fetch: avoid ls-refs only to ask for HEAD symref update

When we fetch from a configured remote, we may try to update the local
refs/remotes/<origin>/HEAD, and so we ask the server to advertise its
HEAD to us.

But if we aren't otherwise asking about any refs at all, then we know
this HEAD update can never happen! To consider a new value for HEAD,
the set_head() function uses guess_remote_head(). And even if it sees an
explicit symref value for HEAD, it will only report that as a match if
we also saw that remote ref advertised, and it mapped to a local
tracking ref via get_fetch_map().

In other words, a fetch like this:

  git fetch origin $exact_oid:refs/heads/foo

can never update HEAD, because we will never have fetched (nor even see
the advertisement for) the ref that HEAD points to.

Currently the command above will still call ls-refs to ask about the
HEAD, even though it is pointless. This patch teaches it to skip the
ls-refs call entirely in this case, which avoids a round-trip to the
server.

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

Author: peff

builtin/fetch.c

@@ -1782,11 +1782,10 @@ static int do_fetch(struct transport *transport,
 			    "refs/tags/");
 	}
 
-	if (uses_remote_tracking(transport, rs)) {
-		must_list_refs = 1;
+	if (must_list_refs &&
+	    uses_remote_tracking(transport, rs))
 		strvec_push(&transport_ls_refs_options.ref_prefixes,
 			    "HEAD");
-	}
 
 	if (must_list_refs) {
 		trace2_region_enter("fetch", "remote_refs", the_repository);

t/t5702-protocol-v2.sh

@@ -708,6 +708,19 @@ test_expect_success 'exact oid fetch with tag following' '
 	git -C exact-oid-tags rev-parse --verify my-tag
 '
 
+test_expect_success 'exact oid fetch avoids pointless HEAD request' '
+	git init exact-oid-head &&
+	git -C exact-oid-head remote add origin ../prefix-parent &&
+
+	commit=$(git -C prefix-parent rev-parse --verify HEAD) &&
+
+	test_when_finished "rm -f log" &&
+	GIT_TRACE_PACKET="$(pwd)/log" \
+		git -C exact-oid-head fetch --no-tags origin \
+			$commit:refs/heads/exact &&
+	test_grep ! command=ls-refs log
+'
+
 test_expect_success 'fetch supports various ways of have lines' '
 	rm -rf server client trace &&
 	git init server &&