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77a42b3b84
Our tempfile struct contains an "active" flag. Long ago, this flag was important: tempfile structs were always allocated for the lifetime of the program and added to a global linked list, and the active flag was what told us whether a struct's tempfile needed to be cleaned up on exit. But since422a21c6a0
(tempfile: remove deactivated list entries, 2017-09-05) and076aa2cbda
(tempfile: auto-allocate tempfiles on heap, 2017-09-05), we actually remove items from the list, and the active flag is generally always set to true for any allocated struct. We set it to true in all of the creation functions, and in the normal code flow it becomes false only in deactivate_tempfile(), which then immediately frees the struct. So the flag isn't performing that role anymore, and in fact makes things more confusing. Dscho noted that delete_tempfile() is a noop for an inactive struct. Since076aa2cbda
taught it to free the struct when deactivating, we'd leak any struct whose active flag is unset. But in practice it's not a leak, because again, we'll free when we unset the flag, and never see the allocated-but-inactive state. Can we just get rid of the flag? The answer is yes, but it requires looking at a few other spots: 1. I said above that the flag only becomes false before we deallocate, but there's one exception: when we call remove_tempfiles() from a signal or atexit handler, we unset the active flag as we remove each file. This isn't important for delete_tempfile(), as nobody would call it anymore, since we're exiting. It does in theory provide us some protection against racily double-removing a tempfile. If we receive a second signal while we are already in the cleanup routines, we'll start the cleanup loop again, and may visit the same tempfile. But this race already exists, because calling unlink() and unsetting the active flag aren't atomic! And it's OK in practice, because unlink() is idempotent (barring the unlikely event that some other process chooses our exact temp filename in that instant). So dropping the active flag widens the race a bit, but it was already there, and is fairly harmless in practice. If we really care about addressing it, the right thing is probably to block further signals while we're doing our cleanup (which we could actually do atomically). 2. The active flag is declared as "volatile sig_atomic_t". The idea is that it's the final bit that gets set to tell the cleanup routines that the tempfile is ready to be used (or not used), and it's safe to receive a signal racing with regular code which adds or removes a tempfile from the list. In practice, I don't think this is buying us anything. The presence on the linked list is really what tells the cleanup routines to look at the struct. That is already marked as "volatile". It's not a sig_atomic_t, so it's possible that we could see a sheared write there as an entry is added or removed. But that is true of the current code, too! Before we can even look at the "active" flag, we'd have to follow a link to the struct itself. If we see a sheared write in the pointer to the struct, then we'll look at garbage memory anyway, and there's not much we can do. This patch removes the active flag entirely, using presence on the global linked list as an indicator that a tempfile ought to be cleaned up. We are already careful to add to the list as the final step in activating. On deactivation, we'll make sure to remove from the list as the first step, before freeing any fields. The use of the volatile keyword should mean that those things happen in the expected order. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>
375 lines
9.5 KiB
C
375 lines
9.5 KiB
C
/*
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* State diagram and cleanup
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* -------------------------
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*
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* If the program exits while a temporary file is active, we want to
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* make sure that we remove it. This is done by remembering the active
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* temporary files in a linked list, `tempfile_list`. An `atexit(3)`
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* handler and a signal handler are registered, to clean up any active
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* temporary files.
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*
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* Because the signal handler can run at any time, `tempfile_list` and
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* the `tempfile` objects that comprise it must be kept in
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* self-consistent states at all times.
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*
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* The possible states of a `tempfile` object are as follows:
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*
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* - Uninitialized. In this state the object's `on_list` field must be
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* zero but the rest of its contents need not be initialized. As
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* soon as the object is used in any way, it is irrevocably
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* registered in `tempfile_list`, and `on_list` is set.
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*
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* - Active, file open (after `create_tempfile()` or
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* `reopen_tempfile()`). In this state:
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*
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* - the temporary file exists
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* - `active` is set
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* - `filename` holds the filename of the temporary file
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* - `fd` holds a file descriptor open for writing to it
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* - `fp` holds a pointer to an open `FILE` object if and only if
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* `fdopen_tempfile()` has been called on the object
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* - `owner` holds the PID of the process that created the file
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*
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* - Active, file closed (after `close_tempfile_gently()`). Same
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* as the previous state, except that the temporary file is closed,
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* `fd` is -1, and `fp` is `NULL`.
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*
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* - Inactive (after `delete_tempfile()`, `rename_tempfile()`, or a
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* failed attempt to create a temporary file). In this state:
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*
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* - `active` is unset
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* - `filename` is empty (usually, though there are transitory
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* states in which this condition doesn't hold). Client code should
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* *not* rely on the filename being empty in this state.
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* - `fd` is -1 and `fp` is `NULL`
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* - the object is removed from `tempfile_list` (but could be used again)
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*
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* A temporary file is owned by the process that created it. The
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* `tempfile` has an `owner` field that records the owner's PID. This
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* field is used to prevent a forked process from deleting a temporary
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* file created by its parent.
