os: deflake TestFdReadRace

The test would hang if the call to Fd set the pipe to be non-blocking
before the Read entered the first read system call. Avoid that problem
by writing data to the pipe to wake up the read.

For #24481
Fixes #44818

Change-Id: I0b798874c7b81e7308a38ebbf657efc4392ffacd
Reviewed-on: https://go-review.googlesource.com/c/go/+/322893
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>

src/os/pipe_test.go

@@ -442,12 +442,14 @@ func TestFdReadRace(t *testing.T) {
 	defer r.Close()
 	defer w.Close()
 
-	c := make(chan bool)
+	const count = 10
+
+	c := make(chan bool, 1)
 	var wg sync.WaitGroup
 	wg.Add(1)
 	go func() {
 		defer wg.Done()
-		var buf [10]byte
+		var buf [count]byte
 		r.SetReadDeadline(time.Now().Add(time.Minute))
 		c <- true
 		if _, err := r.Read(buf[:]); os.IsTimeout(err) {
@@ -466,8 +468,9 @@ func TestFdReadRace(t *testing.T) {
 		r.Fd()
 
 		// The bug was that Fd would hang until Read timed out.
-		// If the bug is fixed, then closing r here will cause
+		// If the bug is fixed, then writing to w and closing r here
-		// the Read to exit before the timeout expires.
+		// will cause the Read to exit before the timeout expires.
+		w.Write(make([]byte, count))
 		r.Close()
 	}()