tests: define GIT_TEST_PROTOCOL_VERSION

Define a GIT_TEST_PROTOCOL_VERSION environment variable meant to be used
from tests. When set, this ensures protocol.version is at least the
given value, allowing the entire test suite to be run as if this
configuration is in place for all repositories.

As of this patch, all tests pass whether GIT_TEST_PROTOCOL_VERSION is
unset or set to 0. Some tests fail when GIT_TEST_PROTOCOL_VERSION is set
to 1 or 2, but this will be dealt with in subsequent patches.

This is based on work by Ævar Arnfjörð Bjarmason.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

protocol.c

@@ -17,17 +17,30 @@ static enum protocol_version parse_protocol_version(const char *value)
 enum protocol_version get_protocol_version_config(void)
 {
 	const char *value;
+	enum protocol_version retval = protocol_v0;
+	const char *git_test_k = "GIT_TEST_PROTOCOL_VERSION";
+	const char *git_test_v = getenv(git_test_k);
+
 	if (!git_config_get_string_const("protocol.version", &value)) {
 		enum protocol_version version = parse_protocol_version(value);
 
 		if (version == protocol_unknown_version)
 			die("unknown value for config 'protocol.version': %s",
 			    value);
 
-		return version;
+		retval = version;
+	}
+
+	if (git_test_v && *git_test_v) {
+		enum protocol_version env = parse_protocol_version(git_test_v);
+
+		if (env == protocol_unknown_version)
+			die("unknown value for %s: %s", git_test_k, git_test_v);
+		if (retval < env)
+			retval = env;
 	}
 
-	return protocol_v0;
+	return retval;
 }
 
 enum protocol_version determine_protocol_version_server(void)

t/README

@@ -327,6 +327,9 @@ marked strings" in po/README for details.
 GIT_TEST_SPLIT_INDEX=<boolean> forces split-index mode on the whole
 test suite. Accept any boolean values that are accepted by git-config.
 
+GIT_TEST_PROTOCOL_VERSION=<n>, when set, overrides the
+'protocol.version' setting to n if it is less than n.
+
 GIT_TEST_FULL_IN_PACK_ARRAY=<boolean> exercises the uncommon
 pack-objects code path where there are more than 1024 packs even if
 the actual number of packs in repository is below this limit. Accept

t/t5400-send-pack.sh

@@ -288,7 +288,7 @@ test_expect_success 'receive-pack de-dupes .have lines' '
 	$shared .have
 	EOF
 
-	GIT_TRACE_PACKET=$(pwd)/trace \
+	GIT_TRACE_PACKET=$(pwd)/trace GIT_TEST_PROTOCOL_VERSION= \
 	    git push \
 		--receive-pack="unset GIT_TRACE_PACKET; git-receive-pack" \
 		fork HEAD:foo &&

t/t5551-http-fetch-smart.sh

@@ -43,7 +43,8 @@ test_expect_success 'clone http repository' '
 	< Cache-Control: no-cache, max-age=0, must-revalidate
 	< Content-Type: application/x-git-upload-pack-result
 	EOF
-	GIT_TRACE_CURL=true git clone --quiet $HTTPD_URL/smart/repo.git clone 2>err &&
+	GIT_TRACE_CURL=true GIT_TEST_PROTOCOL_VERSION= \
+		git clone --quiet $HTTPD_URL/smart/repo.git clone 2>err &&
 	test_cmp file clone/file &&
 	tr '\''\015'\'' Q <err |
 	sed -e "