate, namelen);
	copy_cache_entry(new_entry, old_entry);
	new_entry->ce_flags &= ~CE_HASHED;
	new_entry->ce_namelen = namelen;
	new_entry->index = 0;
	memcpy(new_entry->name, new_name, namelen + 1);

	cache_tree_invalidate_path(istate, old_entry->name);
	untracked_cache_remove_from_index(istate, old_entry->name);
	remove_index_entry_at(istate, nr);
	add_index_entry(istate, new_entry, ADD_CACHE_OK_TO_ADD|ADD_CACHE_OK_TO_REPLACE);
}

void fill_stat_data(struct stat_data *sd, struct stat *st)
{
	sd->sd_ctime.sec = (unsigned int)st->st_ctime;
	sd->sd_mtime.sec = (unsigned int)st->st_mtime;
	sd->sd_ctime.nsec = ST_CTIME_NSEC(*st);
	sd->sd_mtime.nsec = ST_MTIME_NSEC(*st);
	sd->sd_dev = st->st_dev;
	sd->sd_ino = st->st_ino;
	sd->sd_uid = st->st_uid;
	sd->sd_gid = st->st_gid;
	sd->sd_size = st->st_size;
}

int match_stat_data(const struct stat_data *sd, struct stat *st)
{
	int changed = 0;

	if (sd->sd_mtime.sec != (unsigned int)st->st_mtime)
		changed |= MTIME_CHANGED;
	if (trust_ctime && check_stat &&
	    sd->sd_ctime.sec != (unsigned int)st->st_ctime)
		changed |= CTIME_CHANGED;

#ifdef USE_NSEC
	if (check_stat && sd->sd_mtime.nsec != ST_MTIME_NSEC(*st))
		changed |= MTIME_CHANGED;
	if (trust_ctime && check_stat &&
	    sd->sd_ctime.nsec != ST_CTIME_NSEC(*st))
		changed |= CTIME_CHANGED;
#endif

	if (check_stat) {
		if (sd->sd_uid != (unsigned int) st->st_uid ||
			sd->sd_gid != (unsigned int) st->st_gid)
			changed |= OWNER_CHANGED;
		if (sd->sd_ino != (unsigned int) st->st_ino)
			changed |= INODE_CHANGED;
	}

#ifdef USE_STDEV
	/*
	 * st_dev breaks on network filesystems where different
	 * clients will have different views of what "device"
	 * the filesystem is on
	 */
	if (check_stat && sd->sd_dev != (unsigned int) st->st_dev)
			changed |= INODE_CHANGED;
#endif

	if (sd->sd_size != (unsigned int) st->st_size)
		changed |= DATA_CHANGED;

	return changed;
}

/*
 * This only updates the "non-critical" parts of the directory
 * cache, ie the parts that aren't tracked by GIT, and only used
 * to validate the cache.
 */
void fill_stat_cache_info(struct cache_entry *ce, struct stat *st)
{
	fill_stat_data(&ce->ce_stat_data, st);

	if (assume_unchanged)
		ce->ce_flags |= CE_VALID;

	if (S_ISREG(st->st_mode)) {
		ce_mark_uptodate(ce);
		mark_fsmonitor_valid(ce);
	}
}

static int ce_compare_data(const struct cache_entry *ce, struct stat *st)
{
	int match = -1;
	int fd = git_open_cloexec(ce->name, O_RDONLY);

	if (fd >= 0) {
		struct object_id oid;
		if (!index_fd(&oid, fd, st, OBJ_BLOB, ce->name, 0))
			match = !oideq(&oid, &ce->oid);
		/* index_fd() closed the file descriptor already */
	}
	return match;
}

static int ce_compare_link(const struct cache_entry *ce, size_t expected_size)
{
	int match = -1;
	void *buffer;
	unsigned long size;
	enum object_type type;
	struct strbuf sb = STRBUF_INIT;

	if (strbuf_readlink(&sb, ce->name, expected_size))
		return -1;

	buffer = read_object_file(&ce->oid, &type, &size);
	if (buffer) {
		if (size == sb.len)
			match = memcmp(buffer, sb.buf, size);
		free(buffer);
	}
	strbuf_release(&sb);
	return match;
}

static int ce_compare_gitlink(const struct cache_entry *ce)
{
	struct object_id oid;

	/*
	 * We don't actually require that the .git directory
	 * under GITLINK directory be a valid git directory. It
	 * might even be missing (in case nobody populated that
	 * sub-project).
	 *
	 * If so, we consider it always to match.
	 */
	if (resolve_gitlink_ref(ce->name, "HEAD", &oid) < 0)
		return 0;
	return !oideq(&oid, &ce->oid);
}

static int ce_modified_check_fs(const struct cache_entry *ce, struct stat *st)
{
	switch (st->st_mode & S_IFMT) {
	case S_IFREG:
		if (ce_compare_data(ce, st))
			return DATA_CHANGED;
		break;
	case S_IFLNK:
		if (ce_compare_link(ce, xsize_t(st->st_size)))
			return DATA_CHANGED;
		break;
	case S_IFDIR:
		if (S_ISGITLINK(ce->ce_mode))
			return ce_compare_gitlink(ce) ? DATA_CHANGED : 0;
		/* else fallthrough */
	default:
		return TYPE_CHANGED;
	}
	return 0;
}

static int ce_match_stat_basic(const struct cache_entry *ce, struct stat *st)
{
	unsigned int changed = 0;

	if (ce->ce_flags & CE_REMOVE)
		return MODE_CHANGED | DATA_CHANGED | TYPE_CHANGED;

	switch (ce->ce_mode & S_IFMT) {
	case S_IFREG:
		changed |= !S_ISREG(st->st_mode) ? TYPE_CHANGED : 0;
		/* We consider only the owner x bit to be relevant for
		 * "mode changes"
		 */
		if (trust_executable_bit &&
		    (0100 & (ce->ce_mode ^ st->st_mode)))
			changed |= MODE_CHANGED;
		break;
	case S_IFLNK:
		if (!S_ISLNK(st->st_mode) &&
		    (has_symlinks || !S_ISREG(st->st_mode)))
			changed |= TYPE_CHANGED;
		break;
	case S_IFGITLINK:
		/* We ignore most of the st_xxx fields for gitlinks */
		if (!S_ISDIR(st->st_mode))
			changed |= TYPE_CHANGED;
		else if (ce_compare_gitlink(ce))
			changed |= DATA_CHANGED;
		return changed;
	default:
		die("internal error: ce_mode is %o", ce->ce_mode);
	}

	changed |= match_stat_data(&ce->ce_stat_data, st);

	/* Racily smudged entry? */
	if (!ce->ce_stat_data.sd_size) {
		if (!is_empty_blob_sha1(ce->oid.hash))
			changed |= DATA_CHANGED;
	}

	return changed;
}

static int is_racy_stat(const struct index_state *istate,
			const struct stat_data *sd)
{
	return (istate->timestamp.sec &&
#ifdef USE_NSEC
		 /* nanosecond timestamped files can also be racy! */
		(istate->timestamp.sec < sd->sd_mtime.sec ||
		 (istate->timestamp.sec == sd->sd_mtime.sec &&
		  istate->timestamp.nsec <= sd->sd_mtime.nsec))
#else
		istate->timestamp.sec <= sd->sd_mtime.sec
#endif
		);
}

static int is_racy_timestamp(const struct index_state *istate,
			     const struct cache_entry *ce)
{
	return (!S_ISGITLINK(ce->ce_mode) &&
		is_racy_stat(istate, &ce->ce_stat_data));
}

int match_stat_data_racy(const struct index_state *istate,
			 const struct stat_data *sd, struct stat *st)
{
	if (is_racy_stat(istate, sd))
		return MTIME_CHANGED;
	return match_stat_data(sd, st);
}

int ie_match_stat(struct index_state *istate,
		  const struct cache_entry *ce, struct stat *st,
		  unsigned int options)
{
	unsigned int changed;
	int ignore_valid = options & CE_MATCH_IGNORE_VALID;
	int ignore_skip_worktree = options & CE_MATCH_IGNORE_SKIP_WORKTREE;
	int assume_racy_is_modified = options & CE_MATCH_RACY_IS_DIRTY;
	int ignore_fsmonitor = options & CE_MATCH_IGNORE_FSMONITOR;

	if (!ignore_fsmonitor)
		refresh_fsmonitor(istate);
	/*
	 * If it's marked as always valid in the index, it's
	 * valid whatever the checked-out copy says.
	 *
	 * skip-worktree has the same effect with higher precedence
	 */
	if (!ignore_skip_worktree && ce_skip_worktree(ce))
		return 0;
	if (!ignore_valid && (ce->ce_flags & CE_VALID))
		return 0;
	if (!ignore_fsmonitor && (ce->ce_flags & CE_FSMONITOR_VALID))
		return 0;

	/*
	 * Intent-to-add entries have not been added, so the index entry
	 * by definition never matches what is in the work tree until it
	 * actually gets added.
	 */
	if (ce_intent_to_add(ce))
		return DATA_CHANGED | TYPE_CHANGED | MODE_CHANGED;

	changed = ce_match_stat_basic(ce, st);

	/*
	 * Within 1 second of this sequence:
	 * 	echo xyzzy >file && git-update-index --add file
	 * running this command:
	 * 	echo frotz >file
	 * would give a falsely clean cache entry.  The mtime and
	 * length match the cache, and other stat fields do not change.
	 *
	 * We could detect this at update-index time (the cache entry
	 * being registered/updated records the same time as "now")
	 * and delay the return from git-update-index, but that would
	 * effectively mean we can make at most one commit per second,
	 * which is not acceptable.  Instead, we check cache entries
	 * whose mtime are the same as the index file timestamp more
	 * carefully than others.
	 */
	if (!changed && is_racy_timestamp(istate, ce)) {
		if (assume_racy_is_modified)
			changed |= DATA_CHANGED;
		else
			changed |= ce_modified_check_fs(ce, st);
	}

	return changed;
}

int ie_modified(struct index_state *istate,
		const struct cache_entry *ce,
		struct stat *st, unsigned int options)
{
	int changed, changed_fs;

	changed = ie_match_stat(istate, ce, st, options);
	if (!changed)
		return 0;
	/*
	 * If the mode or type has changed, there's no point in trying
	 * to refresh the entry - it's not going to match
	 */
	if (changed & (MODE_CHANGED | TYPE_CHANGED))
		return changed;

	/*
	 * Immediately after read-tree or update-index --cacheinfo,
	 * the length field is zero, as we have never even read the
	 * lstat(2) information once, and we cannot trust DATA_CHANGED
	 * returned by ie_match_stat() which in turn was returned by
	 * ce_match_stat_basic() to signal that the filesize of the
	 * blob changed.  We have to actually go to the filesystem to
	 * see if the contents match, and if so, should answer "unchanged".
	 *
	 * The logic does not apply to gitlinks, as ce_match_stat_basic()
	 * already has checked the actual HEAD from the filesystem in the
	 * subproject.  If ie_match_stat() already said it is different,
	 * then we know it is.
	 */
	if ((changed & DATA_CHANGED) &&
	    (S_ISGITLINK(ce->ce_mode) || ce->ce_stat_data.sd_size != 0))
		return changed;

	changed_fs = ce_modified_check_fs(ce, st);
	if (changed_fs)
		return changed | changed_fs;
	return 0;
}

int base_name_compare(const char *name1, int len1, int mode1,
		      const char *name2, int len2, int mode2)
{
	unsigned char c1, c2;
	int len = len1 < len2 ? len1 : len2;
	int cmp;

	cmp = memcmp(name1, name2, len);
	if (cmp)
		return cmp;
	c1 = name1[len];
	c2 = name2[len];
	if (!c1 && S_ISDIR(mode1))
		c1 = '/';
	if (!c2 && S_ISDIR(mode2))
		c2 = '/';
	return (c1 < c2) ? -1 : (c1 > c2) ? 1 : 0;
}

