wrapper: use trace2 counters to collect fsync stats

As mentioned in the thread starting at [1], trace2 counters should be
used to count events instead of ad-hoc static variables.

Convert the two fsync static variables to trace2 counters, reducing the
coupling between wrapper.c and the trace2 subsystem. Adjust t/t5351 to
match the trace2 counter output format.

The counters are not per-thread because the ones being replaced also
were not.

[1] https://lore.kernel.org/git/20230627195251.1973421-2-calvinwan@google.com/

Signed-off-by: Beat Bolli <dev+git@drbeat.li>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5351-unpack-large-objects.sh

@@ -55,7 +55,7 @@ check_fsync_events () {
 
 	cat >expect &&
 	sed -n \
-		-e '/^{"event":"data",.*"category":"fsync",/ {
+		-e '/^{"event":"counter",.*"category":"fsync",/ {
 			s/.*"category":"fsync",//;
 			s/}$//;
 			p;
@@ -78,8 +78,8 @@ test_expect_success 'unpack big object in stream (core.fsyncmethod=batch)' '
 		flush_count=1
 	fi &&
 	check_fsync_events trace2.txt <<-EOF &&
-	"key":"fsync/writeout-only","value":"6"
+	"name":"writeout-only","count":6
-	"key":"fsync/hardware-flush","value":"$flush_count"
+	"name":"hardware-flush","count":$flush_count
 	EOF
 
 	test_dir_is_empty dest.git/objects/pack &&

trace2.c

@@ -276,7 +276,6 @@ void trace2_cmd_exit_fl(const char *file, int line, int code)
 	if (!trace2_enabled)
 		return;
 
-	trace_git_fsync_stats();
 	trace2_collect_process_info(TRACE2_PROCESS_INFO_EXIT);
 
 	tr2main_exit_code = code;

trace2.h

@@ -552,6 +552,10 @@ enum trace2_counter_id {
 	TRACE2_COUNTER_ID_TEST1 = 0, /* emits summary event only */
 	TRACE2_COUNTER_ID_TEST2,     /* emits summary and thread events */
 
+	/* counts number of fsyncs */
+	TRACE2_COUNTER_ID_FSYNC_WRITEOUT_ONLY,
+	TRACE2_COUNTER_ID_FSYNC_HARDWARE_FLUSH,
+
 	/* Add additional counter definitions before here. */
 	TRACE2_NUMBER_OF_COUNTERS
 };

trace2/tr2_ctr.c

@@ -27,6 +27,16 @@ static struct tr2_counter_metadata tr2_counter_metadata[TRACE2_NUMBER_OF_COUNTER
 		.name = "test2",
 		.want_per_thread_events = 1,
 	},
+	[TRACE2_COUNTER_ID_FSYNC_WRITEOUT_ONLY] = {
+		.category = "fsync",
+		.name = "writeout-only",
+		.want_per_thread_events = 0,
+	},
+	[TRACE2_COUNTER_ID_FSYNC_HARDWARE_FLUSH] = {
+		.category = "fsync",
+		.name = "hardware-flush",
+		.want_per_thread_events = 0,
+	},
 
 	/* Add additional metadata before here. */
 };

wrapper.c

@@ -10,9 +10,6 @@
 #include "strbuf.h"
 #include "trace2.h"
 
-static intmax_t count_fsync_writeout_only;
-static intmax_t count_fsync_hardware_flush;
-
 #ifdef HAVE_RTLGENRANDOM
 /* This is required to get access to RtlGenRandom. */
 #define SystemFunction036 NTAPI SystemFunction036
@@ -551,7 +548,7 @@ int git_fsync(int fd, enum fsync_action action)
 {
 	switch (action) {
 	case FSYNC_WRITEOUT_ONLY:
-		count_fsync_writeout_only += 1;
+		trace2_counter_add(TRACE2_COUNTER_ID_FSYNC_WRITEOUT_ONLY, 1);
 
 #ifdef __APPLE__
 		/*
@@ -583,7 +580,7 @@ int git_fsync(int fd, enum fsync_action action)
 		return -1;
 
 	case FSYNC_HARDWARE_FLUSH:
-		count_fsync_hardware_flush += 1;
+		trace2_counter_add(TRACE2_COUNTER_ID_FSYNC_HARDWARE_FLUSH, 1);
 
 		/*
 		 * On macOS, a special fcntl is required to really flush the
@@ -600,18 +597,6 @@ int git_fsync(int fd, enum fsync_action action)
 	}
 }
 
-static void log_trace_fsync_if(const char *key, intmax_t value)
-{
-	if (value)
-		trace2_data_intmax("fsync", the_repository, key, value);
-}
-
-void trace_git_fsync_stats(void)
-{
-	log_trace_fsync_if("fsync/writeout-only", count_fsync_writeout_only);
-	log_trace_fsync_if("fsync/hardware-flush", count_fsync_hardware_flush);
-}
-
 static int warn_if_unremovable(const char *op, const char *file, int rc)
 {
 	int err;

wrapper.h

@@ -87,11 +87,6 @@ enum fsync_action {
  */
 int git_fsync(int fd, enum fsync_action action);
 
-/*
- * Writes out trace statistics for fsync using the trace2 API.
- */
-void trace_git_fsync_stats(void);
-
 /*
  * Preserves errno, prints a message, but gives no warning for ENOENT.
  * Returns 0 on success, which includes trying to unlink an object that does