gopls/internal/regtest: fix data race when printing logs

Lock around log access in regtests. This flake was relatively rare in
regtests because of the serial nature of our jsonrpc2 implementation,
but was nonetheless racy.

Fixes golang/go#41653

Change-Id: I08674f42b05624a69d33885c2232c9e31866375b
Reviewed-on: https://go-review.googlesource.com/c/tools/+/257957
Trust: Rebecca Stambler <[email protected]>
Trust: Robert Findley <[email protected]>
Run-TryBot: Rebecca Stambler <[email protected]>
Run-TryBot: Robert Findley <[email protected]>
gopls-CI: kokoro <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Rebecca Stambler <[email protected]>

Author: findleyr

gopls/internal/regtest/runner.go

@@ -288,17 +288,29 @@ func (r *Runner) Run(t *testing.T, files string, test TestFunc, opts ...RunOptio
 
 type loggingFramer struct {
 	mu      sync.Mutex
-	buffers []*bytes.Buffer
+	buffers []*safeBuffer
+}
+
+// safeBuffer is a threadsafe buffer for logs.
+type safeBuffer struct {
+	mu  sync.Mutex
+	buf bytes.Buffer
+}
+
+func (b *safeBuffer) Write(p []byte) (int, error) {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+	return b.buf.Write(p)
 }
 
 func (s *loggingFramer) framer(f jsonrpc2.Framer) jsonrpc2.Framer {
 	return func(nc net.Conn) jsonrpc2.Stream {
 		s.mu.Lock()
-		var buf bytes.Buffer
-		s.buffers = append(s.buffers, &buf)
+		buf := &safeBuffer{buf: bytes.Buffer{}}
+		s.buffers = append(s.buffers, buf)
 		s.mu.Unlock()
 		stream := f(nc)
-		return protocol.LoggingStream(stream, &buf)
+		return protocol.LoggingStream(stream, buf)
 	}
 }
 
@@ -308,9 +320,9 @@ func (s *loggingFramer) printBuffers(testname string, w io.Writer) {
 
 	for i, buf := range s.buffers {
 		fmt.Fprintf(os.Stderr, "#### Start Gopls Test Logs %d of %d for %q\n", i+1, len(s.buffers), testname)
-		// Re-buffer buf to avoid a data rate (io.Copy mutates src).
-		writeBuf := bytes.NewBuffer(buf.Bytes())
-		io.Copy(w, writeBuf)
+		buf.mu.Lock()
+		io.Copy(w, &buf.buf)
+		buf.mu.Unlock()
 		fmt.Fprintf(os.Stderr, "#### End Gopls Test Logs %d of %d for %q\n", i+1, len(s.buffers), testname)
 	}
 }