/*
 * df_name_compare() is identical to base_name_compare(), except it
 * compares conflicting directory/file entries as equal. Note that
 * while a directory name compares as equal to a regular file, they
 * then individually compare _differently_ to a filename that has
 * a dot after the basename (because '\0' < '.' < '/').
 *
 * This is used by routines that want to traverse the git namespace
 * but then handle conflicting entries together when possible.
 */
int df_name_compare(const char *name1, int len1, int mode1,
		    const char *name2, int len2, int mode2)
{
	int len = len1 < len2 ? len1 : len2, cmp;
	unsigned char c1, c2;

	cmp = memcmp(name1, name2, len);
	if (cmp)
		return cmp;
	/* Directories and files compare equal (same length, same name) */
	if (len1 == len2)
		return 0;
	c1 = name1[len];
	if (!c1 && S_ISDIR(mode1))
		c1 = '/';
	c2 = name2[len];
	if (!c2 && S_ISDIR(mode2))
		c2 = '/';
	if (c1 == '/' && !c2)
		return 0;
	if (c2 == '/' && !c1)
		return 0;
	return c1 - c2;
}

int name_compare(const char *name1, size_t len1, const char *name2, size_t len2)
{
	size_t min_len = (len1 < len2) ? len1 : len2;
	int cmp = memcmp(name1, name2, min_len);
	if (cmp)
		return cmp;
	if (len1 < len2)
		return -1;
	if (len1 > len2)
		return 1;
	return 0;
}

int cache_name_stage_compare(const char *name1, int len1, int stage1, const char *name2, int len2, int stage2)
{
	int cmp;

	cmp = name_compare(name1, len1, name2, len2);
	if (cmp)
		return cmp;

	if (stage1 < stage2)
		return -1;
	if (stage1 > stage2)
		return 1;
	return 0;
}

static int index_name_stage_pos(const struct index_state *istate, const char *name, int namelen, int stage)
{
	int first, last;

	first = 0;
	last = istate->cache_nr;
	while (last > first) {
		int next = (last + first) >> 1;
		struct cache_entry *ce = istate->cache[next];
		int cmp = cache_name_stage_compare(name, namelen, stage, ce->name, ce_namelen(ce), ce_stage(ce));
		if (!cmp)
			return next;
		if (cmp < 0) {
			last = next;
			continue;
		}
		first = next+1;
	}
	return -first-1;
}

int index_name_pos(const struct index_state *istate, const char *name, int namelen)
{
	return index_name_stage_pos(istate, name, namelen, 0);
}

int remove_index_entry_at(struct index_state *istate, int pos)
{
	struct cache_entry *ce = istate->cache[pos];

	record_resolve_undo(istate, ce);
	remove_name_hash(istate, ce);
	save_or_free_index_entry(istate, ce);
	istate->cache_changed |= CE_ENTRY_REMOVED;
	istate->cache_nr--;
	if (pos >= istate->cache_nr)
		return 0;
	MOVE_ARRAY(istate->cache + pos, istate->cache + pos + 1,
		   istate->cache_nr - pos);
	return 1;
}

/*
 * Remove all cache entries marked for removal, that is where
 * CE_REMOVE is set in ce_flags.  This is much more effective than
 * calling remove_index_entry_at() for each entry to be removed.
 */
void remove_marked_cache_entries(struct index_state *istate)
{
	struct cache_entry **ce_array = istate->cache;
	unsigned int i, j;

	for (i = j = 0; i < istate->cache_nr; i++) {
		if (ce_array[i]->ce_flags & CE_REMOVE) {
			remove_name_hash(istate, ce_array[i]);
			save_or_free_index_entry(istate, ce_array[i]);
		}
		else
			ce_array[j++] = ce_array[i];
	}
	if (j == istate->cache_nr)
		return;
	istate->cache_changed |= CE_ENTRY_REMOVED;
	istate->cache_nr = j;
}

int remove_file_from_index(struct index_state *istate, const char *path)
{
	int pos = index_name_pos(istate, path, strlen(path));
	if (pos < 0)
		pos = -pos-1;
	cache_tree_invalidate_path(istate, path);
	untracked_cache_remove_from_index(istate, path);
	while (pos < istate->cache_nr && !strcmp(istate->cache[pos]->name, path))
		remove_index_entry_at(istate, pos);
	return 0;
}

static int compare_name(struct cache_entry *ce, const char *path, int namelen)
{
	return namelen != ce_namelen(ce) || memcmp(path, ce->name, namelen);
}

static int index_name_pos_also_unmerged(struct index_state *istate,
	const char *path, int namelen)
{
	int pos = index_name_pos(istate, path, namelen);
	struct cache_entry *ce;

	if (pos >= 0)
		return pos;

	/* maybe unmerged? */
	pos = -1 - pos;
	if (pos >= istate->cache_nr ||
			compare_name((ce = istate->cache[pos]), path, namelen))
		return -1;

	/* order of preference: stage 2, 1, 3 */
	if (ce_stage(ce) == 1 && pos + 1 < istate->cache_nr &&
			ce_stage((ce = istate->cache[pos + 1])) == 2 &&
			!compare_name(ce, path, namelen))
		pos++;
	return pos;
}

static int different_name(struct cache_entry *ce, struct cache_entry *alias)
{
	int len = ce_namelen(ce);
	return ce_namelen(alias) != len || memcmp(ce->name, alias->name, len);
}

/*
 * If we add a filename that aliases in the cache, we will use the
 * name that we already have - but we don't want to update the same
 * alias twice, because that implies that there were actually two
 * different files with aliasing names!
 *
 * So we use the CE_ADDED flag to verify that the alias was an old
 * one before we accept it as
 */
static struct cache_entry *create_alias_ce(struct index_state *istate,
					   struct cache_entry *ce,
					   struct cache_entry *alias)
{
	int len;
	struct cache_entry *new_entry;

	if (alias->ce_flags & CE_ADDED)
		die("Will not add file alias '%s' ('%s' already exists in index)", ce->name, alias->name);

	/* Ok, create the new entry using the name of the existing alias */
	len = ce_namelen(alias);
	new_entry = make_empty_cache_entry(istate, len);
	memcpy(new_entry->name, alias->name, len);
	copy_cache_entry(new_entry, ce);
	save_or_free_index_entry(istate, ce);
	return new_entry;
}

void set_object_name_for_intent_to_add_entry(struct cache_entry *ce)
{
	struct object_id oid;
	if (write_object_file("", 0, blob_type, &oid))
		die("cannot create an empty blob in the object database");
	oidcpy(&ce->oid, &oid);
}

int add_to_index(struct index_state *istate, const char *path, struct stat *st, int flags)
{
	int namelen, was_same;
	mode_t st_mode = st->st_mode;
	struct cache_entry *ce, *alias = NULL;
	unsigned ce_option = CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_SKIP_WORKTREE|CE_MATCH_RACY_IS_DIRTY;
	int verbose = flags & (ADD_CACHE_VERBOSE | ADD_CACHE_PRETEND);
	int pretend = flags & ADD_CACHE_PRETEND;
	int intent_only = flags & ADD_CACHE_INTENT;
	int add_option = (ADD_CACHE_OK_TO_ADD|ADD_CACHE_OK_TO_REPLACE|
			  (intent_only ? ADD_CACHE_NEW_ONLY : 0));
	int newflags = HASH_WRITE_OBJECT;

	if (flags & HASH_RENORMALIZE)
		newflags |= HASH_RENORMALIZE;

	if (!S_ISREG(st_mode) && !S_ISLNK(st_mode) && !S_ISDIR(st_mode))
		return error("%s: can only add regular files, symbolic links or git-directories", path);

	namelen = strlen(path);
	if (S_ISDIR(st_mode)) {
		while (namelen && path[namelen-1] == '/')
			namelen--;
	}
	ce = make_empty_cache_entry(istate, namelen);
	memcpy(ce->name, path, namelen);
	ce->ce_namelen = namelen;
	if (!intent_only)
		fill_stat_cache_info(ce, st);
	else
		ce->ce_flags |= CE_INTENT_TO_ADD;


	if (trust_executable_bit && has_symlinks) {
		ce->ce_mode = create_ce_mode(st_mode);
	} else {
		/* If there is an existing entry, pick the mode bits and type
		 * from it, otherwise assume unexecutable regular file.
		 */
		struct cache_entry *ent;
		int pos = index_name_pos_also_unmerged(istate, path, namelen);

		ent = (0 <= pos) ? istate->cache[pos] : NULL;
		ce->ce_mode = ce_mode_from_stat(ent, st_mode);
	}

	/* When core.ignorecase=true, determine if a directory of the same name but differing
	 * case already exists within the Git repository.  If it does, ensure the directory
	 * case of the file being added to the repository matches (is folded into) the existing
	 * entry's directory case.
	 */
	if (ignore_case) {
		adjust_dirname_case(istate, ce->name);
	}
	if (!(flags & HASH_RENORMALIZE)) {
		alias = index_file_exists(istate, ce->name,
					  ce_namelen(ce), ignore_case);
		if (alias &&
		    !ce_stage(alias) &&
		    !ie_match_stat(istate, alias, st, ce_option)) {
			/* Nothing changed, really */
			if (!S_ISGITLINK(alias->ce_mode))
				ce_mark_uptodate(alias);
			alias->ce_flags |= CE_ADDED;

			discard_cache_entry(ce);
			return 0;
		}
	}
	if (!intent_only) {
		if (index_path(&ce->oid, path, st, newflags)) {
			discard_cache_entry(ce);
			return error("unable to index file %s", path);
		}
	} else
		set_object_name_for_intent_to_add_entry(ce);

	if (ignore_case && alias && different_name(ce, alias))
		ce = create_alias_ce(istate, ce, alias);
	ce->ce_flags |= CE_ADDED;

	/* It was suspected to be racily clean, but it turns out to be Ok */
	was_same = (alias &&
		    !ce_stage(alias) &&
		    oideq(&alias->oid, &ce->oid) &&
		    ce->ce_mode == alias->ce_mode);

	if (pretend)
		discard_cache_entry(ce);
	else if (add_index_entry(istate, ce, add_option)) {
		discard_cache_entry(ce);
		return error("unable to add %s to index", path);
	}
	if (verbose && !was_same)
		printf("add '%s'\n", path);
	return 0;
}

int add_file_to_index(struct index_state *istate, const char *path, int flags)
{
	struct stat st;
	if (lstat(path, &st))
		die_errno("unable to stat '%s'", path);
	return add_to_index(istate, path, &st, flags);
}

struct cache_entry *make_empty_cache_entry(struct index_state *istate, size_t len)
{
	return mem_pool__ce_calloc(find_mem_pool(istate), len);
}

struct cache_entry *make_empty_transient_cache_entry(size_t len)
{
	return xcalloc(1, cache_entry_size(len));
}

struct cache_entry *make_cache_entry(struct index_state *istate,
				     unsigned int mode,
				     const struct object_id *oid,
				     const char *path,
				     int stage,
				     unsigned int refresh_options)
{
	struct cache_entry *ce, *ret;
	int len;

	if (!verify_path(path, mode)) {
		error("Invalid path '%s'", path);
		return NULL;
	}

	len = strlen(path);
	ce = make_empty_cache_entry(istate, len);

	oidcpy(&ce->oid, oid);
	memcpy(ce->name, path, len);
	ce->ce_flags = create_ce_flags(stage);
	ce->ce_namelen = len;
	ce->ce_mode = create_ce_mode(mode);

	ret = refresh_cache_entry(&the_index, ce, refresh_options);
	if (ret != ce)
		discard_cache_entry(ce);
	return ret;
}

struct cache_entry *make_transient_cache_entry(unsigned int mode, const struct object_id *oid,
					       const char *path, int stage)
{
	struct cache_entry *ce;
	int len;

	if (!verify_path(path, mode)) {
		error("Invalid path '%s'", path);
		return NULL;
	}

	len = strlen(path);
	ce = make_empty_transient_cache_entry(len);

	oidcpy(&ce->oid, oid);
	memcpy(ce->name, path, len);
	ce->ce_flags = create_ce_flags(stage);
	ce->ce_namelen = len;
	ce->ce_mode = create_ce_mode(mode);

	return ce;
}

/*
 * Chmod an index entry with either +x or -x.
 *
 * Returns -1 if the chmod for the particular cache entry failed (if it's
 * not a regular file), -2 if an invalid flip argument is passed in, 0
 * otherwise.
 */
int chmod_index_entry(struct index_state *istate, struct cache_entry *ce,
		      char flip)
{
	if (!S_ISREG(ce->ce_mode))
		return -1;
	switch (flip) {
	case '+':
		ce->ce_mode |= 0111;
		break;
	case '-':
		ce->ce_mode &= ~0111;
		break;
	default:
		return -2;
	}
	cache_tree_invalidate_path(istate, ce->name);
	ce->ce_flags |= CE_UPDATE_IN_BASE;
	mark_fsmonitor_invalid(istate, ce);
	istate->cache_changed |= CE_ENTRY_CHANGED;

	return 0;
}

int ce_same_name(const struct cache_entry *a, const struct cache_entry *b)
{
	int len = ce_namelen(a);
	return ce_namelen(b) == len && !memcmp(a->name, b->name, len);
}

