Merge branch 'jc/merge-base-reflog'

Code the logic in "pull --rebase" that figures out a fork point
from reflog entries in C.

* jc/merge-base-reflog:
  merge-base: teach "--fork-point" mode
  merge-base: use OPT_CMDMODE and clarify the command line parsing

Documentation/git-merge-base.txt

@@ -13,6 +13,7 @@ SYNOPSIS
 'git merge-base' [-a|--all] --octopus <commit>...
 'git merge-base' --is-ancestor <commit> <commit>
 'git merge-base' --independent <commit>...
+'git merge-base' --fork-point <ref> [<commit>]
 
 DESCRIPTION
 -----------
@@ -24,8 +25,8 @@ that does not have any better common ancestor is a 'best common
 ancestor', i.e. a 'merge base'.  Note that there can be more than one
 merge base for a pair of commits.
 
-OPERATION MODE
---------------
+OPERATION MODES
+---------------
 
 As the most common special case, specifying only two commits on the
 command line means computing the merge base between the given two commits.
@@ -56,6 +57,14 @@ from linkgit:git-show-branch[1] when used with the `--merge-base` option.
 	and exit with status 0 if true, or with status 1 if not.
 	Errors are signaled by a non-zero status that is not 1.
 
+--fork-point::
+	Find the point at which a branch (or any history that leads
+	to <commit>) forked from another branch (or any reference)
+	<ref>. This does not just look for the common ancestor of
+	the two commits, but also takes into account the reflog of
+	<ref> to see if the history leading to <commit> forked from
+	an earlier incarnation of the branch <ref> (see discussion
+	on this mode below).
 
 OPTIONS
 -------
@@ -137,6 +146,31 @@ In modern git, you can say this in a more direct way:
 
 instead.
 
+Discussion on fork-point mode
+-----------------------------
+
+After working on the `topic` branch created with `git checkout -b
+topic origin/master`, the history of remote-tracking branch
+`origin/master` may have been rewound and rebuilt, leading to a
+history of this shape:
+
+			 o---B1
+			/
+	---o---o---B2--o---o---o---B (origin/master)
+		\
+		 B3
+		  \
+		   Derived (topic)
+
+where `origin/master` used to point at commits B3, B2, B1 and now it
+points at B, and your `topic` branch was started on top of it back
+when `origin/master` was at B3. This mode uses the reflog of
+`origin/master` to find B3 as the fork point, so that the `topic`
+can be rebased on top of the updated `origin/master` by:
+
+    $ fork_point=$(git merge-base --fork-point origin/master topic)
+    $ git rebase --onto origin/master $fork_point topic
+
 
 See also
 --------

builtin/merge-base.c

@@ -1,6 +1,9 @@
 #include "builtin.h"
 #include "cache.h"
 #include "commit.h"
+#include "refs.h"
+#include "diff.h"
+#include "revision.h"
 #include "parse-options.h"
 
 static int show_merge_base(struct commit **rev, int rev_nr, int show_all)
@@ -27,6 +30,7 @@ static const char * const merge_base_usage[] = {
 	N_("git merge-base [-a|--all] --octopus <commit>..."),
 	N_("git merge-base --independent <commit>..."),
 	N_("git merge-base --is-ancestor <commit> <commit>"),
+	N_("git merge-base --fork-point <ref> [<commit>]"),
 	NULL
 };
 
@@ -85,37 +89,148 @@ static int handle_is_ancestor(int argc, const char **argv)
 		return 1;
 }
 
+struct rev_collect {
+	struct commit **commit;
+	int nr;
+	int alloc;
+	unsigned int initial : 1;
+};
+
+static void add_one_commit(unsigned char *sha1, struct rev_collect *revs)
+{
+	struct commit *commit;
+
+	if (is_null_sha1(sha1))
+		return;
+
+	commit = lookup_commit(sha1);
+	if (!commit ||
+	    (commit->object.flags & TMP_MARK) ||
+	    parse_commit(commit))
+		return;
+
+	ALLOC_GROW(revs->commit, revs->nr + 1, revs->alloc);
+	revs->commit[revs->nr++] = commit;
+	commit->object.flags |= TMP_MARK;
+}
+
+static int collect_one_reflog_ent(unsigned char *osha1, unsigned char *nsha1,
+				  const char *ident, unsigned long timestamp,
+				  int tz, const char *message, void *cbdata)
+{
+	struct rev_collect *revs = cbdata;
+
+	if (revs->initial) {
+		revs->initial = 0;
+		add_one_commit(osha1, revs);
+	}
+	add_one_commit(nsha1, revs);
+	return 0;
+}
+
+static int handle_fork_point(int argc, const char **argv)
+{
+	unsigned char sha1[20];
+	char *refname;
+	const char *commitname;
+	struct rev_collect revs;
+	struct commit *derived;
+	struct commit_list *bases;
+	int i, ret = 0;
+
+	switch (dwim_ref(argv[0], strlen(argv[0]), sha1, &refname)) {
+	case 0:
+		die("No such ref: '%s'", argv[0]);
+	case 1:
+		break; /* good */
+	default:
+		die("Ambiguous refname: '%s'", argv[0]);
+	}
+
+	commitname = (argc == 2) ? argv[1] : "HEAD";
+	if (get_sha1(commitname, sha1))
+		die("Not a valid object name: '%s'", commitname);
+
+	derived = lookup_commit_reference(sha1);
+	memset(&revs, 0, sizeof(revs));
+	revs.initial = 1;
+	for_each_reflog_ent(refname, collect_one_reflog_ent, &revs);
+
+	for (i = 0; i < revs.nr; i++)
+		revs.commit[i]->object.flags &= ~TMP_MARK;
+
+	bases = get_merge_bases_many(derived, revs.nr, revs.commit, 0);
+
+	/*
+	 * There should be one and only one merge base, when we found
+	 * a common ancestor among reflog entries.
+	 */
+	if (!bases || bases->next) {
+		ret = 1;
+		goto cleanup_return;
+	}
+
+	/* And the found one must be one of the reflog entries */
+	for (i = 0; i < revs.nr; i++)
+		if (&bases->item->object == &revs.commit[i]->object)
+			break; /* found */
+	if (revs.nr <= i) {
+		ret = 1; /* not found */
+		goto cleanup_return;
+	}
+
+	printf("%s\n", sha1_to_hex(bases->item->object.sha1));
+
+cleanup_return:
+	free_commit_list(bases);
+	return ret;
+}
+
 int cmd_merge_base(int argc, const char **argv, const char *prefix)
 {
 	struct commit **rev;
 	int rev_nr = 0;
 	int show_all = 0;
-	int octopus = 0;
-	int reduce = 0;
-	int is_ancestor = 0;
+	int cmdmode = 0;
 