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*/
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#include "cache.h"
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#include "tempfile.h"
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#include "sigchain.h"
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static VOLATILE_LIST_HEAD(tempfile_list);
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static void remove_template_directory(struct tempfile *tempfile,
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int in_signal_handler)
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{
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if (tempfile->directory) {
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if (in_signal_handler)
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rmdir(tempfile->directory);
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else
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rmdir_or_warn(tempfile->directory);
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}
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}
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static void remove_tempfiles(int in_signal_handler)
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{
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pid_t me = getpid();
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volatile struct volatile_list_head *pos;
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list_for_each(pos, &tempfile_list) {
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struct tempfile *p = list_entry(pos, struct tempfile, list);
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if (!is_tempfile_active(p) || p->owner != me)
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continue;
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if (p->fd >= 0)
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close(p->fd);
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if (in_signal_handler)
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unlink(p->filename.buf);
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else
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unlink_or_warn(p->filename.buf);
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remove_template_directory(p, in_signal_handler);
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}
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}
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static void remove_tempfiles_on_exit(void)
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{
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remove_tempfiles(0);
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}
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static void remove_tempfiles_on_signal(int signo)
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{
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remove_tempfiles(1);
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sigchain_pop(signo);
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raise(signo);
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}
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static struct tempfile *new_tempfile(void)
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{
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struct tempfile *tempfile = xmalloc(sizeof(*tempfile));
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tempfile->fd = -1;
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tempfile->fp = NULL;
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tempfile->owner = 0;
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INIT_LIST_HEAD(&tempfile->list);
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strbuf_init(&tempfile->filename, 0);
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tempfile->directory = NULL;
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return tempfile;
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}
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static void activate_tempfile(struct tempfile *tempfile)
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{
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static int initialized;
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if (!initialized) {
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sigchain_push_common(remove_tempfiles_on_signal);
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atexit(remove_tempfiles_on_exit);
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initialized = 1;
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}
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volatile_list_add(&tempfile->list, &tempfile_list);
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tempfile->owner = getpid();
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}
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static void deactivate_tempfile(struct tempfile *tempfile)
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{
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volatile_list_del(&tempfile->list);
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strbuf_release(&tempfile->filename);
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free(tempfile->directory);
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free(tempfile);
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}
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/* Make sure errno contains a meaningful value on error */
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struct tempfile *create_tempfile_mode(const char *path, int mode)
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{
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struct tempfile *tempfile = new_tempfile();
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strbuf_add_absolute_path(&tempfile->filename, path);
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tempfile->fd = open(tempfile->filename.buf,
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O_RDWR | O_CREAT | O_EXCL | O_CLOEXEC, mode);
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if (O_CLOEXEC && tempfile->fd < 0 && errno == EINVAL)
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/* Try again w/o O_CLOEXEC: the kernel might not support it */
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tempfile->fd = open(tempfile->filename.buf,
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O_RDWR | O_CREAT | O_EXCL, mode);
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if (tempfile->fd < 0) {
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deactivate_tempfile(tempfile);
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return NULL;
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}
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activate_tempfile(tempfile);
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if (adjust_shared_perm(tempfile->filename.buf)) {
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int save_errno = errno;
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error("cannot fix permission bits on %s", tempfile->filename.buf);
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delete_tempfile(&tempfile);
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errno = save_errno;
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return NULL;
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}
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return tempfile;
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}
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struct tempfile *register_tempfile(const char *path)
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{
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struct tempfile *tempfile = new_tempfile();
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strbuf_add_absolute_path(&tempfile->filename, path);
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activate_tempfile(tempfile);
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return tempfile;
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}
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struct tempfile *mks_tempfile_sm(const char *filename_template, int suffixlen, int mode)
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{
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struct tempfile *tempfile = new_tempfile();
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strbuf_add_absolute_path(&tempfile->filename, filename_template);
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tempfile->fd = git_mkstemps_mode(tempfile->filename.buf, suffixlen, mode);
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if (tempfile->fd < 0) {
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deactivate_tempfile(tempfile);
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return NULL;
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}
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activate_tempfile(tempfile);
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return tempfile;
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}
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struct tempfile *mks_tempfile_tsm(const char *filename_template, int suffixlen, int mode)
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{
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struct tempfile *tempfile = new_tempfile();
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const char *tmpdir;
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tmpdir = getenv("TMPDIR");
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if (!tmpdir)
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tmpdir = "/tmp";
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strbuf_addf(&tempfile->filename, "%s/%s", tmpdir, filename_template);
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tempfile->fd = git_mkstemps_mode(tempfile->filename.buf, suffixlen, mode);
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if (tempfile->fd < 0) {
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deactivate_tempfile(tempfile);
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return NULL;
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}
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activate_tempfile(tempfile);
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return tempfile;
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}
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struct tempfile *mks_tempfile_dt(const char *directory_template,
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const char *filename)
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{
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struct tempfile *tempfile;