/*
 * We fundamentally don't like some paths: we don't want
 * dot or dot-dot anywhere, and for obvious reasons don't
 * want to recurse into ".git" either.
 *
 * Also, we don't want double slashes or slashes at the
 * end that can make pathnames ambiguous.
 */
static int verify_dotfile(const char *rest, unsigned mode)
{
	/*
	 * The first character was '.', but that
	 * has already been discarded, we now test
	 * the rest.
	 */

	/* "." is not allowed */
	if (*rest == '\0' || is_dir_sep(*rest))
		return 0;

	switch (*rest) {
	/*
	 * ".git" followed by NUL or slash is bad. Note that we match
	 * case-insensitively here, even if ignore_case is not set.
	 * This outlaws ".GIT" everywhere out of an abundance of caution,
	 * since there's really no good reason to allow it.
	 *
	 * Once we've seen ".git", we can also find ".gitmodules", etc (also
	 * case-insensitively).
	 */
	case 'g':
	case 'G':
		if (rest[1] != 'i' && rest[1] != 'I')
			break;
		if (rest[2] != 't' && rest[2] != 'T')
			break;
		if (rest[3] == '\0' || is_dir_sep(rest[3]))
			return 0;
		if (S_ISLNK(mode)) {
			rest += 3;
			if (skip_iprefix(rest, "modules", &rest) &&
			    (*rest == '\0' || is_dir_sep(*rest)))
				return 0;
		}
		break;
	case '.':
		if (rest[1] == '\0' || is_dir_sep(rest[1]))
			return 0;
	}
	return 1;
}

int verify_path(const char *path, unsigned mode)
{
	char c;

	if (has_dos_drive_prefix(path))
		return 0;

	goto inside;
	for (;;) {
		if (!c)
			return 1;
		if (is_dir_sep(c)) {
inside:
			if (protect_hfs) {
				if (is_hfs_dotgit(path))
					return 0;
				if (S_ISLNK(mode)) {
					if (is_hfs_dotgitmodules(path))
						return 0;
				}
			}
			if (protect_ntfs) {
				if (is_ntfs_dotgit(path))
					return 0;
				if (S_ISLNK(mode)) {
					if (is_ntfs_dotgitmodules(path))
						return 0;
				}
			}

			c = *path++;
			if ((c == '.' && !verify_dotfile(path, mode)) ||
			    is_dir_sep(c) || c == '\0')
				return 0;
		}
		c = *path++;
	}
}

/*
 * Do we have another file that has the beginning components being a
 * proper superset of the name we're trying to add?
 */
static int has_file_name(struct index_state *istate,
			 const struct cache_entry *ce, int pos, int ok_to_replace)
{
	int retval = 0;
	int len = ce_namelen(ce);
	int stage = ce_stage(ce);
	const char *name = ce->name;

	while (pos < istate->cache_nr) {
		struct cache_entry *p = istate->cache[pos++];

		if (len >= ce_namelen(p))
			break;
		if (memcmp(name, p->name, len))
			break;
		if (ce_stage(p) != stage)
			continue;
		if (p->name[len] != '/')
			continue;
		if (p->ce_flags & CE_REMOVE)
			continue;
		retval = -1;
		if (!ok_to_replace)
			break;
		remove_index_entry_at(istate, --pos);
	}
	return retval;
}


/*
 * Like strcmp(), but also return the offset of the first change.
 * If strings are equal, return the length.
 */
int strcmp_offset(const char *s1, const char *s2, size_t *first_change)
{
	size_t k;

	if (!first_change)
		return strcmp(s1, s2);

	for (k = 0; s1[k] == s2[k]; k++)
		if (s1[k] == '\0')
			break;

	*first_change = k;
	return (unsigned char)s1[k] - (unsigned char)s2[k];
}

/*
 * Do we have another file with a pathname that is a proper
 * subset of the name we're trying to add?
 *
 * That is, is there another file in the index with a path
 * that matches a sub-directory in the given entry?
 */
static int has_dir_name(struct index_state *istate,
			const struct cache_entry *ce, int pos, int ok_to_replace)
{
	int retval = 0;
	int stage = ce_stage(ce);
	const char *name = ce->name;
	const char *slash = name + ce_namelen(ce);
	size_t len_eq_last;
	int cmp_last = 0;

	/*
	 * We are frequently called during an iteration on a sorted
	 * list of pathnames and while building a new index.  Therefore,
	 * there is a high probability that this entry will eventually
	 * be appended to the index, rather than inserted in the middle.
	 * If we can confirm that, we can avoid binary searches on the
	 * components of the pathname.
	 *
	 * Compare the entry's full path with the last path in the index.
	 */
	if (istate->cache_nr > 0) {
		cmp_last = strcmp_offset(name,
			istate->cache[istate->cache_nr - 1]->name,
			&len_eq_last);
		if (cmp_last > 0) {
			if (len_eq_last == 0) {
				/*
				 * The entry sorts AFTER the last one in the
				 * index and their paths have no common prefix,
				 * so there cannot be a F/D conflict.
				 */
				return retval;
			} else {
				/*
				 * The entry sorts AFTER the last one in the
				 * index, but has a common prefix.  Fall through
				 * to the loop below to disect the entry's path
				 * and see where the difference is.
				 */
			}
		} else if (cmp_last == 0) {
			/*
			 * The entry exactly matches the last one in the
			 * index, but because of multiple stage and CE_REMOVE
			 * items, we fall through and let the regular search
			 * code handle it.
			 */
		}
	}

	for (;;) {
		size_t len;

		for (;;) {
			if (*--slash == '/')
				break;
			if (slash <= ce->name)
				return retval;
		}
		len = slash - name;

		if (cmp_last > 0) {
			/*
			 * (len + 1) is a directory boundary (including
			 * the trailing slash).  And since the loop is
			 * decrementing "slash", the first iteration is
			 * the longest directory prefix; subsequent
			 * iterations consider parent directories.
			 */

			if (len + 1 <= len_eq_last) {
				/*
				 * The directory prefix (including the trailing
				 * slash) also appears as a prefix in the last
				 * entry, so the remainder cannot collide (because
				 * strcmp said the whole path was greater).
				 *
				 * EQ: last: xxx/A
				 *     this: xxx/B
				 *
				 * LT: last: xxx/file_A
				 *     this: xxx/file_B
				 */
				return retval;
			}

			if (len > len_eq_last) {
				/*
				 * This part of the directory prefix (excluding
				 * the trailing slash) is longer than the known
				 * equal portions, so this sub-directory cannot
				 * collide with a file.
				 *
				 * GT: last: xxxA
				 *     this: xxxB/file
				 */
				return retval;
			}

			if (istate->cache_nr > 0 &&
				ce_namelen(istate->cache[istate->cache_nr - 1]) > len) {
				/*
				 * The directory prefix lines up with part of
				 * a longer file or directory name, but sorts
				 * after it, so this sub-directory cannot
				 * collide with a file.
				 *
				 * last: xxx/yy-file (because '-' sorts before '/')
				 * this: xxx/yy/abc
				 */
				return retval;
			}

			/*
			 * This is a possible collision. Fall through and
			 * let the regular search code handle it.
			 *
			 * last: xxx
			 * this: xxx/file
			 */
		}

		pos = index_name_stage_pos(istate, name, len, stage);
		if (pos >= 0) {
			/*
			 * Found one, but not so fast.  This could
			 * be a marker that says "I was here, but
			 * I am being removed".  Such an entry is
			 * not a part of the resulting tree, and
			 * it is Ok to have a directory at the same
			 * path.
			 */
			if (!(istate->cache[pos]->ce_flags & CE_REMOVE)) {
				retval = -1;
				if (!ok_to_replace)
					break;
				remove_index_entry_at(istate, pos);
				continue;
			}
		}
		else
			pos = -pos-1;

		/*
		 * Trivial optimization: if we find an entry that
		 * already matches the sub-directory, then we know
		 * we're ok, and we can exit.
		 */
		while (pos < istate->cache_nr) {
			struct cache_entry *p = istate->cache[pos];
			if ((ce_namelen(p) <= len) ||
			    (p->name[len] != '/') ||
			    memcmp(p->name, name, len))
				break; /* not our subdirectory */
			if (ce_stage(p) == stage && !(p->ce_flags & CE_REMOVE))
				/*
				 * p is at the same stage as our entry, and
				 * is a subdirectory of what we are looking
				 * at, so we cannot have conflicts at our
				 * level or anything shorter.
				 */
				return retval;
			pos++;
		}
	}
	return retval;
}

/* We may be in a situation where we already have path/file and path
 * is being added, or we already have path and path/file is being
 * added.  Either one would result in a nonsense tree that has path
 * twice when git-write-tree tries to write it out.  Prevent it.
 *
 * If ok-to-replace is specified, we remove the conflicting entries
 * from the cache so the caller should recompute the insert position.
 * When this happens, we return non-zero.
 */
static int check_file_directory_conflict(struct index_state *istate,
					 const struct cache_entry *ce,
					 int pos, int ok_to_replace)
{
	int retval;

	/*
	 * When ce is an "I am going away" entry, we allow it to be added
	 */
	if (ce->ce_flags & CE_REMOVE)
		return 0;

	/*
	 * We check if the path is a sub-path of a subsequent pathname
	 * first, since removing those will not change the position
	 * in the array.
	 */
	retval = has_file_name(istate, ce, pos, ok_to_replace);

	/*
	 * Then check if the path might have a clashing sub-directory
	 * before it.
	 */
	return retval + has_dir_name(istate, ce, pos, ok_to_replace);
}

static int add_index_entry_with_check(struct index_state *istate, struct cache_entry *ce, int option)
{
	int pos;
	int ok_to_add = option & ADD_CACHE_OK_TO_ADD;
	int ok_to_replace = option & ADD_CACHE_OK_TO_REPLACE;
	int skip_df_check = option & ADD_CACHE_SKIP_DFCHECK;
	int new_only = option & ADD_CACHE_NEW_ONLY;

	if (!(option & ADD_CACHE_KEEP_CACHE_TREE))
		cache_tree_invalidate_path(istate, ce->name);

	/*
	 * If this entry's path sorts after the last entry in the index,
	 * we can avoid searching for it.
	 */
	if (istate->cache_nr > 0 &&
		strcmp(ce->name, istate->cache[istate->cache_nr - 1]->name) > 0)
		pos = -istate->cache_nr - 1;
	else
		pos = index_name_stage_pos(istate, ce->name, ce_namelen(ce), ce_stage(ce));

	/* existing match? Just replace it. */
	if (pos >= 0) {
		if (!new_only)
			replace_index_entry(istate, pos, ce);
		return 0;
	}
	pos = -pos-1;

	if (!(option & ADD_CACHE_KEEP_CACHE_TREE))
		untracked_cache_add_to_index(istate, ce->name);

	/*
	 * Inserting a merged entry ("stage 0") into the index
	 * will always replace all non-merged entries..
	 */
	if (pos < istate->cache_nr && ce_stage(ce) == 0) {
		while (ce_same_name(istate->cache[pos], ce)) {
			ok_to_add = 1;
			if (!remove_index_entry_at(istate, pos))
				break;
		}
	}

	if (!ok_to_add)
		return -1;
	if (!verify_path(ce->name, ce->ce_mode))
		return error("Invalid path '%s'", ce->name);

	if (!skip_df_check &&
	    check_file_directory_conflict(istate, ce, pos, ok_to_replace)) {
		if (!ok_to_replace)
			return error("'%s' appears as both a file and as a directory",
				     ce->name);
		pos = index_name_stage_pos(istate, ce->name, ce_namelen(ce), ce_stage(ce));
		pos = -pos-1;
	}
	return pos + 1;
}

int add_index_entry(struct index_state *istate, struct cache_entry *ce, int option)
{
	int pos;

	if (option & ADD_CACHE_JUST_APPEND)
		pos = istate->cache_nr;
	else {
		int ret;
		ret = add_index_entry_with_check(istate, ce, option);
		if (ret <= 0)
			return ret;
		pos = ret - 1;
	}

