[PATCH 2/3] sha1-file: emit error if an alternate looks like a repository

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

Since 26125f6b9b ("detect broken alternates.", 2006-02-22) we've
emitted an error if the alternates directory doesn't exist, but not
for the common misstep of adding a path to another git repository as
an alternate, as opposed to its "objects" directory.

Let's check for this, i.e. whether X/objects or X/.git/objects exists
if the user supplies X and print an error (which as a commit leading
up to this one shows doesn't change the exit code, just "warns").

This check is intentionally not implemented by e.g. requiring that any
of X/?? exists or X/info or X/pack exists. It's a legitimate use-case
to point to an existing alternate that hasn't been populated yet, but
pointing to one where an "X/objects" or "X/.git/objects" directory
exists is definitely a mistake we should warn the user about.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 sha1-file.c               | 10 +++++++++-
 t/t5613-info-alternate.sh | 14 ++++++++++++++
 2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/sha1-file.c b/sha1-file.c
index 5bd11c85bc..f142f81658 100644
--- a/sha1-file.c
+++ b/sha1-file.c
@@ -376,12 +376,20 @@ static int alt_odb_usable(struct raw_object_store *o,
 {
 	struct alternate_object_database *alt;
 
-	/* Detect cases where alternate disappeared */
 	if (!is_directory(path->buf)) {
+		/* Detect cases where alternate disappeared */
 		error(_("object directory %s does not exist; "
 			"check .git/objects/info/alternates"),
 		      path->buf);
 		return 0;
+	} else if (is_directory(mkpath("%s/objects", path->buf)) ||
+		   is_directory(mkpath("%s/.git/objects", path->buf))) {
+		/* Detect cases where alternate is a git repository */
+		error(_("object directory %s looks like a git repository; "
+			"alternates must point to the 'objects' directory. "
+			"check .git/objects/info/alternates"),
+		      path->buf);
+		return 0;
 	}
 
 	/*
diff --git a/t/t5613-info-alternate.sh b/t/t5613-info-alternate.sh
index d2964c57b7..b959e21421 100755
--- a/t/t5613-info-alternate.sh
+++ b/t/t5613-info-alternate.sh
@@ -143,4 +143,18 @@ test_expect_success 'print "error" on non-existing alternate' '
 	test_i18ngrep "does not exist; check" stderr
 '
 
+test_expect_success 'print "error" on alternate that looks like a git repository' '
+	git init --bare J &&
+	git init --bare K &&
+
+	# H is bare, G is not
+	echo ../../H >J/objects/info/alternates &&
+	echo ../../G >K/objects/info/alternates &&
+
+	git -C J fsck 2>stderr &&
+	test_i18ngrep "looks like a git repository; alternates must" stderr &&
+	git -C K fsck 2>stderr &&
+	test_i18ngrep "looks like a git repository; alternates must" stderr
+'
+
 test_done
-- 
2.20.0.rc2.403.gdbc3b29805