trace2: only include "fsync" events if we git_fsync()

Fix the overly verbose trace2 logging added in 9a4987677d (trace2:
add stats for fsync operations, 2022-03-30) (first released with
v2.36.0).

Since that change every single "git" command invocation has included
these "data" events, even though we'll only make use of these with
core.fsyncMethod=batch, and even then only have non-zero values if
we're writing object data to disk. See c0f4752ed2 (core.fsyncmethod:
batched disk flushes for loose-objects, 2022-04-04) for that feature.

As we're needing to indent the trace2_data_intmax() lines let's
introduce helper variables to ensure that our resulting lines (which
were already too) don't exceed the recommendations of the
CodingGuidelines. Doing that requires either wrapping them twice, or
introducing short throwaway variable names, let's do the latter.

The result was that e.g. "git version" would previously emit a total
of 6 trace2 events with the GIT_TRACE2_EVENT target (version, start,
cmd_ancestry, cmd_name, exit, atexit), but afterwards would emit
8. We'd emit 2 "data" events before the "exit" event.

The reason we didn't catch this was that the trace2 unit tests added
in a15860dca3 (trace2: t/helper/test-trace2, t0210.sh, t0211.sh,
t0212.sh, 2019-02-22) would omit any "data" events that weren't the
ones it cared about. Before this change to the C code 6/7 of our
"t/t0212-trace2-event.sh" tests would fail if this change was applied
to "t/t0212/parse_events.perl".

Let's make the trace2 testing more strict, and further append any new
events types we don't know about in "t/t0212/parse_events.perl". Since
we only invoke the "test-tool trace2" there's no guarantee that we'll
catch other overly verbose events in the future, but we'll at least
notice if we start emitting new events that are issues every time we
log anything with trace2's JSON target.

We exclude the "data_json" event type, we'd otherwise would fail on
both "win test" and "win+VS test" CI due to the logging added in
353d3d77f4 (trace2: collect Windows-specific process information,
2019-02-22). It looks like that logging should really be using
trace2_cmd_ancestry() instead, which was introduced later in
2f732bf15e (tr2: log parent process name, 2021-07-21), but let's
leave it for now.

The fix-up to aaf81223f4 (unpack-objects: use stream_loose_object()
to unpack large objects, 2022-06-11) is needed because we're changing
the behavior of these events as discussed above. Since we'd always
emit a "hardware-flush" event the test added in aaf81223f4 wasn't
testing anything except that this trace2 data was unconditionally
logged. Even if "core.fsyncMethod" wasn't set to "batch" we'd pass the
test.

Now we'll check the expected number of "writeout" v.s. "flush" calls
under "core.fsyncMethod=batch", but note that this doesn't actually
test if we carried out the sync using that method, on a platform where
we'd have to fall back to fsync() each of those "writeout" would
really be a "flush" (i.e. a full fsync()).

But in this case what we're testing is that the logic in
"unpack-objects" behaves as expected, not the OS-specific question of
whether we actually were able to use the "bulk" method.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t0212/parse_events.perl

@@ -216,12 +216,19 @@
 
     elsif ($event eq 'data') {
 	my $cat = $line->{'category'};
-	if ($cat eq 'test_category') {
-	    
-	    my $key = $line->{'key'};
-	    my $value = $line->{'value'};
-	    $processes->{$sid}->{'data'}->{$cat}->{$key} = $value;
-	}
+	my $key = $line->{'key'};
+	my $value = $line->{'value'};
+	$processes->{$sid}->{'data'}->{$cat}->{$key} = $value;
+    }
+
+    elsif ($event eq 'data_json') {
+	# NEEDSWORK: Ignore due to
+	# compat/win32/trace2_win32_process_info.c, which should log a
+	# "cmd_ancestry" event instead.
+    }
+
+    else {
+	push @{$processes->{$sid}->{$event}} => $line->{value};
     }
 
     # This trace2 target does not emit 'printf' events.

t/t5351-unpack-large-objects.sh

@@ -48,13 +48,33 @@ test_expect_success 'unpack big object in stream' '
 	test_dir_is_empty dest.git/objects/pack
 '
 
+check_fsync_events () {
+	local trace="$1" &&
+	shift &&
+
+	cat >expect &&
+	sed -n \
+		-e '/^{"event":"data",.*"category":"fsync",/ {
+			s/.*"category":"fsync",//;
+			s/}$//;
+			p;
+		}' \
+		<"$trace" >actual &&
+	test_cmp expect actual
+}
+
 BATCH_CONFIGURATION='-c core.fsync=loose-object -c core.fsyncmethod=batch'
 
 test_expect_success 'unpack big object in stream (core.fsyncmethod=batch)' '
 	prepare_dest 1m &&
 	GIT_TRACE2_EVENT="$(pwd)/trace2.txt" \
+	GIT_TEST_FSYNC=true \
 		git -C dest.git $BATCH_CONFIGURATION unpack-objects <pack-$PACK.pack &&
-	grep fsync/hardware-flush trace2.txt &&
+	check_fsync_events trace2.txt <<-\EOF &&
+	"key":"fsync/writeout-only","value":"6"
+	"key":"fsync/hardware-flush","value":"1"
+	EOF
+
 	test_dir_is_empty dest.git/objects/pack &&
 	git -C dest.git cat-file --batch-check="%(objectname)" <obj-list >current &&
 	cmp obj-list current

wrapper.c

@@ -616,10 +616,16 @@ 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)
 {
-	trace2_data_intmax("fsync", the_repository, "fsync/writeout-only", count_fsync_writeout_only);
-	trace2_data_intmax("fsync", the_repository, "fsync/hardware-flush", count_fsync_hardware_flush);
+	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)