	/* Make sure the array is big enough .. */
	ALLOC_GROW(istate->cache, istate->cache_nr + 1, istate->cache_alloc);

	/* Add it in.. */
	istate->cache_nr++;
	if (istate->cache_nr > pos + 1)
		MOVE_ARRAY(istate->cache + pos + 1, istate->cache + pos,
			   istate->cache_nr - pos - 1);
	set_index_entry(istate, pos, ce);
	istate->cache_changed |= CE_ENTRY_ADDED;
	return 0;
}

/*
 * "refresh" does not calculate a new sha1 file or bring the
 * cache up-to-date for mode/content changes. But what it
 * _does_ do is to "re-match" the stat information of a file
 * with the cache, so that you can refresh the cache for a
 * file that hasn't been changed but where the stat entry is
 * out of date.
 *
 * For example, you'd want to do this after doing a "git-read-tree",
 * to link up the stat cache details with the proper files.
 */
static struct cache_entry *refresh_cache_ent(struct index_state *istate,
					     struct cache_entry *ce,
					     unsigned int options, int *err,
					     int *changed_ret)
{
	struct stat st;
	struct cache_entry *updated;
	int changed;
	int refresh = options & CE_MATCH_REFRESH;
	int ignore_valid = options & CE_MATCH_IGNORE_VALID;
	int ignore_skip_worktree = options & CE_MATCH_IGNORE_SKIP_WORKTREE;
	int ignore_missing = options & CE_MATCH_IGNORE_MISSING;
	int ignore_fsmonitor = options & CE_MATCH_IGNORE_FSMONITOR;

	if (!refresh || ce_uptodate(ce))
		return ce;

	if (!ignore_fsmonitor)
		refresh_fsmonitor(istate);
	/*
	 * CE_VALID or CE_SKIP_WORKTREE means the user promised us
	 * that the change to the work tree does not matter and told
	 * us not to worry.
	 */
	if (!ignore_skip_worktree && ce_skip_worktree(ce)) {
		ce_mark_uptodate(ce);
		return ce;
	}
	if (!ignore_valid && (ce->ce_flags & CE_VALID)) {
		ce_mark_uptodate(ce);
		return ce;
	}
	if (!ignore_fsmonitor && (ce->ce_flags & CE_FSMONITOR_VALID)) {
		ce_mark_uptodate(ce);
		return ce;
	}

	if (has_symlink_leading_path(ce->name, ce_namelen(ce))) {
		if (ignore_missing)
			return ce;
		if (err)
			*err = ENOENT;
		return NULL;
	}

	if (lstat(ce->name, &st) < 0) {
		if (ignore_missing && errno == ENOENT)
			return ce;
		if (err)
			*err = errno;
		return NULL;
	}

	changed = ie_match_stat(istate, ce, &st, options);
	if (changed_ret)
		*changed_ret = changed;
	if (!changed) {
		/*
		 * The path is unchanged.  If we were told to ignore
		 * valid bit, then we did the actual stat check and
		 * found that the entry is unmodified.  If the entry
		 * is not marked VALID, this is the place to mark it
		 * valid again, under "assume unchanged" mode.
		 */
		if (ignore_valid && assume_unchanged &&
		    !(ce->ce_flags & CE_VALID))
			; /* mark this one VALID again */
		else {
			/*
			 * We do not mark the index itself "modified"
			 * because CE_UPTODATE flag is in-core only;
			 * we are not going to write this change out.
			 */
			if (!S_ISGITLINK(ce->ce_mode)) {
				ce_mark_uptodate(ce);
				mark_fsmonitor_valid(ce);
			}
			return ce;
		}
	}

	if (ie_modified(istate, ce, &st, options)) {
		if (err)
			*err = EINVAL;
		return NULL;
	}

	updated = make_empty_cache_entry(istate, ce_namelen(ce));
	copy_cache_entry(updated, ce);
	memcpy(updated->name, ce->name, ce->ce_namelen + 1);
	fill_stat_cache_info(updated, &st);
	/*
	 * If ignore_valid is not set, we should leave CE_VALID bit
	 * alone.  Otherwise, paths marked with --no-assume-unchanged
	 * (i.e. things to be edited) will reacquire CE_VALID bit
	 * automatically, which is not really what we want.
	 */
	if (!ignore_valid && assume_unchanged &&
	    !(ce->ce_flags & CE_VALID))
		updated->ce_flags &= ~CE_VALID;

	/* istate->cache_changed is updated in the caller */
	return updated;
}

static void show_file(const char * fmt, const char * name, int in_porcelain,
		      int * first, const char *header_msg)
{
	if (in_porcelain && *first && header_msg) {
		printf("%s\n", header_msg);
		*first = 0;
	}
	printf(fmt, name);
}

int refresh_index(struct index_state *istate, unsigned int flags,
		  const struct pathspec *pathspec,
		  char *seen, const char *header_msg)
{
	int i;
	int has_errors = 0;
	int really = (flags & REFRESH_REALLY) != 0;
	int allow_unmerged = (flags & REFRESH_UNMERGED) != 0;
	int quiet = (flags & REFRESH_QUIET) != 0;
	int not_new = (flags & REFRESH_IGNORE_MISSING) != 0;
	int ignore_submodules = (flags & REFRESH_IGNORE_SUBMODULES) != 0;
	int first = 1;
	int in_porcelain = (flags & REFRESH_IN_PORCELAIN);
	unsigned int options = (CE_MATCH_REFRESH |
				(really ? CE_MATCH_IGNORE_VALID : 0) |
				(not_new ? CE_MATCH_IGNORE_MISSING : 0));
	const char *modified_fmt;
	const char *deleted_fmt;
	const char *typechange_fmt;
	const char *added_fmt;
	const char *unmerged_fmt;

	trace_performance_enter();
	modified_fmt = (in_porcelain ? "M\t%s\n" : "%s: needs update\n");
	deleted_fmt = (in_porcelain ? "D\t%s\n" : "%s: needs update\n");
	typechange_fmt = (in_porcelain ? "T\t%s\n" : "%s needs update\n");
	added_fmt = (in_porcelain ? "A\t%s\n" : "%s needs update\n");
	unmerged_fmt = (in_porcelain ? "U\t%s\n" : "%s: needs merge\n");
	for (i = 0; i < istate->cache_nr; i++) {
		struct cache_entry *ce, *new_entry;
		int cache_errno = 0;
		int changed = 0;
		int filtered = 0;

		ce = istate->cache[i];
		if (ignore_submodules && S_ISGITLINK(ce->ce_mode))
			continue;

		if (pathspec && !ce_path_match(&the_index, ce, pathspec, seen))
			filtered = 1;

		if (ce_stage(ce)) {
			while ((i < istate->cache_nr) &&
			       ! strcmp(istate->cache[i]->name, ce->name))
				i++;
			i--;
			if (allow_unmerged)
				continue;
			if (!filtered)
				show_file(unmerged_fmt, ce->name, in_porcelain,
					  &first, header_msg);
			has_errors = 1;
			continue;
		}

		if (filtered)
			continue;

		new_entry = refresh_cache_ent(istate, ce, options, &cache_errno, &changed);
		if (new_entry == ce)
			continue;
		if (!new_entry) {
			const char *fmt;

			if (really && cache_errno == EINVAL) {
				/* If we are doing --really-refresh that
				 * means the index is not valid anymore.
				 */
				ce->ce_flags &= ~CE_VALID;
				ce->ce_flags |= CE_UPDATE_IN_BASE;
				mark_fsmonitor_invalid(istate, ce);
				istate->cache_changed |= CE_ENTRY_CHANGED;
			}
			if (quiet)
				continue;

			if (cache_errno == ENOENT)
				fmt = deleted_fmt;
			else if (ce_intent_to_add(ce))
				fmt = added_fmt; /* must be before other checks */
			else if (changed & TYPE_CHANGED)
				fmt = typechange_fmt;
			else
				fmt = modified_fmt;
			show_file(fmt,
				  ce->name, in_porcelain, &first, header_msg);
			has_errors = 1;
			continue;
		}

		replace_index_entry(istate, i, new_entry);
	}
	trace_performance_leave("refresh index");
	return has_errors;
}

struct cache_entry *refresh_cache_entry(struct index_state *istate,
					struct cache_entry *ce,
					unsigned int options)
{
	return refresh_cache_ent(istate, ce, options, NULL, NULL);
}


/*****************************************************************
 * Index File I/O
 *****************************************************************/

#define INDEX_FORMAT_DEFAULT 3

static unsigned int get_index_format_default(void)
{
	char *envversion = getenv("GIT_INDEX_VERSION");
	char *endp;
	int value;
	unsigned int version = INDEX_FORMAT_DEFAULT;

	if (!envversion) {
		if (!git_config_get_int("index.version", &value))
			version = value;
		if (version < INDEX_FORMAT_LB || INDEX_FORMAT_UB < version) {
			warning(_("index.version set, but the value is invalid.\n"
				  "Using version %i"), INDEX_FORMAT_DEFAULT);
			return INDEX_FORMAT_DEFAULT;
		}
		return version;
	}

	version = strtoul(envversion, &endp, 10);
	if (*endp ||
	    version < INDEX_FORMAT_LB || INDEX_FORMAT_UB < version) {
		warning(_("GIT_INDEX_VERSION set, but the value is invalid.\n"
			  "Using version %i"), INDEX_FORMAT_DEFAULT);
		version = INDEX_FORMAT_DEFAULT;
	}
	return version;
}

/*
 * dev/ino/uid/gid/size are also just tracked to the low 32 bits
 * Again - this is just a (very strong in practice) heuristic that
 * the inode hasn't changed.
 *
 * We save the fields in big-endian order to allow using the
 * index file over NFS transparently.
 */
struct ondisk_cache_entry {
	struct cache_time ctime;
	struct cache_time mtime;
	uint32_t dev;
	uint32_t ino;
	uint32_t mode;
	uint32_t uid;
	uint32_t gid;
	uint32_t size;
	unsigned char sha1[20];
	uint16_t flags;
	char name[FLEX_ARRAY]; /* more */
};

/*
 * This struct is used when CE_EXTENDED bit is 1
 * The struct must match ondisk_cache_entry exactly from
 * ctime till flags
 */
struct ondisk_cache_entry_extended {
	struct cache_time ctime;
	struct cache_time mtime;
	uint32_t dev;
	uint32_t ino;
	uint32_t mode;
	uint32_t uid;
	uint32_t gid;
	uint32_t size;
	unsigned char sha1[20];
	uint16_t flags;
	uint16_t flags2;
	char name[FLEX_ARRAY]; /* more */
};

/* These are only used for v3 or lower */
#define align_padding_size(size, len) ((size + (len) + 8) & ~7) - (size + len)
#define align_flex_name(STRUCT,len) ((offsetof(struct STRUCT,name) + (len) + 8) & ~7)
#define ondisk_cache_entry_size(len) align_flex_name(ondisk_cache_entry,len)
#define ondisk_cache_entry_extended_size(len) align_flex_name(ondisk_cache_entry_extended,len)
#define ondisk_ce_size(ce) (((ce)->ce_flags & CE_EXTENDED) ? \
			    ondisk_cache_entry_extended_size(ce_namelen(ce)) : \
			    ondisk_cache_entry_size(ce_namelen(ce)))

/* Allow fsck to force verification of the index checksum. */
int verify_index_checksum;

