Implemented return value for rev-list --quiet

If --quiet is given, the program will return non-zero if the traversed commit set was empty. This way, rev-list can be used to check commit ancestry as described by the documentation for --quiet.

"Non-zero" is implemented as 1 in order to avoid confusion with the usual 128 for die_builtin().

---
 builtin/rev-list.c        |    8 ++++++++
 revision.h                |    1 +
 t/t6041-rev-list-quiet.sh |   27 +++++++++++++++++++++++++++
 3 files changed, 36 insertions(+), 0 deletions(-)
 create mode 100755 t/t6041-rev-list-quiet.sh

index 9bfb942..001d6af 100644
@@ -157,12 +157,17 @@ static void show_commit(struct commit *commit, void *data)
 
 static void finish_commit(struct commit *commit, void *data)
 {
+	struct rev_list_info *info = data;
+	struct rev_info *revs = info->revs;
+
 	if (commit->parents) {
 		free_commit_list(commit->parents);
 		commit->parents = NULL;
 	}
 	free(commit->buffer);
 	commit->buffer = NULL;
+
+	revs->count_finished++;
 }
 
 static void finish_object(struct object *obj, const struct name_path *path, const char *name)

@@ -412,5 +417,8 @@ int cmd_rev_list(int argc, const char **argv, const char *prefix)
 			printf("%d\n", revs.count_left + revs.count_right);
 	}
 
+	if (quiet)
+		/* Return non-zero if no commits have been traversed. */
+		return (revs.count_finished == 0 ? 1 : 0);
 	return 0;
 }

index 9fd8f30..ddc35d2 100644
@@ -141,6 +141,7 @@ struct rev_info {
 	/* commit counts */
 	int count_left;
 	int count_right;
+	int count_finished;
 };
 
 #define REV_TREE_SAME		0

new file mode 100755
index 0000000..6cb9120
@@ -0,0 +1,27 @@
+#!/bin/sh
+
+test_description='Non-zero return value for empty commit sets and --quiet'
+
+. ./test-lib.sh
+
+test_expect_success 'setup' '
+	echo >fileA fileA
+	git add fileA
+	git commit -a -m "Initial"
+	echo >fileA fileA modified
+	git commit -a -m "fileA modified"
+'
+
+test_expect_success 'not quiet' '
+	test $(git rev-list master | wc -l) = 2 &&
+	test $(git rev-list master..master^ | wc -l) = 0 &&
+	test $(git rev-list master^..master | wc -l) = 1
+'
+
+test_expect_success '--quiet' '
+	test $(git rev-list --quiet master; echo $?) = 0 &&
+	test $(git rev-list --quiet master..master^; echo $?) != 0 &&
+	test $(git rev-list --quiet master^..master; echo $?) = 0
+'
+
+test_done

1.7.4.4

Given two commits A and X, "rev-list --objects A..X" is a way to make sure that everything between A and X exists. When your ref is at A, you are trying to fetch from a remote that wants to update you to X, and when you happen to have X already, you run that command and see if it dies due to disconnect in the history. If it doesn't, you know you do not actually have to transfer anything (this is called quickfetch test). For the purpose of this test, the caller is not interested in the output, so it is perfectly OK to give --quiet to the command.

In reality, you would feed all the refs you locally have on the negative side (i.e. "rev-list --objects --quiet X --not A B C D ...") to check if X is connected to something you know you have a connected history behind it. Returning non-zero to a quickfetch test when X is reachable from some of your refs will break existing scripts.

Thank you for this explanation, this really cleared up the intended usage of --quiet for me. I wanted to use it for ancestry testing of two commits, noticed the hard-coded return value of '0' in cmd_rev_list() and thought that the implementation is probably missing.

After some additional digging, I realized that git-merge-base is the right tool for my task.

Regards,
Jonas

Either

	test "$(git merge-base A B)" = "$(git rev-parse A)"

or

	test -z "$(git rev-list -1 A..B)"

would be valid ways to see if A is an ancestor of B.