t7900: exercise detaching via trace2 regions

In t7900, we exercise the `--detach` logic by checking whether the
command ended up writing anything to its output or not. This supposedly
works because we close stdin, stdout and stderr when daemonizing. But
one, it breaks on platforms where daemonize is a no-op, like Windows.
And second, that git-maintenance(1) outputs anything at all in these
tests is a bug in the first place that we'll fix in a subsequent commit.

Introduce a new trace2 region around the detach which allows us to more
explicitly check whether the detaching logic was executed. This is a
much more direct way to exercise the logic, provides a potentially
useful signal to tracing logs and also works alright on platforms which
do not have the ability to daemonize.

Signed-off-by: Patrick Steinhardt <[email protected]>
[jc: dropped a stale in-code comment from a test]
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t

builtin/gc.c

@@ -1428,8 +1428,11 @@ static int maintenance_run_tasks(struct maintenance_run_opts *opts,
 	free(lock_path);
 
 	/* Failure to daemonize is ok, we'll continue in foreground. */
-	if (opts->detach > 0)
+	if (opts->detach > 0) {
+		trace2_region_enter("maintenance", "detach", the_repository);
 		daemonize();
+		trace2_region_leave("maintenance", "detach", the_repository);
+	}
 
 	for (i = 0; !found_selected && i < TASK__COUNT; i++)
 		found_selected = tasks[i].selected_order >= 0;

t/t7900-maintenance.sh

@@ -947,11 +947,9 @@ test_expect_success '--no-detach causes maintenance to not run in background' '
 		git config set maintenance.loose-objects.auto 1 &&
 		git config set maintenance.incremental-repack.enabled true &&
 
-		# We have no better way to check whether or not the task ran in
-		# the background than to verify whether it output anything. The
-		# next testcase checks the reverse, making this somewhat safer.
-		git maintenance run --no-detach >out 2>&1 &&
-		test_line_count = 1 out
+		GIT_TRACE2_EVENT="$(pwd)/trace.txt" \
+			git maintenance run --no-detach >out 2>&1 &&
+		! test_region maintenance detach trace.txt
 	)
 '
 
@@ -971,9 +969,9 @@ test_expect_success '--detach causes maintenance to run in background' '
 		# process, and by reading stdout we thus essentially wait for
 		# that descriptor to get closed, which indicates that the child
 		# is done, too.
-		output=$(git maintenance run --detach 2>&1 9>&1) &&
-		printf "%s" "$output" >output &&
-		test_must_be_empty output
+		does_not_matter=$(GIT_TRACE2_EVENT="$(pwd)/trace.txt" \
+			git maintenance run --detach 9>&1) &&
+		test_region maintenance detach trace.txt
 	)
 '