merge-base: "--is-ancestor A B"

In many scripted Porcelain commands, we find this idiom:

    if test "$(git rev-parse --verify A)" = "$(git merge-base A B)"
    then
    	... A is an ancestor of B ...
    fi

But you do not have to compute exact merge-base only to see if A is
an ancestor of B.  Give them a more direct way to use the underlying
machinery.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

2 files changed, 50 insertions, 0 deletions

diff --git a/Documentation/git-merge-base.txt b/Documentation/git-merge-base.txt
index b295bf8330..87842e33f8 100644
--- a/Documentation/git-merge-base.txt
+++ b/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],
diff --git a/builtin/merge-base.c b/builtin/merge-base.c
index 4f30f1b0c8..0568b07e1d 100644
--- a/builtin/merge-base.c
+++ b/builtin/merge-base.c
@@ -26,6 +26,7 @@ static const char * const merge_base_usage[] = {
 	"git merge-base [-a|--all] <commit> <commit>...",
 	"git merge-base [-a|--all] --octopus <commit>...",
 	"git merge-base --independent <commit>...",
+	"git merge-base --is-ancestor <commit> <commit>",
 	NULL
 };
 
@@ -70,6 +71,20 @@ 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;
@@ -77,11 +92,14 @@ int cmd_merge_base(int argc, const char **argv, const char *prefix)
 	int show_all = 0;
 	int octopus = 0;
 	int reduce = 0;
+	int is_ancestor = 0;
 
 	struct option options[] = {
 		OPT_BOOLEAN('a', "all", &show_all, "output all common ancestors"),
 		OPT_BOOLEAN(0, "octopus", &octopus, "find ancestors for a single n-way merge"),
 		OPT_BOOLEAN(0, "independent", &reduce, "list revs not reachable from others"),
+		OPT_BOOLEAN(0, "is-ancestor", &is_ancestor,
+			    "is the first one ancestor of the other?"),
 		OPT_END()
 	};
 
@@ -89,6 +107,10 @@ int cmd_merge_base(int argc, const char **argv, const char *prefix)
 	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");