Merge branch 'jc/merge-bases'

Optimise the "merge-base" computation a bit, and also update its
users that do not need the full merge-base information to call a
cheaper subset.

* jc/merge-bases:
  reduce_heads(): reimplement on top of remove_redundant()
  merge-base: "--is-ancestor A B"
  get_merge_bases_many(): walk from many tips in parallel
  in_merge_bases(): use paint_down_to_common()
  merge_bases_many(): split out the logic to paint history
  in_merge_bases(): omit unnecessary redundant common ancestor reduction
  http-push: use in_merge_bases() for fast-forward check
  receive-pack: use in_merge_bases() for fast-forward check
  in_merge_bases(): support only one "other" commit

Documentation/git-merge-base.txt

@@ -11,6 +11,7 @@ SYNOPSIS
 [verse]
 'git merge-base' [-a|--all] <commit> <commit>...
 'git merge-base' [-a|--all] --octopus <commit>...
+'git merge-base' --is-ancestor <commit> <commit>
 'git merge-base' --independent <commit>...
 
 DESCRIPTION
@@ -50,6 +51,12 @@ from linkgit:git-show-branch[1] when used with the `--merge-base` option.
 	from any other.  This mimics the behavior of 'git show-branch
 	--independent'.
 
+--is-ancestor::
+	Check if the first <commit> is an ancestor of the second <commit>,
+	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.
+
+
 OPTIONS
 -------
 -a::
@@ -110,6 +117,27 @@ both '1' and '2' are merge-bases of A and B.  Neither one is better than
 the other (both are 'best' merge bases).  When the `--all` option is not given,
 it is unspecified which best one is output.
 
+A common idiom to check "fast-forward-ness" between two commits A
+and B is (or at least used to be) to compute the merge base between
+A and B, and check if it is the same as A, in which case, A is an
+ancestor of B.  You will see this idiom used often in older scripts.
+
+	A=$(git rev-parse --verify A)
+	if test "$A" = "$(git merge-base A B)"
+	then
+		... A is an ancestor of B ...
+	fi
+
+In modern git, you can say this in a more direct way:
+
+	if git merge-base --is-ancestor A B
+	then
+		... A is an ancestor of B ...
+	fi
+
+instead.
+
+
 See also
 --------
 linkgit:git-rev-list[1],

builtin/branch.c

@@ -130,7 +130,7 @@ static int branch_merged(int kind, const char *name,
 	if (!reference_rev)
 		reference_rev = head_rev;
 
-	merged = in_merge_bases(rev, &reference_rev, 1);
+	merged = in_merge_bases(rev, reference_rev);
 
 	/*
 	 * After the safety valve is fully redefined to "check with
@@ -140,7 +140,7 @@ static int branch_merged(int kind, const char *name,
 	 * a gentle reminder is in order.
 	 */
 	if ((head_rev != reference_rev) &&
-	    in_merge_bases(rev, &head_rev, 1) != merged) {
+	    in_merge_bases(rev, head_rev) != merged) {
 		if (merged)
 			warning(_("deleting branch '%s' that has been merged to\n"
 				"         '%s', but not yet merged to HEAD."),

builtin/fetch.c

@@ -323,7 +323,7 @@ static int update_local_ref(struct ref *ref,
 		return r;
 	}
 
-	if (in_merge_bases(current, &updated, 1)) {
+	if (in_merge_bases(current, updated)) {
 		char quickref[83];
 		int r;
 		strcpy(quickref, find_unique_abbrev(current->object.sha1, DEFAULT_ABBREV));

builtin/merge-base.c

@@ -26,6 +26,7 @@ static const char * const merge_base_usage[] = {
 	N_("git merge-base [-a|--all] <commit> <commit>..."),
 	N_("git merge-base [-a|--all] --octopus <commit>..."),
 	N_("git merge-base --independent <commit>..."),
+	N_("git merge-base --is-ancestor <commit> <commit>"),
 	NULL
 };
 
@@ -70,25 +71,46 @@ static int handle_octopus(int count, const char **args, int reduce, int show_all
 	return 0;
 }
 
+static int handle_is_ancestor(int argc, const char **argv)
+{
+	struct commit *one, *two;
+
+	if (argc != 2)
+		die("--is-ancestor takes exactly two commits");
+	one = get_commit_reference(argv[0]);
+	two = get_commit_reference(argv[1]);
+	if (in_merge_bases(one, two))
+		return 0;
+	else
+		return 1;
+}
+
 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;
 
 	struct option options[] = {
 		OPT_BOOLEAN('a', "all", &show_all, N_("output all common ancestors")),
 		OPT_BOOLEAN(0, "octopus", &octopus, N_("find ancestors for a single n-way merge")),
 		OPT_BOOLEAN(0, "independent", &reduce, N_("list revs not reachable from others")),
+		OPT_BOOLEAN(0, "is-ancestor", &is_ancestor,
+			    N_("is the first one ancestor of the other?")),
 		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");
 

builtin/receive-pack.c

@@ -480,7 +480,6 @@ static const char *update(struct command *cmd)
 	    !prefixcmp(name, "refs/heads/")) {
 		struct object *old_object, *new_object;
 		struct commit *old_commit, *new_commit;
-		struct commit_list *bases, *ent;
 
 		old_object = parse_object(old_sha1);
 		new_object = parse_object(new_sha1);
@@ -493,12 +492,7 @@ static const char *update(struct command *cmd)
 		}
 		old_commit = (struct commit *)old_object;
 		new_commit = (struct commit *)new_object;
-		bases = get_merge_bases(old_commit, new_commit, 1);
-		for (ent = bases; ent; ent = ent->next)
-			if (!hashcmp(old_sha1, ent->item->object.sha1))
-				break;
-		free_commit_list(bases);
-		if (!ent) {
+		if (!in_merge_bases(old_commit, new_commit)) {
 			rp_error("denying non-fast-forward %s"
 				 " (you should pull first)", name);
 			return "non-fast-forward";