docker: don't lose a client size that races the exec creation

A browser's terminal-size control frame (NAWS through the console telnet
server) can arrive while client_connected_hook is still creating the
exec; the resize is a no-op then, and the tall default applied after
creation would overwrite it, leaving the session at 511x10000 until the
user resizes.

Record sizes received before the exec exists and prefer them over the
tall default once creation finishes. The recorded size is cleared when
the last client disconnects, together with the restore-to-default.

tests/compute/docker/test_vendor_docker_vm.py

@@ -584,6 +584,24 @@ async def test_last_client_disconnect_restores_tall_default(compute_project, man
     srv._resize_exec.assert_called_once_with(511, 10000)
 
 
+@pytest.mark.asyncio
+async def test_size_arriving_before_exec_wins_over_default(compute_project, manager):
+    """A client size that races the exec creation (WS control frame / NAWS
+    arriving inside client_connected_hook) must not be overwritten by the
+    tall default once the exec exists."""
+
+    srv = _make_lazy_server(compute_project, manager)
+    assert srv._exec_id is None
+    # real _resize_exec (not the mock) records the size when no exec exists
+    srv._resize_exec = _LazyExecTelnetServer._resize_exec.__get__(srv)
+    await srv._on_naws(120, 40)
+    assert srv._client_size == (120, 40)
+
+    srv._resize_exec = AsyncioMagicMock()
+    await srv.client_connected_hook()
+    srv._resize_exec.assert_called_once_with(120, 40)
+
+
 @pytest.mark.asyncio
 async def test_reconnect_live_exec_not_recreated(compute_project, manager):
     """Reconnecting while the exec is alive must NOT recreate it."""