 	struct option options[] = {
 		OPT_BOOL('a', "all", &show_all, N_("output all common ancestors")),
-		OPT_BOOL(0, "octopus", &octopus, N_("find ancestors for a single n-way merge")),
-		OPT_BOOL(0, "independent", &reduce, N_("list revs not reachable from others")),
-		OPT_BOOL(0, "is-ancestor", &is_ancestor,
-			 N_("is the first one ancestor of the other?")),
+		OPT_CMDMODE(0, "octopus", &cmdmode,
+			    N_("find ancestors for a single n-way merge"), 'o'),
+		OPT_CMDMODE(0, "independent", &cmdmode,
+			    N_("list revs not reachable from others"), 'r'),
+		OPT_CMDMODE(0, "is-ancestor", &cmdmode,
+			    N_("is the first one ancestor of the other?"), 'a'),
+		OPT_CMDMODE(0, "fork-point", &cmdmode,
+			    N_("find where <commit> forked from reflog of <ref>"), 'f'),
 		OPT_END()
 	};
 
 	git_config(git_default_config, NULL);
 	argc = parse_options(argc, argv, prefix, options, merge_base_usage, 0);
-	if (!octopus && !reduce && argc < 2)
-		usage_with_options(merge_base_usage, options);
-	if (is_ancestor && (show_all || octopus || reduce))
-		die("--is-ancestor cannot be used with other options");
-	if (is_ancestor)
-		return handle_is_ancestor(argc, argv);
-	if (reduce && (show_all || octopus))
-		die("--independent cannot be used with other options");
 
-	if (octopus || reduce)
-		return handle_octopus(argc, argv, reduce, show_all);
+	if (cmdmode == 'a') {
+		if (argc < 2)
+			usage_with_options(merge_base_usage, options);
+		if (show_all)
+			die("--is-ancestor cannot be used with --all");
+		return handle_is_ancestor(argc, argv);
+	}
+
+	if (cmdmode == 'r' && show_all)
+		die("--independent cannot be used with --all");
+
+	if (cmdmode == 'r' || cmdmode == 'o')
+		return handle_octopus(argc, argv, cmdmode == 'r', show_all);
+
+	if (cmdmode == 'f') {
+		if (argc < 1 || 2 < argc)
+			usage_with_options(merge_base_usage, options);
+		return handle_fork_point(argc, argv);
+	}
+
+	if (argc < 2)
+		usage_with_options(merge_base_usage, options);
 
 	rev = xmalloc(argc * sizeof(*rev));
 	while (argc-- > 0)

t/t6010-merge-base.sh

@@ -230,4 +230,32 @@ test_expect_success 'criss-cross merge-base for octopus-step' '
 	test_cmp expected.sorted actual.sorted
 '
 
+test_expect_success 'using reflog to find the fork point' '
+	git reset --hard &&
+	git checkout -b base $E &&
+
+	(
+		for count in 1 2 3
+		do
+			git commit --allow-empty -m "Base commit #$count" &&
+			git rev-parse HEAD >expect$count &&
+			git checkout -B derived &&
+			git commit --allow-empty -m "Derived #$count" &&
+			git rev-parse HEAD >derived$count &&
+			git checkout -B base $E || exit 1
+		done
+
+		for count in 1 2 3
+		do
+			git merge-base --fork-point base $(cat derived$count) >actual &&
+			test_cmp expect$count actual || exit 1
+		done
+
+	) &&
+	# check that we correctly default to HEAD
+	git checkout derived &&
+	git merge-base --fork-point base >actual &&
+	test_cmp expect3 actual
+'
+
 test_done