fetch-pack: trace packfile URI downloads

When a protocol v2 fetch includes packfile URIs, the client downloads
each advertised pack in a separate http-fetch process. Existing Trace2
regions cover negotiation, but not the time spent downloading these
packs or the number of advertised URIs.

Add a Trace2 region around the packfile URI download loop and record the
number of URIs. This makes the cost of downloading external packs
visible without emitting an event for each pack.

Extend the existing packfile URI test to verify the region and count.

Signed-off-by: Ted Nyman <tnyman@openai.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

fetch-pack.c

@@ -1886,6 +1886,13 @@ static struct ref *do_fetch_pack_v2(struct fetch_pack_args *args,
 		}
 	}
 
+	if (packfile_uris.nr) {
+		trace2_region_enter("fetch-pack", "packfile-uris",
+				    the_repository);
+		trace2_data_intmax("fetch-pack", the_repository,
+				   "packfile-uris/count", packfile_uris.nr);
+	}
+
 	for (i = 0; i < packfile_uris.nr; i++) {
 		int j;
 		struct child_process cmd = CHILD_PROCESS_INIT;
@@ -1936,6 +1943,11 @@ static struct ref *do_fetch_pack_v2(struct fetch_pack_args *args,
 						 repo_get_object_directory(the_repository),
 						 packname));
 	}
+
+	if (packfile_uris.nr)
+		trace2_region_leave("fetch-pack", "packfile-uris",
+				    the_repository);
+
 	string_list_clear(&packfile_uris, 0);
 	strvec_clear(&index_pack_args);
 

t/t5702-protocol-v2.sh

@@ -1223,7 +1223,7 @@ configure_exclusion () {
 
 test_expect_success 'part of packfile response provided as URI' '
 	P="$HTTPD_DOCUMENT_ROOT_PATH/http_parent" &&
-	rm -rf "$P" http_child log &&
+	rm -rf "$P" http_child log trace2 &&
 
 	git init "$P" &&
 	git -C "$P" config "uploadpack.allowsidebandall" "true" &&
@@ -1238,10 +1238,15 @@ test_expect_success 'part of packfile response provided as URI' '
 	configure_exclusion "$P" other-blob >h2 &&
 
 	GIT_TRACE=1 GIT_TRACE_PACKET="$(pwd)/log" GIT_TEST_SIDEBAND_ALL=1 \
+	GIT_TRACE2_EVENT="$(pwd)/trace2" \
 	git -c protocol.version=2 \
 		-c fetch.uriprotocols=http,https \
 		clone "$HTTPD_URL/smart/http_parent" http_child &&
 
+	test_grep \"event\":\"region_enter\".*\"label\":\"packfile-uris\" trace2 &&
+	test_grep \"key\":\"packfile-uris/count\",\"value\":\"2\" trace2 &&
+	test_grep \"event\":\"region_leave\".*\"label\":\"packfile-uris\" trace2 &&
+
 	# Ensure that my-blob and other-blob are in separate packfiles.
 	for idx in http_child/.git/objects/pack/*.idx
 	do