merge-file: clamp exit code to maximum 127

Git-merge-file is documented to return one of three exit
codes:

  - zero means the merge was successful

  - a negative number means an error occurred

  - a positive number indicates the number of conflicts

Unfortunately, this all gets stuffed into an 8-bit return
code. Which means that if you have 256 conflicts, this wraps
to zero, and the merge appears to succeed (and commits a
blob full of conflict-marker cruft!).

This patch clamps the return value to a maximum of 127,
which we should be able to safely represent everywhere. This
also leaves 128-255 for other values. Shells (and some parts
of git) will typically represent signal death as 128 plus
the signal number. And negative values are typically coerced
to an 8-bit unsigned value (so "return -1" ends up as 255).

Technically negative returns have the same problem (e.g.,
"-256" wraps back to 0), but this is not a problem in
practice, as the only negative value we use is "-1".

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-merge-file.txt

@@ -41,7 +41,8 @@ lines from `<other-file>`, or lines from both respectively.  The length of the
 conflict markers can be given with the `--marker-size` option.
 
 The exit value of this program is negative on error, and the number of
-conflicts otherwise. If the merge was clean, the exit value is 0.
+conflicts otherwise (truncated to 127 if there are more than that many
+conflicts). If the merge was clean, the exit value is 0.
 
 'git merge-file' is designed to be a minimal clone of RCS 'merge'; that is, it
 implements all of RCS 'merge''s functionality which is needed by

builtin/merge-file.c

@@ -102,5 +102,8 @@ int cmd_merge_file(int argc, const char **argv, const char *prefix)
 		free(result.ptr);
 	}
 
+	if (ret > 127)
+		ret = 127;
+
 	return ret;
 }

t/t7600-merge.sh

@@ -692,4 +692,37 @@ test_expect_success GPG 'merge --no-edit tag should skip editor' '
 	test_cmp actual expect
 '
 
+test_expect_success 'set up mod-256 conflict scenario' '
+	# 256 near-identical stanzas...
+	for i in $(test_seq 1 256); do
+		for j in 1 2 3 4 5; do
+			echo $i-$j
+		done
+	done >file &&
+	git add file &&
+	git commit -m base &&
+
+	# one side changes the first line of each to "master"
+	sed s/-1/-master/ <file >tmp &&
+	mv tmp file &&
+	git commit -am master &&
+
+	# and the other to "side"; merging the two will
+	# yield 256 separate conflicts
+	git checkout -b side HEAD^ &&
+	sed s/-1/-side/ <file >tmp &&
+	mv tmp file &&
+	git commit -am side
+'
+
+test_expect_success 'merge detects mod-256 conflicts (recursive)' '
+	git reset --hard &&
+	test_must_fail git merge -s recursive master
+'
+
+test_expect_success 'merge detects mod-256 conflicts (resolve)' '
+	git reset --hard &&
+	test_must_fail git merge -s resolve master
+'
+
 test_done