crypto/tls: deflake TestConnReadNonzeroAndEOF

Fixes #7683

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/83080048

tls_test.go

@@ -5,6 +5,7 @@
 package tls
 
 import (
+	"fmt"
 	"io"
 	"net"
 	"strings"
@@ -161,21 +162,41 @@ func TestDialTimeout(t *testing.T) {
 // (non-zero, nil) when a Close (alertCloseNotify) is sitting right
 // behind the application data in the buffer.
 func TestConnReadNonzeroAndEOF(t *testing.T) {
+	// This test is racy: it assumes that after a write to a
+	// localhost TCP connection, the peer TCP connection can
+	// immediately read it.  Because it's racy, we skip this test
+	// in short mode, and then retry it several times with an
+	// increasing sleep in between our final write (via srv.Close
+	// below) and the following read.
+	if testing.Short() {
+		t.Skip("skipping in short mode")
+	}
+	var err error
+	for delay := time.Millisecond; delay <= 64*time.Millisecond; delay *= 2 {
+		if err = testConnReadNonzeroAndEOF(t, delay); err == nil {
+			return
+		}
+	}
+	t.Error(err)
+}
+
+func testConnReadNonzeroAndEOF(t *testing.T, delay time.Duration) error {
 	ln := newLocalListener(t)
 	defer ln.Close()
 
 	srvCh := make(chan *Conn, 1)
+	var serr error
 	go func() {
 		sconn, err := ln.Accept()
 		if err != nil {
-			t.Error(err)
+			serr = err
 			srvCh <- nil
 			return
 		}
 		serverConfig := *testConfig
 		srv := Server(sconn, &serverConfig)
 		if err := srv.Handshake(); err != nil {
-			t.Error("handshake: %v", err)
+			serr = fmt.Errorf("handshake: %v", err)
 			srvCh <- nil
 			return
 		}
@@ -191,7 +212,7 @@ func TestConnReadNonzeroAndEOF(t *testing.T) {
 
 	srv := <-srvCh
 	if srv == nil {
-		return
+		return serr
 	}
 
 	buf := make([]byte, 6)
@@ -199,16 +220,18 @@ func TestConnReadNonzeroAndEOF(t *testing.T) {
 	srv.Write([]byte("foobar"))
 	n, err := conn.Read(buf)
 	if n != 6 || err != nil || string(buf) != "foobar" {
-		t.Fatalf("Read = %d, %v, data %q; want 6, nil, foobar", n, err, buf)
+		return fmt.Errorf("Read = %d, %v, data %q; want 6, nil, foobar", n, err, buf)
 	}
 
 	srv.Write([]byte("abcdef"))
 	srv.Close()
+	time.Sleep(delay)
 	n, err = conn.Read(buf)
 	if n != 6 || string(buf) != "abcdef" {
-		t.Fatalf("Read = %d, buf= %q; want 6, abcdef", n, buf)
+		return fmt.Errorf("Read = %d, buf= %q; want 6, abcdef", n, buf)
 	}
 	if err != io.EOF {
-		t.Errorf("Second Read error = %v; want io.EOF", err)
+		return fmt.Errorf("Second Read error = %v; want io.EOF", err)
 	}
+	return nil
 }