runtime: deflake TestGoroutineProfileTrivial

Failed at https://storage.googleapis.com/go-build-log/9875de36/nacl-amd64p32_931ba6cf.log

Change-Id: I2bc204ed58da543ee2534b69c29c8e8485d54683
Reviewed-on: https://go-review.googlesource.com/19155
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/runtime_test.go

@@ -310,13 +310,22 @@ func TestAppendSliceGrowth(t *testing.T) {
 }
 
 func TestGoroutineProfileTrivial(t *testing.T) {
-	n1, ok := GoroutineProfile(nil) // should fail, there's at least 1 goroutine
-	if n1 < 1 || ok {
-		t.Fatalf("GoroutineProfile(nil) = %d, %v, want >0, false", n1, ok)
-	}
-
-	n2, ok := GoroutineProfile(make([]StackRecord, n1))
-	if n2 != n1 || !ok {
-		t.Fatalf("GoroutineProfile(%d) = %d, %v, want %d, true", n1, n2, ok, n1)
+	// Calling GoroutineProfile twice in a row should find the same number of goroutines,
+	// but it's possible there are goroutines just about to exit, so we might end up
+	// with fewer in the second call. Try a few times; it should converge once those
+	// zombies are gone.
+	for i := 0; ; i++ {
+		n1, ok := GoroutineProfile(nil) // should fail, there's at least 1 goroutine
+		if n1 < 1 || ok {
+			t.Fatalf("GoroutineProfile(nil) = %d, %v, want >0, false", n1, ok)
+		}
+		n2, ok := GoroutineProfile(make([]StackRecord, n1))
+		if n2 == n1 && ok {
+			break
+		}
+		t.Logf("GoroutineProfile(%d) = %d, %v, want %d, true", n1, n2, ok, n1)
+		if i >= 10 {
+			t.Fatalf("GoroutineProfile not converging")
+		}
 	}
 }