runtime: soften up the GCTestIsReachable test a bit

This test can fail due to objects being incorrectly retained due
to conservative scanning. Allow a bit of slop (1 accidentally
retained object) to prevent flaky failures.

Fixes #67204

"fixes" is a bit too strong a word. More like, hopefully reduces
the false positive rate to something approaching 0.

Change-Id: I09984f0cce50d8209aef19f3d89b0e295c86f8d1
Reviewed-on: https://go-review.googlesource.com/c/go/+/590615
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

diff --git a/src/runtime/gc_test.go b/src/runtime/gc_test.go
index 968d5275c5d3b8d505c79f5ad9b89d584794d5ce..908f6322466b17705c418a57fe52b0bb511e5dd8 100644 (file)
--- a/src/runtime/gc_test.go
+++ b/src/runtime/gc_test.go
@@ -6,6 +6,7 @@ package runtime_test
 
 import (
        "fmt"
+       "math/bits"
        "math/rand"
        "os"
        "reflect"
@@ -278,8 +279,17 @@ func TestGCTestIsReachable(t *testing.T) {
        }
 
        got := runtime.GCTestIsReachable(all...)
-       if want != got {
-               t.Fatalf("did not get expected reachable set; want %b, got %b", want, got)
+       if got&want != want {
+               // This is a serious bug - an object is live (due to the KeepAlive
+               // call below), but isn't reported as such.
+               t.Fatalf("live object not in reachable set; want %b, got %b", want, got)
+       }
+       if bits.OnesCount64(got&^want) > 1 {
+               // Note: we can occasionally have a value that is retained even though
+               // it isn't live, due to conservative scanning of stack frames.
+               // See issue 67204. For now, we allow a "slop" of 1 unintentionally
+               // retained object.
+               t.Fatalf("dead object in reachable set; want %b, got %b", want, got)
        }
        runtime.KeepAlive(half)
 }