upload-pack: add tracing for fetches

Information on how users are accessing hosted repositories can be
helpful to server operators. For example, being able to broadly
differentiate between fetches and initial clones; the use of shallow
repository features; or partial clone filters.

a29263c (fetch-pack: add tracing for negotiation rounds, 2022-08-02)
added some information on have counts to fetch-pack itself to help
diagnose negotiation; but from a git-upload-pack (server) perspective,
there's no means of accessing such information without using
GIT_TRACE_PACKET to examine the protocol packets.

Improve this by emitting a Trace2 JSON event from upload-pack with
summary information on the contents of a fetch request.

* haves, wants, and want-ref counts can help determine (broadly) between
  fetches and clones, and the use of single-branch, etc.
* shallow clone depth, tip counts, and deepening options.
* any partial clone filter type.

Signed-off-by: Robert Coup <robert@coup.net.nz>

Author: rcoup

t/t5500-fetch-pack.sh

@@ -132,13 +132,18 @@ test_expect_success 'single branch object count' '
 '
 
 test_expect_success 'single given branch clone' '
-	git clone --single-branch --branch A "file://$(pwd)/." branch-a &&
-	test_must_fail git --git-dir=branch-a/.git rev-parse origin/B
+	GIT_TRACE2_EVENT="$(pwd)/branch-a/trace2_event" \
+		git clone --single-branch --branch A "file://$(pwd)/." branch-a &&
+	test_must_fail git --git-dir=branch-a/.git rev-parse origin/B &&
+	grep \"fetch-info\".*\"haves\":0 branch-a/trace2_event &&
+	grep \"fetch-info\".*\"wants\":1 branch-a/trace2_event
 '
 
 test_expect_success 'clone shallow depth 1' '
-	git clone --no-single-branch --depth 1 "file://$(pwd)/." shallow0 &&
-	test "$(git --git-dir=shallow0/.git rev-list --count HEAD)" = 1
+	GIT_TRACE2_EVENT="$(pwd)/shallow0/trace2_event" \
+		git clone --no-single-branch --depth 1 "file://$(pwd)/." shallow0 &&
+	test "$(git --git-dir=shallow0/.git rev-list --count HEAD)" = 1 &&
+	grep \"fetch-info\".*\"depth\":1 shallow0/trace2_event
 '
 
 test_expect_success 'clone shallow depth 1 with fsck' '
@@ -235,7 +240,10 @@ test_expect_success 'add two more (part 2)' '
 test_expect_success 'deepening pull in shallow repo' '
 	(
 		cd shallow &&
-		git pull --depth 4 .. B
+		GIT_TRACE2_EVENT="$(pwd)/trace2_event" \
+			git pull --depth 4 .. B &&
+		grep \"fetch-info\".*\"depth\":4 trace2_event &&
+		grep \"fetch-info\".*\"shallows\":2 trace2_event
 	)
 '
 
@@ -306,9 +314,12 @@ test_expect_success 'fetch --depth --no-shallow' '
 test_expect_success 'turn shallow to complete repository' '
 	(
 		cd shallow &&
-		git fetch --unshallow &&
+		GIT_TRACE2_EVENT="$(pwd)/trace2_event" \
+			git fetch --unshallow &&
 		! test -f .git/shallow &&
-		git fsck --full
+		git fsck --full &&
+		grep \"fetch-info\".*\"shallows\":2 trace2_event &&
+		grep \"fetch-info\".*\"depth\":2147483647 trace2_event
 	)
 '
 
@@ -826,13 +837,15 @@ test_expect_success 'clone shallow since ...' '
 '
 
 test_expect_success 'fetch shallow since ...' '
-	git -C shallow11 fetch --shallow-since "200000000 +0700" origin &&
+	GIT_TRACE2_EVENT=$(pwd)/shallow11/trace2_event \
+		git -C shallow11 fetch --shallow-since "200000000 +0700" origin &&
 	git -C shallow11 log --pretty=tformat:%s origin/main >actual &&
 	cat >expected <<-\EOF &&
 	three
 	two
 	EOF
-	test_cmp expected actual
+	test_cmp expected actual &&
+	grep \"fetch-info\".*\"deepen-since\":true shallow11/trace2_event
 '
 
 test_expect_success 'clone shallow since selects no commits' '
@@ -987,13 +1000,16 @@ test_expect_success 'filtering by size' '
 	test_config -C server uploadpack.allowfilter 1 &&
 
 	test_create_repo client &&
-	git -C client fetch-pack --filter=blob:limit=0 ../server HEAD &&
+	GIT_TRACE2_EVENT=$(pwd)/client/trace2_event \
+		git -C client fetch-pack --filter=blob:limit=0 ../server HEAD &&
 
 	# Ensure that object is not inadvertently fetched
 	commit=$(git -C server rev-parse HEAD) &&
 	blob=$(git hash-object server/one.t) &&
 	git -C client rev-list --objects --missing=allow-any "$commit" >oids &&
-	! grep "$blob" oids
+	! grep "$blob" oids &&
+
+	grep \"fetch-info\".*\"filter\":\"blob:limit\" client/trace2_event
 '
 
 test_expect_success 'filtering by size has no effect if support for it is not advertised' '

upload-pack.c

@@ -33,6 +33,7 @@
 #include "commit-reach.h"
 #include "shallow.h"
 #include "write-or-die.h"
+#include "json-writer.h"
 
 /* Remember to update object flag allocation in object.h */
 #define THEY_HAVE	(1u << 11)
@@ -1552,6 +1553,30 @@ static int parse_have(const char *line, struct oid_array *haves)
 	return 0;
 }
 
+static void trace2_fetch_info(struct upload_pack_data *data)
+{
+	struct json_writer jw = JSON_WRITER_INIT;
+
+	jw_object_begin(&jw, 0);
+	jw_object_intmax(&jw, "haves", data->haves.nr);
+	jw_object_intmax(&jw, "wants", data->want_obj.nr);
+	jw_object_intmax(&jw, "want-refs", data->wanted_refs.nr);
+	jw_object_intmax(&jw, "depth", data->depth);
+	jw_object_intmax(&jw, "shallows", data->shallows.nr);
+	jw_object_bool(&jw, "deepen-since", data->deepen_since);
+	jw_object_intmax(&jw, "deepen-not", data->deepen_not.nr);
+	jw_object_bool(&jw, "deepen-relative", data->deepen_relative);
+	if (data->filter_options.choice)
+		jw_object_string(&jw, "filter", list_object_filter_config_name(data->filter_options.choice));
+	else
+		jw_object_null(&jw, "filter");
+	jw_end(&jw);
+
+	trace2_data_json("upload-pack", the_repository, "fetch-info", &jw);
+
+	jw_release(&jw);
+}
+
 static void process_args(struct packet_reader *request,
 			 struct upload_pack_data *data)
 {
@@ -1640,6 +1665,9 @@ static void process_args(struct packet_reader *request,
 
 	if (request->status != PACKET_READ_FLUSH)
 		die(_("expected flush after fetch arguments"));
+
+	if (trace2_is_enabled())
+		trace2_fetch_info(data);
 }
 
 static int process_haves(struct upload_pack_data *data, struct oid_array *common)