/* Allow fsck to force verification of the cache entry order. */
int verify_ce_order;

static int verify_hdr(const struct cache_header *hdr, unsigned long size)
{
	git_hash_ctx c;
	unsigned char hash[GIT_MAX_RAWSZ];
	int hdr_version;

	if (hdr->hdr_signature != htonl(CACHE_SIGNATURE))
		return error("bad signature");
	hdr_version = ntohl(hdr->hdr_version);
	if (hdr_version < INDEX_FORMAT_LB || INDEX_FORMAT_UB < hdr_version)
		return error("bad index version %d", hdr_version);

	if (!verify_index_checksum)
		return 0;

	the_hash_algo->init_fn(&c);
	the_hash_algo->update_fn(&c, hdr, size - the_hash_algo->rawsz);
	the_hash_algo->final_fn(hash, &c);
	if (!hasheq(hash, (unsigned char *)hdr + size - the_hash_algo->rawsz))
		return error("bad index file sha1 signature");
	return 0;
}

static int read_index_extension(struct index_state *istate,
				const char *ext, const char *data, unsigned long sz)
{
	switch (CACHE_EXT(ext)) {
	case CACHE_EXT_TREE:
		istate->cache_tree = cache_tree_read(data, sz);
		break;
	case CACHE_EXT_RESOLVE_UNDO:
		istate->resolve_undo = resolve_undo_read(data, sz);
		break;
	case CACHE_EXT_LINK:
		if (read_link_extension(istate, data, sz))
			return -1;
		break;
	case CACHE_EXT_UNTRACKED:
		istate->untracked = read_untracked_extension(data, sz);
		break;
	case CACHE_EXT_FSMONITOR:
		read_fsmonitor_extension(istate, data, sz);
		break;
	case CACHE_EXT_ENDOFINDEXENTRIES:
		/* already handled in do_read_index() */
		break;
	default:
		if (*ext < 'A' || 'Z' < *ext)
			return error("index uses %.4s extension, which we do not understand",
				     ext);
		fprintf(stderr, "ignoring %.4s extension\n", ext);
		break;
	}
	return 0;
}

int hold_locked_index(struct lock_file *lk, int lock_flags)
{
	return hold_lock_file_for_update(lk, get_index_file(), lock_flags);
}

int read_index(struct index_state *istate)
{
	return read_index_from(istate, get_index_file(), get_git_dir());
}

static struct cache_entry *create_from_disk(struct index_state *istate,
					    struct ondisk_cache_entry *ondisk,
					    unsigned long *ent_size,
					    const struct cache_entry *previous_ce)
{
	struct cache_entry *ce;
	size_t len;
	const char *name;
	unsigned int flags;
	size_t copy_len;
	/*
	 * Adjacent cache entries tend to share the leading paths, so it makes
	 * sense to only store the differences in later entries.  In the v4
	 * on-disk format of the index, each on-disk cache entry stores the
	 * number of bytes to be stripped from the end of the previous name,
	 * and the bytes to append to the result, to come up with its name.
	 */
	int expand_name_field = istate->version == 4;

	/* On-disk flags are just 16 bits */
	flags = get_be16(&ondisk->flags);
	len = flags & CE_NAMEMASK;

	if (flags & CE_EXTENDED) {
		struct ondisk_cache_entry_extended *ondisk2;
		int extended_flags;
		ondisk2 = (struct ondisk_cache_entry_extended *)ondisk;
		extended_flags = get_be16(&ondisk2->flags2) << 16;
		/* We do not yet understand any bit out of CE_EXTENDED_FLAGS */
		if (extended_flags & ~CE_EXTENDED_FLAGS)
			die("Unknown index entry format %08x", extended_flags);
		flags |= extended_flags;
		name = ondisk2->name;
	}
	else
		name = ondisk->name;

	if (expand_name_field) {
		const unsigned char *cp = (const unsigned char *)name;
		size_t strip_len, previous_len;

		previous_len = previous_ce ? previous_ce->ce_namelen : 0;
		strip_len = decode_varint(&cp);
		if (previous_len < strip_len) {
			if (previous_ce)
				die(_("malformed name field in the index, near path '%s'"),
				    previous_ce->name);
			else
				die(_("malformed name field in the index in the first path"));
		}
		copy_len = previous_len - strip_len;
		name = (const char *)cp;
	}

	if (len == CE_NAMEMASK) {
		len = strlen(name);
		if (expand_name_field)
			len += copy_len;
	}

	ce = mem_pool__ce_alloc(istate->ce_mem_pool, len);

	ce->ce_stat_data.sd_ctime.sec = get_be32(&ondisk->ctime.sec);
	ce->ce_stat_data.sd_mtime.sec = get_be32(&ondisk->mtime.sec);
	ce->ce_stat_data.sd_ctime.nsec = get_be32(&ondisk->ctime.nsec);
	ce->ce_stat_data.sd_mtime.nsec = get_be32(&ondisk->mtime.nsec);
	ce->ce_stat_data.sd_dev   = get_be32(&ondisk->dev);
	ce->ce_stat_data.sd_ino   = get_be32(&ondisk->ino);
	ce->ce_mode  = get_be32(&ondisk->mode);
	ce->ce_stat_data.sd_uid   = get_be32(&ondisk->uid);
	ce->ce_stat_data.sd_gid   = get_be32(&ondisk->gid);
	ce->ce_stat_data.sd_size  = get_be32(&ondisk->size);
	ce->ce_flags = flags & ~CE_NAMEMASK;
	ce->ce_namelen = len;
	ce->index = 0;
	hashcpy(ce->oid.hash, ondisk->sha1);

	if (expand_name_field) {
		if (copy_len)
			memcpy(ce->name, previous_ce->name, copy_len);
		memcpy(ce->name + copy_len, name, len + 1 - copy_len);
		*ent_size = (name - ((char *)ondisk)) + len + 1 - copy_len;
	} else {
		memcpy(ce->name, name, len + 1);
		*ent_size = ondisk_ce_size(ce);
	}
	return ce;
}

static void check_ce_order(struct index_state *istate)
{
	unsigned int i;

	if (!verify_ce_order)
		return;

	for (i = 1; i < istate->cache_nr; i++) {
		struct cache_entry *ce = istate->cache[i - 1];
		struct cache_entry *next_ce = istate->cache[i];
		int name_compare = strcmp(ce->name, next_ce->name);

		if (0 < name_compare)
			die("unordered stage entries in index");
		if (!name_compare) {
			if (!ce_stage(ce))
				die("multiple stage entries for merged file '%s'",
				    ce->name);
			if (ce_stage(ce) > ce_stage(next_ce))
				die("unordered stage entries for '%s'",
				    ce->name);
		}
	}
}

static void tweak_untracked_cache(struct index_state *istate)
{
	switch (git_config_get_untracked_cache()) {
	case -1: /* keep: do nothing */
		break;
	case 0: /* false */
		remove_untracked_cache(istate);
		break;
	case 1: /* true */
		add_untracked_cache(istate);
		break;
	default: /* unknown value: do nothing */
		break;
	}
}

static void tweak_split_index(struct index_state *istate)
{
	switch (git_config_get_split_index()) {
	case -1: /* unset: do nothing */
		break;
	case 0: /* false */
		remove_split_index(istate);
		break;
	case 1: /* true */
		add_split_index(istate);
		break;
	default: /* unknown value: do nothing */
		break;
	}
}

static void post_read_index_from(struct index_state *istate)
{
	check_ce_order(istate);
	tweak_untracked_cache(istate);
	tweak_split_index(istate);
	tweak_fsmonitor(istate);
}

static size_t estimate_cache_size_from_compressed(unsigned int entries)
{
	return entries * (sizeof(struct cache_entry) + CACHE_ENTRY_PATH_LENGTH);
}

static size_t estimate_cache_size(size_t ondisk_size, unsigned int entries)
{
	long per_entry = sizeof(struct cache_entry) - sizeof(struct ondisk_cache_entry);

	/*
	 * Account for potential alignment differences.
	 */
	per_entry += align_padding_size(sizeof(struct cache_entry), -sizeof(struct ondisk_cache_entry));
	return ondisk_size + entries * per_entry;
}

static size_t read_eoie_extension(const char *mmap, size_t mmap_size);
static void write_eoie_extension(struct strbuf *sb, git_hash_ctx *eoie_context, size_t offset);

struct load_index_extensions
{
#ifndef NO_PTHREADS
	pthread_t pthread;
#endif
	struct index_state *istate;
	const char *mmap;
	size_t mmap_size;
	unsigned long src_offset;
};

static void *load_index_extensions(void *_data)
{
	struct load_index_extensions *p = _data;
	unsigned long src_offset = p->src_offset;

	while (src_offset <= p->mmap_size - the_hash_algo->rawsz - 8) {
		/* After an array of active_nr index entries,
		 * there can be arbitrary number of extended
		 * sections, each of which is prefixed with
		 * extension name (4-byte) and section length
		 * in 4-byte network byte order.
		 */
		uint32_t extsize = get_be32(p->mmap + src_offset + 4);
		if (read_index_extension(p->istate,
					 p->mmap + src_offset,
					 p->mmap + src_offset + 8,
					 extsize) < 0) {
			munmap((void *)p->mmap, p->mmap_size);
			die(_("index file corrupt"));
		}
		src_offset += 8;
		src_offset += extsize;
	}

	return NULL;
}

/* remember to discard_cache() before reading a different cache! */
int do_read_index(struct index_state *istate, const char *path, int must_exist)
{
	int fd, i;
	struct stat st;
	unsigned long src_offset;
	const struct cache_header *hdr;
	const char *mmap;
	size_t mmap_size;
	const struct cache_entry *previous_ce = NULL;
	struct load_index_extensions p;
	size_t extension_offset = 0;
#ifndef NO_PTHREADS
	int nr_threads;
#endif

	if (istate->initialized)
		return istate->cache_nr;

	istate->timestamp.sec = 0;
	istate->timestamp.nsec = 0;
	fd = open(path, O_RDONLY);
	if (fd < 0) {
		if (!must_exist && errno == ENOENT)
			return 0;
		die_errno("%s: index file open failed", path);
	}

	if (fstat(fd, &st))
		die_errno("cannot stat the open index");

	mmap_size = xsize_t(st.st_size);
	if (mmap_size < sizeof(struct cache_header) + the_hash_algo->rawsz)
		die("index file smaller than expected");

	mmap = xmmap(NULL, mmap_size, PROT_READ, MAP_PRIVATE, fd, 0);
	if (mmap == MAP_FAILED)
		die_errno("unable to map index file");
	close(fd);

	hdr = (const struct cache_header *)mmap;
	if (verify_hdr(hdr, mmap_size) < 0)
		goto unmap;

	hashcpy(istate->oid.hash, (const unsigned char *)hdr + mmap_size - the_hash_algo->rawsz);
	istate->version = ntohl(hdr->hdr_version);
	istate->cache_nr = ntohl(hdr->hdr_entries);
	istate->cache_alloc = alloc_nr(istate->cache_nr);
	istate->cache = xcalloc(istate->cache_alloc, sizeof(*istate->cache));
	istate->initialized = 1;

	p.istate = istate;
	p.mmap = mmap;
	p.mmap_size = mmap_size;

#ifndef NO_PTHREADS
	nr_threads = git_config_get_index_threads();
	if (!nr_threads)
		nr_threads = online_cpus();

	if (nr_threads > 1) {
		extension_offset = read_eoie_extension(mmap, mmap_size);
		if (extension_offset) {
			int err;

			p.src_offset = extension_offset;
			err = pthread_create(&p.pthread, NULL, load_index_extensions, &p);
			if (err)
				die(_("unable to create load_index_extensions thread: %s"), strerror(err));

			nr_threads--;
		}
	}
#endif

	if (istate->version == 4) {
		mem_pool_init(&istate->ce_mem_pool,
			      estimate_cache_size_from_compressed(istate->cache_nr));
	} else {
		mem_pool_init(&istate->ce_mem_pool,
			      estimate_cache_size(mmap_size, istate->cache_nr));
	}

	src_offset = sizeof(*hdr);
	for (i = 0; i < istate->cache_nr; i++) {
		struct ondisk_cache_entry *disk_ce;
		struct cache_entry *ce;
		unsigned long consumed;

		disk_ce = (struct ondisk_cache_entry *)(mmap + src_offset);
		ce = create_from_disk(istate, disk_ce, &consumed, previous_ce);
		set_index_entry(istate, i, ce);

		src_offset += consumed;
		previous_ce = ce;
	}
	istate->timestamp.sec = st.st_mtime;
	istate->timestamp.nsec = ST_MTIME_NSEC(st);

