testing: detect a stopped timer in B.Loop

Currently, if the user stops the timer in a B.Loop benchmark loop, the
benchmark will run until it hits the timeout and fails.

Fix this by detecting that the timer is stopped and failing the
benchmark right away. We avoid making the fast path more expensive for
this check by "poisoning" the B.Loop iteration counter when the timer
is stopped so that it falls back to the slow path, which can check the
timer.

This causes b to escape from B.Loop, which is totally harmless because
it was already definitely heap-allocated. But it causes the
test/inline_testingbloop.go errorcheck test to fail. I don't think the
escape messages actually mattered to that test, they just had to be
matched. To fix this, we drop the debug level to -m=1, since -m=2
prints a lot of extra information for escaping parameters that we
don't want to deal with, and change one error check to allow b to
escape.

Fixes #72971.

Change-Id: I7d4abbb1ec1e096685514536f91ba0d581cca6b7
Reviewed-on: https://go-review.googlesource.com/c/go/+/659657
Auto-Submit: Austin Clements <austin@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

1 files changed, 23 insertions, 0 deletions

diff --git a/src/testing/loop_test.go b/src/testing/loop_test.go
index 423094fbbd..743cbe64f0 100644
--- a/src/testing/loop_test.go
+++ b/src/testing/loop_test.go
@@ -129,3 +129,26 @@ func TestBenchmarkBLoopError(t *T) {
 		t.Errorf("want N == 0, got %d", bRet.N)
 	}
 }
+
+func TestBenchmarkBLoopStop(t *T) {
+	var bState *B
+	var bLog bytes.Buffer
+	bRet := Benchmark(func(b *B) {
+		bState = b
+		b.common.w = &bLog
+
+		for i := 0; b.Loop(); i++ {
+			b.StopTimer()
+		}
+	})
+	if !bState.failed {
+		t.Errorf("benchmark should have failed")
+	}
+	const wantLog = "B.Loop called with timer stopped"
+	if log := bLog.String(); !strings.Contains(log, wantLog) {
+		t.Errorf("missing error %q in output:\n%s", wantLog, log)
+	}
+	if bRet.N != 0 {
+		t.Errorf("want N == 0, got %d", bRet.N)
+	}
+}