runtime: deflake TestNumGoroutine

Fixes #14107.

Change-Id: Icd9463b1a77b139c7ebc2d8732482d704ea332d0
Reviewed-on: https://go-review.googlesource.com/19002Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/proc_test.go

@@ -345,12 +345,27 @@ func TestNumGoroutine(t *testing.T) {
 	}
 
 	buf := make([]byte, 1<<20)
-	buf = buf[:runtime.Stack(buf, true)]
 
-	n := runtime.NumGoroutine()
+	// Try up to 10 times for a match before giving up.
+	// This is a fundamentally racy check but it's important
+	// to notice if NumGoroutine and Stack are _always_ out of sync.
+	for i := 0; ; i++ {
+		// Give goroutines about to exit a chance to exit.
+		// The NumGoroutine and Stack below need to see
+		// the same state of the world, so anything we can do
+		// to keep it quiet is good.
+		runtime.Gosched()
+
+		n := runtime.NumGoroutine()
+		buf = buf[:runtime.Stack(buf, true)]
 
-	if nstk := strings.Count(string(buf), "goroutine "); n != nstk {
-		t.Fatalf("NumGoroutine=%d, but found %d goroutines in stack dump: %s", n, nstk, buf)
+		nstk := strings.Count(string(buf), "goroutine ")
+		if n == nstk {
+			break
+		}
+		if i >= 10 {
+			t.Fatalf("NumGoroutine=%d, but found %d goroutines in stack dump: %s", n, nstk, buf)
+		}
 	}
 }