transport/bufWriter: fast-fail on error returned from flushKeepBuffer…

…() (#7394)

internal/transport/http_util.go

@@ -317,28 +317,32 @@ func newBufWriter(conn net.Conn, batchSize int, pool *sync.Pool) *bufWriter {
 	return w
 }
 
-func (w *bufWriter) Write(b []byte) (n int, err error) {
+func (w *bufWriter) Write(b []byte) (int, error) {
 	if w.err != nil {
 		return 0, w.err
 	}
 	if w.batchSize == 0 { // Buffer has been disabled.
-		n, err = w.conn.Write(b)
+		n, err := w.conn.Write(b)
 		return n, toIOError(err)
 	}
 	if w.buf == nil {
 		b := w.pool.Get().(*[]byte)
 		w.buf = *b
 	}
+	written := 0
 	for len(b) > 0 {
-		nn := copy(w.buf[w.offset:], b)
-		b = b[nn:]
-		w.offset += nn
-		n += nn
-		if w.offset >= w.batchSize {
-			err = w.flushKeepBuffer()
+		copied := copy(w.buf[w.offset:], b)
+		b = b[copied:]
+		written += copied
+		w.offset += copied
+		if w.offset < w.batchSize {
+			continue
+		}
+		if err := w.flushKeepBuffer(); err != nil {
+			return written, err
 		}
 	}
-	return n, err
+	return written, nil
 }
 
 func (w *bufWriter) Flush() error {

internal/transport/http_util_test.go

@@ -19,7 +19,10 @@
 package transport
 
 import (
+	"errors"
 	"fmt"
+	"io"
+	"net"
 	"reflect"
 	"testing"
 	"time"
@@ -215,6 +218,39 @@ func (s) TestParseDialTarget(t *testing.T) {
 	}
 }
 
+type badNetworkConn struct {
+	net.Conn
+}
+
+func (c *badNetworkConn) Write([]byte) (int, error) {
+	return 0, io.EOF
+}
+
+// This test ensures Write() on a broken network connection does not lead to
+// an infinite loop. See https://github.com/grpc/grpc-go/issues/7389 for more details.
+func (s) TestWriteBadConnection(t *testing.T) {
+	data := []byte("test_data")
+	// Configure the bufWriter with a batchsize that results in data being flushed
+	// to the underlying conn, midway through Write().
+	writeBufferSize := (len(data) - 1) / 2
+	writer := newBufWriter(&badNetworkConn{}, writeBufferSize, getWriteBufferPool(writeBufferSize))
+
+	errCh := make(chan error, 1)
+	go func() {
+		_, err := writer.Write(data)
+		errCh <- err
+	}()
+
+	select {
+	case <-time.After(time.Second):
+		t.Fatalf("Write() did not return in time")
+	case err := <-errCh:
+		if !errors.Is(err, io.EOF) {
+			t.Fatalf("Write() = %v, want error presence = %v", err, io.EOF)
+		}
+	}
+}
+
 func BenchmarkDecodeGrpcMessage(b *testing.B) {
 	input := "Hello, %E4%B8%96%E7%95%8C"
 	want := "Hello, 世界"