packer/rpc: fix crashes with big file uploads [GH-897]

CHANGELOG.md

@@ -14,6 +14,7 @@ IMPROVEMENTS:
 BUG FIXES:
 
 * core: Fix crash case if blank parameters are given to Packer. [GH-832]
+* core: Fix crash if big file uploads are done. [GH-897]
 * builders/docker: user variables work properly. [GH-777]
 * builder/virtualbox,vmware: iso\_checksum is not required if the
   checksum type is "none"

packer/rpc/muxconn.go

@@ -400,6 +400,8 @@ func (m *MuxConn) loop() {
 			stream.mu.Unlock()
 
 		case muxPacketData:
+			unlocked := false
+
 			stream.mu.Lock()
 			switch stream.state {
 			case streamStateFinWait1:
@@ -408,17 +410,25 @@ func (m *MuxConn) loop() {
 				fallthrough
 			case streamStateEstablished:
 				if len(data) > 0 {
-					select {
-					case stream.writeCh <- data:
-					default:
-						panic(fmt.Sprintf(
-							"Failed to write data, buffer full for stream %d", id))
-					}
+					// Get a reference to the write channel while we have
+					// the lock because otherwise the field might change.
+					// We unlock early here because the write might block
+					// for a long time.
+					writeCh := stream.writeCh
+					stream.mu.Unlock()
+					unlocked = true
+
+					// Blocked write, this provides some backpressure on
+					// the connection if there is a lot of data incoming.
+					writeCh <- data
 				}
 			default:
 				log.Printf("[ERR] Data received for stream in state: %d", stream.state)
 			}
-			stream.mu.Unlock()
+
+			if !unlocked {
+				stream.mu.Unlock()
+			}
 		}
 	}
 }

packer/rpc/muxconn_test.go

@@ -114,6 +114,52 @@ func TestMuxConn(t *testing.T) {
 	<-doneCh
 }
 
+func TestMuxConn_lotsOfData(t *testing.T) {
+	client, server := testMux(t)
+	defer client.Close()
+	defer server.Close()
+
+	// When the server is done
+	doneCh := make(chan struct{})
+
+	// The server side
+	go func() {
+		defer close(doneCh)
+
+		s0, err := server.Accept(0)
+		if err != nil {
+			t.Fatalf("err: %s", err)
+		}
+
+		var wg sync.WaitGroup
+		wg.Add(1)
+
+		go func() {
+			defer wg.Done()
+			data := readStream(t, s0)
+			if data != "hello" {
+				t.Fatalf("bad: %#v", data)
+			}
+		}()
+
+		wg.Wait()
+	}()
+
+	s0, err := client.Dial(0)
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	for i := 0; i < 4096 * 4; i++ {
+		if _, err := s0.Write([]byte("hello")); err != nil {
+			t.Fatalf("err: %s", err)
+		}
+	}
+
+	// Wait for the server to be done
+	<-doneCh
+}
+
 // This tests that even when the client end is closed, data can be
 // read from the server.
 func TestMuxConn_clientCloseRead(t *testing.T) {