	/* if we created a thread, join it otherwise load the extensions on the primary thread */
#ifndef NO_PTHREADS
	if (extension_offset) {
		int ret = pthread_join(p.pthread, NULL);
		if (ret)
			die(_("unable to join load_index_extensions thread: %s"), strerror(ret));
	}
#endif
	if (!extension_offset) {
		p.src_offset = src_offset;
		load_index_extensions(&p);
	}
	munmap((void *)mmap, mmap_size);
	return istate->cache_nr;

unmap:
	munmap((void *)mmap, mmap_size);
	die("index file corrupt");
}

/*
 * Signal that the shared index is used by updating its mtime.
 *
 * This way, shared index can be removed if they have not been used
 * for some time.
 */
static void freshen_shared_index(const char *shared_index, int warn)
{
	if (!check_and_freshen_file(shared_index, 1) && warn)
		warning("could not freshen shared index '%s'", shared_index);
}

int read_index_from(struct index_state *istate, const char *path,
		    const char *gitdir)
{
	struct split_index *split_index;
	int ret;
	char *base_oid_hex;
	char *base_path;

	/* istate->initialized covers both .git/index and .git/sharedindex.xxx */
	if (istate->initialized)
		return istate->cache_nr;

	trace_performance_enter();
	ret = do_read_index(istate, path, 0);
	trace_performance_leave("read cache %s", path);

	split_index = istate->split_index;
	if (!split_index || is_null_oid(&split_index->base_oid)) {
		post_read_index_from(istate);
		return ret;
	}

	trace_performance_enter();
	if (split_index->base)
		discard_index(split_index->base);
	else
		split_index->base = xcalloc(1, sizeof(*split_index->base));

	base_oid_hex = oid_to_hex(&split_index->base_oid);
	base_path = xstrfmt("%s/sharedindex.%s", gitdir, base_oid_hex);
	ret = do_read_index(split_index->base, base_path, 1);
	if (!oideq(&split_index->base_oid, &split_index->base->oid))
		die("broken index, expect %s in %s, got %s",
		    base_oid_hex, base_path,
		    oid_to_hex(&split_index->base->oid));

	freshen_shared_index(base_path, 0);
	merge_base_index(istate);
	post_read_index_from(istate);
	free(base_path);
	trace_performance_leave("read cache %s", base_path);
	return ret;
}

int is_index_unborn(struct index_state *istate)
{
	return (!istate->cache_nr && !istate->timestamp.sec);
}

int discard_index(struct index_state *istate)
{
	/*
	 * Cache entries in istate->cache[] should have been allocated
	 * from the memory pool associated with this index, or from an
	 * associated split_index. There is no need to free individual
	 * cache entries. validate_cache_entries can detect when this
	 * assertion does not hold.
	 */
	validate_cache_entries(istate);

	resolve_undo_clear_index(istate);
	istate->cache_nr = 0;
	istate->cache_changed = 0;
	istate->timestamp.sec = 0;
	istate->timestamp.nsec = 0;
	free_name_hash(istate);
	cache_tree_free(&(istate->cache_tree));
	istate->initialized = 0;
	FREE_AND_NULL(istate->cache);
	istate->cache_alloc = 0;
	discard_split_index(istate);
	free_untracked_cache(istate->untracked);
	istate->untracked = NULL;

	if (istate->ce_mem_pool) {
		mem_pool_discard(istate->ce_mem_pool, should_validate_cache_entries());
		istate->ce_mem_pool = NULL;
	}

	return 0;
}

/*
 * Validate the cache entries of this index.
 * All cache entries associated with this index
 * should have been allocated by the memory pool
 * associated with this index, or by a referenced
 * split index.
 */
void validate_cache_entries(const struct index_state *istate)
{
	int i;

	if (!should_validate_cache_entries() ||!istate || !istate->initialized)
		return;

	for (i = 0; i < istate->cache_nr; i++) {
		if (!istate) {
			die("internal error: cache entry is not allocated from expected memory pool");
		} else if (!istate->ce_mem_pool ||
			!mem_pool_contains(istate->ce_mem_pool, istate->cache[i])) {
			if (!istate->split_index ||
				!istate->split_index->base ||
				!istate->split_index->base->ce_mem_pool ||
				!mem_pool_contains(istate->split_index->base->ce_mem_pool, istate->cache[i])) {
				die("internal error: cache entry is not allocated from expected memory pool");
			}
		}
	}

	if (istate->split_index)
		validate_cache_entries(istate->split_index->base);
}

int unmerged_index(const struct index_state *istate)
{
	int i;
	for (i = 0; i < istate->cache_nr; i++) {
		if (ce_stage(istate->cache[i]))
			return 1;
	}
	return 0;
}

int index_has_changes(const struct index_state *istate,
		      struct tree *tree,
		      struct strbuf *sb)
{
	struct object_id cmp;
	int i;

	if (istate != &the_index) {
		BUG("index_has_changes cannot yet accept istate != &the_index; do_diff_cache needs updating first.");
	}
	if (tree)
		cmp = tree->object.oid;
	if (tree || !get_oid_tree("HEAD", &cmp)) {
		struct diff_options opt;

		diff_setup(&opt);
		opt.flags.exit_with_status = 1;
		if (!sb)
			opt.flags.quick = 1;
		do_diff_cache(&cmp, &opt);
		diffcore_std(&opt);
		for (i = 0; sb && i < diff_queued_diff.nr; i++) {
			if (i)
				strbuf_addch(sb, ' ');
			strbuf_addstr(sb, diff_queued_diff.queue[i]->two->path);
		}
		diff_flush(&opt);
		return opt.flags.has_changes != 0;
	} else {
		for (i = 0; sb && i < istate->cache_nr; i++) {
			if (i)
				strbuf_addch(sb, ' ');
			strbuf_addstr(sb, istate->cache[i]->name);
		}
		return !!istate->cache_nr;
	}
}

#define WRITE_BUFFER_SIZE 8192
static unsigned char write_buffer[WRITE_BUFFER_SIZE];
static unsigned long write_buffer_len;

static int ce_write_flush(git_hash_ctx *context, int fd)
{
	unsigned int buffered = write_buffer_len;
	if (buffered) {
		the_hash_algo->update_fn(context, write_buffer, buffered);
		if (write_in_full(fd, write_buffer, buffered) < 0)
			return -1;
		write_buffer_len = 0;
	}
	return 0;
}

static int ce_write(git_hash_ctx *context, int fd, void *data, unsigned int len)
{
	while (len) {
		unsigned int buffered = write_buffer_len;
		unsigned int partial = WRITE_BUFFER_SIZE - buffered;
		if (partial > len)
			partial = len;
		memcpy(write_buffer + buffered, data, partial);
		buffered += partial;
		if (buffered == WRITE_BUFFER_SIZE) {
			write_buffer_len = buffered;
			if (ce_write_flush(context, fd))
				return -1;
			buffered = 0;
		}
		write_buffer_len = buffered;
		len -= partial;
		data = (char *) data + partial;
	}
	return 0;
}

static int write_index_ext_header(git_hash_ctx *context, git_hash_ctx *eoie_context,
				  int fd, unsigned int ext, unsigned int sz)
{
	ext = htonl(ext);
	sz = htonl(sz);
	if (eoie_context) {
		the_hash_algo->update_fn(eoie_context, &ext, 4);
		the_hash_algo->update_fn(eoie_context, &sz, 4);
	}
	return ((ce_write(context, fd, &ext, 4) < 0) ||
		(ce_write(context, fd, &sz, 4) < 0)) ? -1 : 0;
}

static int ce_flush(git_hash_ctx *context, int fd, unsigned char *hash)
{
	unsigned int left = write_buffer_len;

	if (left) {
		write_buffer_len = 0;
		the_hash_algo->update_fn(context, write_buffer, left);
	}

	/* Flush first if not enough space for hash signature */
	if (left + the_hash_algo->rawsz > WRITE_BUFFER_SIZE) {
		if (write_in_full(fd, write_buffer, left) < 0)
			return -1;
		left = 0;
	}

	/* Append the hash signature at the end */
	the_hash_algo->final_fn(write_buffer + left, context);
	hashcpy(hash, write_buffer + left);
	left += the_hash_algo->rawsz;
	return (write_in_full(fd, write_buffer, left) < 0) ? -1 : 0;
}

static void ce_smudge_racily_clean_entry(struct cache_entry *ce)
{
	/*
	 * The only thing we care about in this function is to smudge the
	 * falsely clean entry due to touch-update-touch race, so we leave
	 * everything else as they are.  We are called for entries whose
	 * ce_stat_data.sd_mtime match the index file mtime.
	 *
	 * Note that this actually does not do much for gitlinks, for
	 * which ce_match_stat_basic() always goes to the actual
	 * contents.  The caller checks with is_racy_timestamp() which
	 * always says "no" for gitlinks, so we are not called for them ;-)
	 */
	struct stat st;

	if (lstat(ce->name, &st) < 0)
		return;
	if (ce_match_stat_basic(ce, &st))
		return;
	if (ce_modified_check_fs(ce, &st)) {
		/* This is "racily clean"; smudge it.  Note that this
		 * is a tricky code.  At first glance, it may appear
		 * that it can break with this sequence:
		 *
		 * $ echo xyzzy >frotz
		 * $ git-update-index --add frotz
		 * $ : >frotz
		 * $ sleep 3
		 * $ echo filfre >nitfol
		 * $ git-update-index --add nitfol
		 *
		 * but it does not.  When the second update-index runs,
		 * it notices that the entry "frotz" has the same timestamp
		 * as index, and if we were to smudge it by resetting its
		 * size to zero here, then the object name recorded
		 * in index is the 6-byte file but the cached stat information
		 * becomes zero --- which would then match what we would
		 * obtain from the filesystem next time we stat("frotz").
		 *
		 * However, the second update-index, before calling
		 * this function, notices that the cached size is 6
		 * bytes and what is on the filesystem is an empty
		 * file, and never calls us, so the cached size information
		 * for "frotz" stays 6 which does not match the filesystem.
		 */
		ce->ce_stat_data.sd_size = 0;
	}
}

/* Copy miscellaneous fields but not the name */
static void copy_cache_entry_to_ondisk(struct ondisk_cache_entry *ondisk,
				       struct cache_entry *ce)
{
	short flags;

	ondisk->ctime.sec = htonl(ce->ce_stat_data.sd_ctime.sec);
	ondisk->mtime.sec = htonl(ce->ce_stat_data.sd_mtime.sec);
	ondisk->ctime.nsec = htonl(ce->ce_stat_data.sd_ctime.nsec);
	ondisk->mtime.nsec = htonl(ce->ce_stat_data.sd_mtime.nsec);
	ondisk->dev  = htonl(ce->ce_stat_data.sd_dev);
	ondisk->ino  = htonl(ce->ce_stat_data.sd_ino);
	ondisk->mode = htonl(ce->ce_mode);
	ondisk->uid  = htonl(ce->ce_stat_data.sd_uid);
	ondisk->gid  = htonl(ce->ce_stat_data.sd_gid);
	ondisk->size = htonl(ce->ce_stat_data.sd_size);
	hashcpy(ondisk->sha1, ce->oid.hash);

	flags = ce->ce_flags & ~CE_NAMEMASK;
	flags |= (ce_namelen(ce) >= CE_NAMEMASK ? CE_NAMEMASK : ce_namelen(ce));
	ondisk->flags = htons(flags);
	if (ce->ce_flags & CE_EXTENDED) {
		struct ondisk_cache_entry_extended *ondisk2;
		ondisk2 = (struct ondisk_cache_entry_extended *)ondisk;
		ondisk2->flags2 = htons((ce->ce_flags & CE_EXTENDED_FLAGS) >> 16);
	}
}

static int ce_write_entry(git_hash_ctx *c, int fd, struct cache_entry *ce,
			  struct strbuf *previous_name, struct ondisk_cache_entry *ondisk)
{
	int size;
	int result;
	unsigned int saved_namelen;
	int stripped_name = 0;
	static unsigned char padding[8] = { 0x00 };

	if (ce->ce_flags & CE_STRIP_NAME) {
		saved_namelen = ce_namelen(ce);
		ce->ce_namelen = 0;
		stripped_name = 1;
	}

	if (ce->ce_flags & CE_EXTENDED)
		size = offsetof(struct ondisk_cache_entry_extended, name);
	else
		size = offsetof(struct ondisk_cache_entry, name);

