Fix black captures from servers announcing DesktopSize before pixel data (#90)

CHANGELOG.rst

@@ -1,5 +1,6 @@
 1.4.0 (UNRELEASED)
 ----------------------
+  - Fix black screen captures from servers that announce DesktopSize before sending pixel data, e.g. TightVNC (@sibson, #90)
   - Fix the dead protocol reference in the published ``rfb`` module documentation, which pointed at a RealVNC PDF that has been 403 for years; RFC 6143 and the rfbproto community document replace it (@sibson)
   - Declare python_requires >=3.10, matching the versions CI tests and the development requirements. 3.9 was advertised but neither tested nor able to install the dev environment (@sibson, #357)
   - [BREAKING] vncdo exit codes now say what went wrong: single digits for bad input, including 3 for authentication, and tens grouped by cause out on the wire, 10s connection, 20s protocol, 30s command, 40s timeout, documented in docs/usage.rst.  Scripts reading the exit code see new values: authentication failure is now 3 and a session cut short 11, both of which used to be 0; an unknown action is now 2, previously 1 (@sibson, #345)

tests/unit/test_client.py

@@ -206,6 +206,56 @@ def test_commitUpdate(self) -> None:
 
         self.deferred.callback.assert_called_once_with(self.client)
 
+    # A framebuffer update whose only rectangle is the DesktopSize
+    # pseudo-encoding carries no pixel data.
+    MSG_FBU_DESKTOP_SIZE_ONLY = (
+        b"\x00"  # FRAMEBUFFER_UPDATE
+        b"\x00"  # padding
+        b"\x00\x01"  # number-of-rectangles
+        b"\x00\x00\x00\x00\x07\x80\x04\xb0"  # x=0 y=0 w=1920 h=1200
+        b"\xff\xff\xff\x21"  # PSEUDO_DESKTOP_SIZE (-223)
+    )
+    MSG_FBU_ONE_PIXEL = (
+        b"\x00"  # FRAMEBUFFER_UPDATE
+        b"\x00"  # padding
+        b"\x00\x01"  # number-of-rectangles
+        b"\x00\x00\x00\x00\x00\x01\x00\x01"  # x=0 y=0 w=1 h=1
+        b"\x00\x00\x00\x00"  # Encoding.RAW
+        b"\xff\x00\x00\x00"  # one RGBX pixel
+    )
+
+    def _connect(self) -> None:
+        self.client._packet = bytearray(self.MSG_HANDSHAKE)
+        self.client._handleInitial()
+        self.client._handleServerInit(self.MSG_INIT)
+
+    def test_desktop_size_only_update_rerequests_instead_of_completing(self) -> None:
+        cli = self.client
+        self._connect()
+        d = cli.refreshScreen()
+        fired: list = []
+        d.addCallback(fired.append)
+        cli.framebufferUpdateRequest.reset_mock()
+
+        cli.dataReceived(self.MSG_FBU_DESKTOP_SIZE_ONLY)
+
+        self.assertEqual(fired, [])
+        cli.framebufferUpdateRequest.assert_called_once_with()
+
+    def test_refresh_completes_once_pixel_data_arrives(self) -> None:
+        cli = self.client
+        self._connect()
+        d = cli.refreshScreen()
+        fired: list = []
+        d.addCallback(fired.append)
+
+        cli.dataReceived(self.MSG_FBU_DESKTOP_SIZE_ONLY)
+        cli.dataReceived(self.MSG_FBU_ONE_PIXEL)
+
+        self.assertEqual(fired, [cli])
+        assert cli.screen is not None
+        self.assertEqual(cli.screen.size, (1920, 1200))
+
     def test_vncRequestPassword_attribute(self):
         cli = self.client
         cli.sendPassword = mock.Mock()

vncdotool/client.py

@@ -393,6 +393,11 @@ def updateRectangle(
 
     def commitUpdate(self, rectangles: list[rfb.Rect] | None = None) -> None:
         if self.deferred:
+            if not rectangles:
+                # No rectangle in this update painted self.screen; wait for
+                # one that does before completing the refresh.
+                self.framebufferUpdateRequest()
+                return
             d = self.deferred
             self.deferred = None
             d.callback(self)

vncdotool/rfb.py

@@ -440,6 +440,7 @@ def _handleRectangle(self, block: bytes) -> None:
                 length += ((width + 7) // 8) * height
                 self.expect(self._handleDecodePsuedoCursor, length, x, y, width, height)
             elif encoding == Encoding.PSEUDO_DESKTOP_SIZE:
+                del self.rectanglePos[-1]  # undo append as this carries no pixel data
                 self._handleDecodeDesktopSize(width, height)
             elif encoding == Encoding.PSEUDO_QEMU_EXTENDED_KEY_EVENT:
                 self.negotiated_encodings.add(Encoding.PSEUDO_QEMU_EXTENDED_KEY_EVENT)