connect: also update offset for features without values

parse_feature_value() takes an offset, and uses it to seek past the
point in features_list that we've already seen. However if the feature
being searched for does not specify a value, the offset is not
updated. Therefore if we call parse_feature_value() in a loop on a
value-less feature, we'll keep on parsing the same feature over and over
again. This usually isn't an issue: there's no point in using
next_server_feature_value() to search for repeated instances of the same
capability unless that capability typically specifies a value - but a
broken server could send a response that omits the value for a feature
even when we are expecting a value.

Therefore we add an offset update calculation for the no-value case,
which helps ensure that loops using next_server_feature_value() will
always terminate.

next_server_feature_value(), and the offset calculation, were first
added in 2.28 in 2c6a403 (connect: add function to parse multiple
v1 capability values, 2020-05-25).

Thanks to Peff for authoring the test.

Co-authored-by: Jeff King <peff@peff.net>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Andrzej Hunt <andrzej@ahunt.org>

connect.c

@@ -557,6 +557,8 @@ const char *parse_feature_value(const char *feature_list, const char *feature, i
 			if (!*value || isspace(*value)) {
 				if (lenp)
 					*lenp = 0;
+				if (offset)
+					*offset = found + len - feature_list;
 				return value;
 			}
 			/* feature with a value (e.g., "agent=git/1.2.3") */

t/t5704-protocol-violations.sh

@@ -32,4 +32,19 @@ test_expect_success 'extra delim packet in v2 fetch args' '
 	test_i18ngrep "expected flush after fetch arguments" err
 '
 
+test_expect_success 'bogus symref in v0 capabilities' '
+	test_commit foo &&
+	oid=$(git rev-parse HEAD) &&
+	dst=refs/heads/foo &&
+	{
+		printf "%s HEAD\0symref object-format=%s symref=HEAD:%s\n" \
+			"$oid" "$GIT_DEFAULT_HASH" "$dst" |
+			test-tool pkt-line pack-raw-stdin &&
+		printf "0000"
+	} >input &&
+	git ls-remote --symref --upload-pack="cat input; read junk;:" . >actual &&
+	printf "ref: %s\tHEAD\n%s\tHEAD\n" "$dst" "$oid" >expect &&
+	test_cmp expect actual
+'
+
 test_done