	if (!previous_name) {
		int len = ce_namelen(ce);
		copy_cache_entry_to_ondisk(ondisk, ce);
		result = ce_write(c, fd, ondisk, size);
		if (!result)
			result = ce_write(c, fd, ce->name, len);
		if (!result)
			result = ce_write(c, fd, padding, align_padding_size(size, len));
	} else {
		int common, to_remove, prefix_size;
		unsigned char to_remove_vi[16];
		for (common = 0;
		     (ce->name[common] &&
		      common < previous_name->len &&
		      ce->name[common] == previous_name->buf[common]);
		     common++)
			; /* still matching */
		to_remove = previous_name->len - common;
		prefix_size = encode_varint(to_remove, to_remove_vi);

		copy_cache_entry_to_ondisk(ondisk, ce);
		result = ce_write(c, fd, ondisk, size);
		if (!result)
			result = ce_write(c, fd, to_remove_vi, prefix_size);
		if (!result)
			result = ce_write(c, fd, ce->name + common, ce_namelen(ce) - common);
		if (!result)
			result = ce_write(c, fd, padding, 1);

		strbuf_splice(previous_name, common, to_remove,
			      ce->name + common, ce_namelen(ce) - common);
	}
	if (stripped_name) {
		ce->ce_namelen = saved_namelen;
		ce->ce_flags &= ~CE_STRIP_NAME;
	}

	return result;
}

/*
 * This function verifies if index_state has the correct sha1 of the
 * index file.  Don't die if we have any other failure, just return 0.
 */
static int verify_index_from(const struct index_state *istate, const char *path)
{
	int fd;
	ssize_t n;
	struct stat st;
	unsigned char hash[GIT_MAX_RAWSZ];

	if (!istate->initialized)
		return 0;

	fd = open(path, O_RDONLY);
	if (fd < 0)
		return 0;

	if (fstat(fd, &st))
		goto out;

	if (st.st_size < sizeof(struct cache_header) + the_hash_algo->rawsz)
		goto out;

	n = pread_in_full(fd, hash, the_hash_algo->rawsz, st.st_size - the_hash_algo->rawsz);
	if (n != the_hash_algo->rawsz)
		goto out;

	if (!hasheq(istate->oid.hash, hash))
		goto out;

	close(fd);
	return 1;

out:
	close(fd);
	return 0;
}

static int verify_index(const struct index_state *istate)
{
	return verify_index_from(istate, get_index_file());
}

static int has_racy_timestamp(struct index_state *istate)
{
	int entries = istate->cache_nr;
	int i;

	for (i = 0; i < entries; i++) {
		struct cache_entry *ce = istate->cache[i];
		if (is_racy_timestamp(istate, ce))
			return 1;
	}
	return 0;
}

void update_index_if_able(struct index_state *istate, struct lock_file *lockfile)
{
	if ((istate->cache_changed || has_racy_timestamp(istate)) &&
	    verify_index(istate))
		write_locked_index(istate, lockfile, COMMIT_LOCK);
	else
		rollback_lock_file(lockfile);
}

/*
 * On success, `tempfile` is closed. If it is the temporary file
 * of a `struct lock_file`, we will therefore effectively perform
 * a 'close_lock_file_gently()`. Since that is an implementation
 * detail of lockfiles, callers of `do_write_index()` should not
 * rely on it.
 */
static int do_write_index(struct index_state *istate, struct tempfile *tempfile,
			  int strip_extensions)
{
	uint64_t start = getnanotime();
	int newfd = tempfile->fd;
	git_hash_ctx c, eoie_c;
	struct cache_header hdr;
	int i, err = 0, removed, extended, hdr_version;
	struct cache_entry **cache = istate->cache;
	int entries = istate->cache_nr;
	struct stat st;
	struct ondisk_cache_entry_extended ondisk;
	struct strbuf previous_name_buf = STRBUF_INIT, *previous_name;
	int drop_cache_tree = istate->drop_cache_tree;
	off_t offset;

	for (i = removed = extended = 0; i < entries; i++) {
		if (cache[i]->ce_flags & CE_REMOVE)
			removed++;

		/* reduce extended entries if possible */
		cache[i]->ce_flags &= ~CE_EXTENDED;
		if (cache[i]->ce_flags & CE_EXTENDED_FLAGS) {
			extended++;
			cache[i]->ce_flags |= CE_EXTENDED;
		}
	}

	if (!istate->version) {
		istate->version = get_index_format_default();
		if (git_env_bool("GIT_TEST_SPLIT_INDEX", 0))
			init_split_index(istate);
	}

	/* demote version 3 to version 2 when the latter suffices */
	if (istate->version == 3 || istate->version == 2)
		istate->version = extended ? 3 : 2;

	hdr_version = istate->version;

	hdr.hdr_signature = htonl(CACHE_SIGNATURE);
	hdr.hdr_version = htonl(hdr_version);
	hdr.hdr_entries = htonl(entries - removed);

	the_hash_algo->init_fn(&c);
	if (ce_write(&c, newfd, &hdr, sizeof(hdr)) < 0)
		return -1;

	offset = lseek(newfd, 0, SEEK_CUR);
	if (offset < 0)
		return -1;
	offset += write_buffer_len;
	previous_name = (hdr_version == 4) ? &previous_name_buf : NULL;

	for (i = 0; i < entries; i++) {
		struct cache_entry *ce = cache[i];
		if (ce->ce_flags & CE_REMOVE)
			continue;
		if (!ce_uptodate(ce) && is_racy_timestamp(istate, ce))
			ce_smudge_racily_clean_entry(ce);
		if (is_null_oid(&ce->oid)) {
			static const char msg[] = "cache entry has null sha1: %s";
			static int allow = -1;

			if (allow < 0)
				allow = git_env_bool("GIT_ALLOW_NULL_SHA1", 0);
			if (allow)
				warning(msg, ce->name);
			else
				err = error(msg, ce->name);

			drop_cache_tree = 1;
		}
		if (ce_write_entry(&c, newfd, ce, previous_name, (struct ondisk_cache_entry *)&ondisk) < 0)
			err = -1;

		if (err)
			break;
	}
	strbuf_release(&previous_name_buf);

	if (err)
		return err;

	/* Write extension data here */
	offset = lseek(newfd, 0, SEEK_CUR);
	if (offset < 0)
		return -1;
	offset += write_buffer_len;
	the_hash_algo->init_fn(&eoie_c);

	if (!strip_extensions && istate->split_index) {
		struct strbuf sb = STRBUF_INIT;

		err = write_link_extension(&sb, istate) < 0 ||
			write_index_ext_header(&c, &eoie_c, newfd, CACHE_EXT_LINK,
					       sb.len) < 0 ||
			ce_write(&c, newfd, sb.buf, sb.len) < 0;
		strbuf_release(&sb);
		if (err)
			return -1;
	}
	if (!strip_extensions && !drop_cache_tree && istate->cache_tree) {
		struct strbuf sb = STRBUF_INIT;

		cache_tree_write(&sb, istate->cache_tree);
		err = write_index_ext_header(&c, &eoie_c, newfd, CACHE_EXT_TREE, sb.len) < 0
			|| ce_write(&c, newfd, sb.buf, sb.len) < 0;
		strbuf_release(&sb);
		if (err)
			return -1;
	}
	if (!strip_extensions && istate->resolve_undo) {
		struct strbuf sb = STRBUF_INIT;

		resolve_undo_write(&sb, istate->resolve_undo);
		err = write_index_ext_header(&c, &eoie_c, newfd, CACHE_EXT_RESOLVE_UNDO,
					     sb.len) < 0
			|| ce_write(&c, newfd, sb.buf, sb.len) < 0;
		strbuf_release(&sb);
		if (err)
			return -1;
	}
	if (!strip_extensions && istate->untracked) {
		struct strbuf sb = STRBUF_INIT;

		write_untracked_extension(&sb, istate->untracked);
		err = write_index_ext_header(&c, &eoie_c, newfd, CACHE_EXT_UNTRACKED,
					     sb.len) < 0 ||
			ce_write(&c, newfd, sb.buf, sb.len) < 0;
		strbuf_release(&sb);
		if (err)
			return -1;
	}
	if (!strip_extensions && istate->fsmonitor_last_update) {
		struct strbuf sb = STRBUF_INIT;

		write_fsmonitor_extension(&sb, istate);
		err = write_index_ext_header(&c, &eoie_c, newfd, CACHE_EXT_FSMONITOR, sb.len) < 0
			|| ce_write(&c, newfd, sb.buf, sb.len) < 0;
		strbuf_release(&sb);
		if (err)
			return -1;
	}

	/*
	 * CACHE_EXT_ENDOFINDEXENTRIES must be written as the last entry before the SHA1
	 * so that it can be found and processed before all the index entries are
	 * read.  Write it out regardless of the strip_extensions parameter as we need it
	 * when loading the shared index.
	 */
	if (offset) {
		struct strbuf sb = STRBUF_INIT;

		write_eoie_extension(&sb, &eoie_c, offset);
		err = write_index_ext_header(&c, NULL, newfd, CACHE_EXT_ENDOFINDEXENTRIES, sb.len) < 0
			|| ce_write(&c, newfd, sb.buf, sb.len) < 0;
		strbuf_release(&sb);
		if (err)
			return -1;
	}

	if (ce_flush(&c, newfd, istate->oid.hash))
		return -1;
	if (close_tempfile_gently(tempfile)) {
		error(_("could not close '%s'"), tempfile->filename.buf);
		return -1;
	}
	if (stat(tempfile->filename.buf, &st))
		return -1;
	istate->timestamp.sec = (unsigned int)st.st_mtime;
	istate->timestamp.nsec = ST_MTIME_NSEC(st);
	trace_performance_since(start, "write index, changed mask = %x", istate->cache_changed);
	return 0;
}

void set_alternate_index_output(const char *name)
{
	alternate_index_output = name;
}

static int commit_locked_index(struct lock_file *lk)
{
	if (alternate_index_output)
		return commit_lock_file_to(lk, alternate_index_output);
	else
		return commit_lock_file(lk);
}

static int do_write_locked_index(struct index_state *istate, struct lock_file *lock,
				 unsigned flags)
{
	int ret = do_write_index(istate, lock->tempfile, 0);
	if (ret)
		return ret;
	if (flags & COMMIT_LOCK)
		return commit_locked_index(lock);
	return close_lock_file_gently(lock);
}

static int write_split_index(struct index_state *istate,
			     struct lock_file *lock,
			     unsigned flags)
{
	int ret;
	prepare_to_write_split_index(istate);
	ret = do_write_locked_index(istate, lock, flags);
	finish_writing_split_index(istate);
	return ret;
}

static const char *shared_index_expire = "2.weeks.ago";

static unsigned long get_shared_index_expire_date(void)
{
	static unsigned long shared_index_expire_date;
	static int shared_index_expire_date_prepared;

	if (!shared_index_expire_date_prepared) {
		git_config_get_expiry("splitindex.sharedindexexpire",
				      &shared_index_expire);
		shared_index_expire_date = approxidate(shared_index_expire);
		shared_index_expire_date_prepared = 1;
	}

	return shared_index_expire_date;
}

static int should_delete_shared_index(const char *shared_index_path)
{
	struct stat st;
	unsigned long expiration;

	/* Check timestamp */
	expiration = get_shared_index_expire_date();
	if (!expiration)
		return 0;
	if (stat(shared_index_path, &st))
		return error_errno(_("could not stat '%s'"), shared_index_path);
	if (st.st_mtime > expiration)
		return 0;

	return 1;
}

static int clean_shared_index_files(const char *current_hex)
{
	struct dirent *de;
	DIR *dir = opendir(get_git_dir());

	if (!dir)
		return error_errno(_("unable to open git dir: %s"), get_git_dir());

	while ((de = readdir(dir)) != NULL) {
		const char *sha1_hex;
		const char *shared_index_path;
		if (!skip_prefix(de->d_name, "sharedindex.", &sha1_hex))
			continue;
		if (!strcmp(sha1_hex, current_hex))
			continue;
		shared_index_path = git_path("%s", de->d_name);
		if (should_delete_shared_index(shared_index_path) > 0 &&
		    unlink(shared_index_path))
			warning_errno(_("unable to unlink: %s"), shared_index_path);
	}
	closedir(dir);

	return 0;
}

static int write_shared_index(struct index_state *istate,
			      struct tempfile **temp)
{
	struct split_index *si = istate->split_index;
	int ret;

	move_cache_to_base_index(istate);
	ret = do_write_index(si->base, *temp, 1);
	if (ret)
		return ret;
	ret = adjust_shared_perm(get_tempfile_path(*temp));
	if (ret) {
		error("cannot fix permission bits on %s", get_tempfile_path(*temp));
		return ret;
	}
	ret = rename_tempfile(temp,
			      git_path("sharedindex.%s", oid_to_hex(&si->base->oid)));
	if (!ret) {
		oidcpy(&si->base_oid, &si->base->oid);
		clean_shared_index_files(oid_to_hex(&si->base->oid));
	}

	return ret;
}

static const int default_max_percent_split_change = 20;

static int too_many_not_shared_entries(struct index_state *istate)
{
	int i, not_shared = 0;
	int max_split = git_config_get_max_percent_split_change();

	switch (max_split) {
	case -1:
		/* not or badly configured: use the default value */
		max_split = default_max_percent_split_change;
		break;
	case 0:
		return 1; /* 0% means always write a new shared index */
	case 100:
		return 0; /* 100% means never write a new shared index */
	default:
		break; /* just use the configured value */
	}

