net/http: deflake TestRequestBodyLimit

This test can return with a Transport still processing
an in-flight request, resulting in a test failure due
to the leaked Transport.

Avoid this by waiting for the Transport to close the
request body before returning.

Fixes #60264

Change-Id: I8d8b54f633c2e28da2b1bf1bc01ce09dd77769de
Reviewed-on: https://go-review.googlesource.com/c/go/+/522695
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Damien Neil <dneil@google.com>

src/net/http/serve_test.go

@@ -2967,15 +2967,36 @@ func (b neverEnding) Read(p []byte) (n int, err error) {
 	return len(p), nil
 }
 
-type countReader struct {
-	r io.Reader
-	n *int64
+type bodyLimitReader struct {
+	mu     sync.Mutex
+	count  int
+	limit  int
+	closed chan struct{}
 }
 
-func (cr countReader) Read(p []byte) (n int, err error) {
-	n, err = cr.r.Read(p)
-	atomic.AddInt64(cr.n, int64(n))
-	return
+func (r *bodyLimitReader) Read(p []byte) (int, error) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	select {
+	case <-r.closed:
+		return 0, errors.New("closed")
+	default:
+	}
+	if r.count > r.limit {
+		return 0, errors.New("at limit")
+	}
+	r.count += len(p)
+	for i := range p {
+		p[i] = 'a'
+	}
+	return len(p), nil
+}
+
+func (r *bodyLimitReader) Close() error {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	close(r.closed)
+	return nil
 }
 
 func TestRequestBodyLimit(t *testing.T) { run(t, testRequestBodyLimit) }
@@ -2999,8 +3020,11 @@ func testRequestBodyLimit(t *testing.T, mode testMode) {
 		}
 	}))
 
-	nWritten := new(int64)
-	req, _ := NewRequest("POST", cst.ts.URL, io.LimitReader(countReader{neverEnding('a'), nWritten}, limit*200))
+	body := &bodyLimitReader{
+		closed: make(chan struct{}),
+		limit:  limit * 200,
+	}
+	req, _ := NewRequest("POST", cst.ts.URL, body)
 
 	// Send the POST, but don't care it succeeds or not. The
 	// remote side is going to reply and then close the TCP
@@ -3015,10 +3039,13 @@ func testRequestBodyLimit(t *testing.T, mode testMode) {
 	if err == nil {
 		resp.Body.Close()
 	}
+	// Wait for the Transport to finish writing the request body.
+	// It will close the body when done.
+	<-body.closed
 
-	if atomic.LoadInt64(nWritten) > limit*100 {
+	if body.count > limit*100 {
 		t.Errorf("handler restricted the request body to %d bytes, but client managed to write %d",
-			limit, nWritten)
+			limit, body.count)
 	}
 }