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 <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: randall77

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)
 }