	/* Count not shared entries */
	for (i = 0; i < istate->cache_nr; i++) {
		struct cache_entry *ce = istate->cache[i];
		if (!ce->index)
			not_shared++;
	}

	return (int64_t)istate->cache_nr * max_split < (int64_t)not_shared * 100;
}

int write_locked_index(struct index_state *istate, struct lock_file *lock,
		       unsigned flags)
{
	int new_shared_index, ret;
	struct split_index *si = istate->split_index;

	if (git_env_bool("GIT_TEST_CHECK_CACHE_TREE", 0))
		cache_tree_verify(istate);

	if ((flags & SKIP_IF_UNCHANGED) && !istate->cache_changed) {
		if (flags & COMMIT_LOCK)
			rollback_lock_file(lock);
		return 0;
	}

	if (istate->fsmonitor_last_update)
		fill_fsmonitor_bitmap(istate);

	if (!si || alternate_index_output ||
	    (istate->cache_changed & ~EXTMASK)) {
		if (si)
			oidclr(&si->base_oid);
		ret = do_write_locked_index(istate, lock, flags);
		goto out;
	}

	if (git_env_bool("GIT_TEST_SPLIT_INDEX", 0)) {
		int v = si->base_oid.hash[0];
		if ((v & 15) < 6)
			istate->cache_changed |= SPLIT_INDEX_ORDERED;
	}
	if (too_many_not_shared_entries(istate))
		istate->cache_changed |= SPLIT_INDEX_ORDERED;

	new_shared_index = istate->cache_changed & SPLIT_INDEX_ORDERED;

	if (new_shared_index) {
		struct tempfile *temp;
		int saved_errno;

		temp = mks_tempfile(git_path("sharedindex_XXXXXX"));
		if (!temp) {
			oidclr(&si->base_oid);
			ret = do_write_locked_index(istate, lock, flags);
			goto out;
		}
		ret = write_shared_index(istate, &temp);

		saved_errno = errno;
		if (is_tempfile_active(temp))
			delete_tempfile(&temp);
		errno = saved_errno;

		if (ret)
			goto out;
	}

	ret = write_split_index(istate, lock, flags);

	/* Freshen the shared index only if the split-index was written */
	if (!ret && !new_shared_index) {
		const char *shared_index = git_path("sharedindex.%s",
						    oid_to_hex(&si->base_oid));
		freshen_shared_index(shared_index, 1);
	}

out:
	if (flags & COMMIT_LOCK)
		rollback_lock_file(lock);
	return ret;
}

/*
 * Read the index file that is potentially unmerged into given
 * index_state, dropping any unmerged entries to stage #0 (potentially
 * resulting in a path appearing as both a file and a directory in the
 * index; the caller is responsible to clear out the extra entries
 * before writing the index to a tree).  Returns true if the index is
 * unmerged.  Callers who want to refuse to work from an unmerged
 * state can call this and check its return value, instead of calling
 * read_cache().
 */
int read_index_unmerged(struct index_state *istate)
{
	int i;
	int unmerged = 0;

	read_index(istate);
	for (i = 0; i < istate->cache_nr; i++) {
		struct cache_entry *ce = istate->cache[i];
		struct cache_entry *new_ce;
		int len;

		if (!ce_stage(ce))
			continue;
		unmerged = 1;
		len = ce_namelen(ce);
		new_ce = make_empty_cache_entry(istate, len);
		memcpy(new_ce->name, ce->name, len);
		new_ce->ce_flags = create_ce_flags(0) | CE_CONFLICTED;
		new_ce->ce_namelen = len;
		new_ce->ce_mode = ce->ce_mode;
		if (add_index_entry(istate, new_ce, ADD_CACHE_SKIP_DFCHECK))
			return error("%s: cannot drop to stage #0",
				     new_ce->name);
	}
	return unmerged;
}

/*
 * Returns 1 if the path is an "other" path with respect to
 * the index; that is, the path is not mentioned in the index at all,
 * either as a file, a directory with some files in the index,
 * or as an unmerged entry.
 *
 * We helpfully remove a trailing "/" from directories so that
 * the output of read_directory can be used as-is.
 */
int index_name_is_other(const struct index_state *istate, const char *name,
		int namelen)
{
	int pos;
	if (namelen && name[namelen - 1] == '/')
		namelen--;
	pos = index_name_pos(istate, name, namelen);
	if (0 <= pos)
		return 0;	/* exact match */
	pos = -pos - 1;
	if (pos < istate->cache_nr) {
		struct cache_entry *ce = istate->cache[pos];
		if (ce_namelen(ce) == namelen &&
		    !memcmp(ce->name, name, namelen))
			return 0; /* Yup, this one exists unmerged */
	}
	return 1;
}

void *read_blob_data_from_index(const struct index_state *istate,
				const char *path, unsigned long *size)
{
	int pos, len;
	unsigned long sz;
	enum object_type type;
	void *data;

	len = strlen(path);
	pos = index_name_pos(istate, path, len);
	if (pos < 0) {
		/*
		 * We might be in the middle of a merge, in which
		 * case we would read stage #2 (ours).
		 */
		int i;
		for (i = -pos - 1;
		     (pos < 0 && i < istate->cache_nr &&
		      !strcmp(istate->cache[i]->name, path));
		     i++)
			if (ce_stage(istate->cache[i]) == 2)
				pos = i;
	}
	if (pos < 0)
		return NULL;
	data = read_object_file(&istate->cache[pos]->oid, &type, &sz);
	if (!data || type != OBJ_BLOB) {
		free(data);
		return NULL;
	}
	if (size)
		*size = sz;
	return data;
}

void stat_validity_clear(struct stat_validity *sv)
{
	FREE_AND_NULL(sv->sd);
}

int stat_validity_check(struct stat_validity *sv, const char *path)
{
	struct stat st;

	if (stat(path, &st) < 0)
		return sv->sd == NULL;
	if (!sv->sd)
		return 0;
	return S_ISREG(st.st_mode) && !match_stat_data(sv->sd, &st);
}

void stat_validity_update(struct stat_validity *sv, int fd)
{
	struct stat st;

	if (fstat(fd, &st) < 0 || !S_ISREG(st.st_mode))
		stat_validity_clear(sv);
	else {
		if (!sv->sd)
			sv->sd = xcalloc(1, sizeof(struct stat_data));
		fill_stat_data(sv->sd, &st);
	}
}

void move_index_extensions(struct index_state *dst, struct index_state *src)
{
	dst->untracked = src->untracked;
	src->untracked = NULL;
	dst->cache_tree = src->cache_tree;
	src->cache_tree = NULL;
}

struct cache_entry *dup_cache_entry(const struct cache_entry *ce,
				    struct index_state *istate)
{
	unsigned int size = ce_size(ce);
	int mem_pool_allocated;
	struct cache_entry *new_entry = make_empty_cache_entry(istate, ce_namelen(ce));
	mem_pool_allocated = new_entry->mem_pool_allocated;

	memcpy(new_entry, ce, size);
	new_entry->mem_pool_allocated = mem_pool_allocated;
	return new_entry;
}

void discard_cache_entry(struct cache_entry *ce)
{
	if (ce && should_validate_cache_entries())
		memset(ce, 0xCD, cache_entry_size(ce->ce_namelen));

	if (ce && ce->mem_pool_allocated)
		return;

	free(ce);
}

int should_validate_cache_entries(void)
{
	static int validate_index_cache_entries = -1;

	if (validate_index_cache_entries < 0) {
		if (getenv("GIT_TEST_VALIDATE_INDEX_CACHE_ENTRIES"))
			validate_index_cache_entries = 1;
		else
			validate_index_cache_entries = 0;
	}

	return validate_index_cache_entries;
}

#define EOIE_SIZE (4 + GIT_SHA1_RAWSZ) /* <4-byte offset> + <20-byte hash> */
#define EOIE_SIZE_WITH_HEADER (4 + 4 + EOIE_SIZE) /* <4-byte signature> + <4-byte length> + EOIE_SIZE */

static size_t read_eoie_extension(const char *mmap, size_t mmap_size)
{
	/*
	 * The end of index entries (EOIE) extension is guaranteed to be last
	 * so that it can be found by scanning backwards from the EOF.
	 *
	 * "EOIE"
	 * <4-byte length>
	 * <4-byte offset>
	 * <20-byte hash>
	 */
	const char *index, *eoie;
	uint32_t extsize;
	size_t offset, src_offset;
	unsigned char hash[GIT_MAX_RAWSZ];
	git_hash_ctx c;

	/* ensure we have an index big enough to contain an EOIE extension */
	if (mmap_size < sizeof(struct cache_header) + EOIE_SIZE_WITH_HEADER + the_hash_algo->rawsz)
		return 0;

	/* validate the extension signature */
	index = eoie = mmap + mmap_size - EOIE_SIZE_WITH_HEADER - the_hash_algo->rawsz;
	if (CACHE_EXT(index) != CACHE_EXT_ENDOFINDEXENTRIES)
		return 0;
	index += sizeof(uint32_t);

	/* validate the extension size */
	extsize = get_be32(index);
	if (extsize != EOIE_SIZE)
		return 0;
	index += sizeof(uint32_t);

	/*
	 * Validate the offset we're going to look for the first extension
	 * signature is after the index header and before the eoie extension.
	 */
	offset = get_be32(index);
	if (mmap + offset < mmap + sizeof(struct cache_header))
		return 0;
	if (mmap + offset >= eoie)
		return 0;
	index += sizeof(uint32_t);

	/*
	 * The hash is computed over extension types and their sizes (but not
	 * their contents).  E.g. if we have "TREE" extension that is N-bytes
	 * long, "REUC" extension that is M-bytes long, followed by "EOIE",
	 * then the hash would be:
	 *
	 * SHA-1("TREE" + <binary representation of N> +
	 *	 "REUC" + <binary representation of M>)
	 */
	src_offset = offset;
	the_hash_algo->init_fn(&c);
	while (src_offset < mmap_size - the_hash_algo->rawsz - EOIE_SIZE_WITH_HEADER) {
		/* After an array of active_nr index entries,
		 * there can be arbitrary number of extended
		 * sections, each of which is prefixed with
		 * extension name (4-byte) and section length
		 * in 4-byte network byte order.
		 */
		uint32_t extsize;
		memcpy(&extsize, mmap + src_offset + 4, 4);
		extsize = ntohl(extsize);

		/* verify the extension size isn't so large it will wrap around */
		if (src_offset + 8 + extsize < src_offset)
			return 0;

		the_hash_algo->update_fn(&c, mmap + src_offset, 8);

		src_offset += 8;
		src_offset += extsize;
	}
	the_hash_algo->final_fn(hash, &c);
	if (!hasheq(hash, (const unsigned char *)index))
		return 0;

	/* Validate that the extension offsets returned us back to the eoie extension. */
	if (src_offset != mmap_size - the_hash_algo->rawsz - EOIE_SIZE_WITH_HEADER)
		return 0;

	return offset;
}

static void write_eoie_extension(struct strbuf *sb, git_hash_ctx *eoie_context, size_t offset)
{
	uint32_t buffer;
	unsigned char hash[GIT_MAX_RAWSZ];

	/* offset */
	put_be32(&buffer, offset);
	strbuf_add(sb, &buffer, sizeof(uint32_t));

	/* hash */
	the_hash_algo->final_fn(hash, eoie_context);
	strbuf_add(sb, hash, the_hash_algo->rawsz);
}