Merge pull request moby#39968 from samuelkarp/issue-39857

awslogs: fix flaky TestLogBlocking unit test

daemon/logger/awslogs/cloudwatchlogs_test.go

@@ -24,7 +24,6 @@ import (
 	"github.com/docker/docker/dockerversion"
 	"gotest.tools/assert"
 	is "gotest.tools/assert/cmp"
-	"gotest.tools/skip"
 )
 
 const (
@@ -286,8 +285,10 @@ func TestLogClosed(t *testing.T) {
 	}
 }
 
+// TestLogBlocking tests that the Log method blocks appropriately when
+// non-blocking behavior is not enabled.  Blocking is achieved through an
+// internal channel that must be drained for Log to return.
 func TestLogBlocking(t *testing.T) {
-	skip.If(t, runtime.GOOS == "windows", "FIXME: test is flaky on Windows. See #39857")
 	mockClient := newMockClient()
 	stream := &logStream{
 		client:   mockClient,
@@ -301,18 +302,20 @@ func TestLogBlocking(t *testing.T) {
 		err := stream.Log(&logger.Message{})
 		errorCh <- err
 	}()
+	// block until the goroutine above has started
 	<-started
 	select {
 	case err := <-errorCh:
 		t.Fatal("Expected stream.Log to block: ", err)
 	default:
-		break
 	}
+	// assuming it is blocked, we can now try to drain the internal channel and
+	// unblock it
 	select {
-	case <-stream.messages:
-		break
-	default:
+	case <-time.After(10 * time.Millisecond):
+		// if we're unable to drain the channel within 10ms, something seems broken
 		t.Fatal("Expected to be able to read from stream.messages but was unable to")
+	case <-stream.messages:
 	}
 	select {
 	case err := <-errorCh: