[PATCH 2/2] connect: know that zero-ID is not a ref

[Jonathan Tan <jonathantanmy@google.com>]

connect.c, when processing packfiles, treats a zero ID (with
`capabilities^{}` in place of the refname) as an actual ref instead of a
placeholder for a capability declaration, contrary to the specification
in Reference Discovery in Documentation/technical/pack-protocol.txt.
This is an issue when interacting with Git implementations that follow
this specification. For example, `ls-remote` (against a git://
repository served by such a Git implementation) will report a ref when
there are none, and `clone` (against something similar) will fail its
checkout.

Make connect.c follow the specification with respect to this, while
maintaining compatibility with existing implementations that do not
serve the zero ID when a repository has no refs.

(git-daemon should probably also be changed to serve zero IDs, but such
a change can be considered independently from this change; even if both
the client and server changes were made in one commit, it is nearly
impossible that all Git installations are updated at the same time - an
updated client would still need to deal with unupdated servers and vice
versa.)

The test uses JGit's daemon feature, which is specification-compliant.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
---
 connect.c            |  7 +++++++
 t/t5512-ls-remote.sh | 22 ++++++++++++++++++++++
 2 files changed, 29 insertions(+)

diff --git a/connect.c b/connect.c
index 722dc3f..d4a58de 100644
--- a/connect.c
+++ b/connect.c
@@ -165,6 +165,13 @@ struct ref **get_remote_heads(int in, char *src_buf, size_t src_len,
 			continue;
 		}
 
+		if (is_null_oid(&old_oid)) {
+			if (strcmp(name, "capabilities^{}"))
+				warning("zero object ID received that is not accompanied by a "
+				        "capability declaration, ignoring and continuing anyway");
+			continue;
+		}
+
 		if (!check_ref(name, flags))
 			continue;
 		ref = alloc_ref(buffer + GIT_SHA1_HEXSZ + 1);
diff --git a/t/t5512-ls-remote.sh b/t/t5512-ls-remote.sh
index 819b9dd..c6f8b6f 100755
--- a/t/t5512-ls-remote.sh
+++ b/t/t5512-ls-remote.sh
@@ -207,5 +207,27 @@ test_expect_success 'ls-remote --symref omits filtered-out matches' '
 	test_cmp expect actual
 '
 
+test_lazy_prereq GIT_DAEMON '
+	test_have_prereq JGIT &&
+	test_tristate GIT_TEST_GIT_DAEMON &&
+	test "$GIT_TEST_GIT_DAEMON" != false
+'
+
+JGIT_DAEMON_PORT=${JGIT_DAEMON_PORT-${this_test#t}}
+
+# This test spawns a daemon, so run it only if the user would be OK with
+# testing with git-daemon.
+test_expect_success JGIT,GIT_DAEMON 'indicate no refs in standards-compliant empty remote' '
+	JGIT_DAEMON_PID= &&
+	git init --bare empty.git &&
+	touch empty.git/git-daemon-export-ok &&
+	{
+		jgit daemon --port="$JGIT_DAEMON_PORT" . &
+		JGIT_DAEMON_PID=$!
+	} &&
+	test_when_finished kill "$JGIT_DAEMON_PID" &&
+	sleep 1 && # allow jgit daemon some time to set up
+	test_expect_code 2 git ls-remote --exit-code git://localhost:$JGIT_DAEMON_PORT/empty.git
+'
 
 test_done
-- 
2.8.0.rc3.226.g39d4020