dir.c(common_prefix): Fix two bugs

The function common_prefix() is used to find the common subdirectory of
a couple of pathnames. When checking if the next pathname matches up with
the prefix, it incorrectly checked the whole path, not just the prefix
(including the slash). Thus, the expensive part of the loop was executed
always.

The other bug is more serious: if the first and the last pathname in the
list have a longer common prefix than the common prefix for _all_ pathnames
in the list, the longer one would be chosen. This bug was probably hidden
by the fact that bash's wildcard expansion sorts the results, and the code
just so happens to work with sorted input.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>

2 files changed, 8 insertions, 1 deletions

diff --git a/dir.c b/dir.c
index b48e19dc0..602282bd1 100644
--- a/dir.c
+++ b/dir.c
@@ -24,8 +24,9 @@ int common_prefix(const char **pathspec)
 	prefix = slash - path + 1;
 	while ((next = *++pathspec) != NULL) {
 		int len = strlen(next);
-		if (len >= prefix && !memcmp(path, next, len))
+		if (len >= prefix && !memcmp(path, next, prefix))
 			continue;
+		len = prefix - 1;
 		for (;;) {
 			if (!len)
 				return 0;
diff --git a/t/t3700-add.sh b/t/t3700-add.sh
index 08e035220..ad8cc7d4a 100755
--- a/t/t3700-add.sh
+++ b/t/t3700-add.sh
@@ -104,4 +104,10 @@ test_expect_success 'add ignored ones with -f' '
 	git-ls-files --error-unmatch d.ig/d.if d.ig/d.ig
 '
 
+mkdir 1 1/2 1/3
+touch 1/2/a 1/3/b 1/2/c
+test_expect_success 'check correct prefix detection' '
+	git add 1/2/a 1/3/b 1/2/c
+'
+
 test_done