runtime: deflake TestNumGoroutine

Fixes #14107.

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

diff --git a/src/runtime/proc_test.go b/src/runtime/proc_test.go
index 34d90a1c9bfe95a0afe1bcc60eedbec980eafc78..9e5960bd2af612232e58e172d7071d3a1ae7cb98 100644 (file)
--- a/src/runtime/proc_test.go
+++ b/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)
+               }
        }
 }