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const char *tmpdir;
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struct strbuf sb = STRBUF_INIT;
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int fd;
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size_t directorylen;
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if (!ends_with(directory_template, "XXXXXX")) {
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errno = EINVAL;
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return NULL;
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}
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tmpdir = getenv("TMPDIR");
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if (!tmpdir)
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tmpdir = "/tmp";
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strbuf_addf(&sb, "%s/%s", tmpdir, directory_template);
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directorylen = sb.len;
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if (!mkdtemp(sb.buf)) {
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int orig_errno = errno;
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strbuf_release(&sb);
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errno = orig_errno;
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return NULL;
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}
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strbuf_addf(&sb, "/%s", filename);
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fd = open(sb.buf, O_CREAT | O_EXCL | O_RDWR, 0600);
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if (fd < 0) {
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int orig_errno = errno;
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strbuf_setlen(&sb, directorylen);
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rmdir(sb.buf);
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strbuf_release(&sb);
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errno = orig_errno;
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return NULL;
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}
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tempfile = new_tempfile();
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strbuf_swap(&tempfile->filename, &sb);
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tempfile->directory = xmemdupz(tempfile->filename.buf, directorylen);
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tempfile->fd = fd;
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activate_tempfile(tempfile);
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return tempfile;
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}
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struct tempfile *xmks_tempfile_m(const char *filename_template, int mode)
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{
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struct tempfile *tempfile;
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struct strbuf full_template = STRBUF_INIT;
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strbuf_add_absolute_path(&full_template, filename_template);
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tempfile = mks_tempfile_m(full_template.buf, mode);
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if (!tempfile)
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die_errno("Unable to create temporary file '%s'",
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full_template.buf);
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strbuf_release(&full_template);
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return tempfile;
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}
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FILE *fdopen_tempfile(struct tempfile *tempfile, const char *mode)
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{
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if (!is_tempfile_active(tempfile))
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BUG("fdopen_tempfile() called for inactive object");
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if (tempfile->fp)
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BUG("fdopen_tempfile() called for open object");
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tempfile->fp = fdopen(tempfile->fd, mode);
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return tempfile->fp;
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}
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const char *get_tempfile_path(struct tempfile *tempfile)
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{
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if (!is_tempfile_active(tempfile))
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BUG("get_tempfile_path() called for inactive object");
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return tempfile->filename.buf;
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}
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int get_tempfile_fd(struct tempfile *tempfile)
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{
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if (!is_tempfile_active(tempfile))
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BUG("get_tempfile_fd() called for inactive object");
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return tempfile->fd;
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}
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FILE *get_tempfile_fp(struct tempfile *tempfile)
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{
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if (!is_tempfile_active(tempfile))
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BUG("get_tempfile_fp() called for inactive object");
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return tempfile->fp;
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}
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int close_tempfile_gently(struct tempfile *tempfile)
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{
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int fd;
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FILE *fp;
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int err;
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if (!is_tempfile_active(tempfile) || tempfile->fd < 0)
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return 0;
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fd = tempfile->fd;
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fp = tempfile->fp;
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tempfile->fd = -1;
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if (fp) {
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tempfile->fp = NULL;
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if (ferror(fp)) {
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err = -1;
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if (!fclose(fp))
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errno = EIO;
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} else {
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err = fclose(fp);
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}
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} else {
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err = close(fd);
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}
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return err ? -1 : 0;
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}
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int reopen_tempfile(struct tempfile *tempfile)
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{
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if (!is_tempfile_active(tempfile))
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BUG("reopen_tempfile called for an inactive object");
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if (0 <= tempfile->fd)
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BUG("reopen_tempfile called for an open object");
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tempfile->fd = open(tempfile->filename.buf, O_WRONLY|O_TRUNC);
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return tempfile->fd;
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}
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int rename_tempfile(struct tempfile **tempfile_p, const char *path)
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{
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struct tempfile *tempfile = *tempfile_p;
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if (!is_tempfile_active(tempfile))
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BUG("rename_tempfile called for inactive object");
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if (close_tempfile_gently(tempfile)) {
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delete_tempfile(tempfile_p);
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return -1;
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}
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if (rename(tempfile->filename.buf, path)) {
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int save_errno = errno;
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delete_tempfile(tempfile_p);
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errno = save_errno;
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return -1;
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}
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deactivate_tempfile(tempfile);
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*tempfile_p = NULL;
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return 0;
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}
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void delete_tempfile(struct tempfile **tempfile_p)
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{
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struct tempfile *tempfile = *tempfile_p;
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if (!is_tempfile_active(tempfile))
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return;
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close_tempfile_gently(tempfile);
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unlink_or_warn(tempfile->filename.buf);
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remove_template_directory(tempfile, 0);
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deactivate_tempfile(tempfile);
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*tempfile_p = NULL;
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}
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