Revert "ipc: Remove _old_socket handling"

This reverts commit c6cf3067e158265096410afee98407826b19c472.

Seems to cause segfaults:

  #0  0x00007ffff5cecbcc in void doActivate<false>(QObject*, int, void**) () from /home/florian/proj/qutebrowser/git/.tox/py39-pyqt515/lib/python3.9/site-packages/PyQt5/Qt5/lib/libQt5Core.so.5
  #1  0x00007ffff5be4e31 in QIODevice::channelReadyRead(int) () from /home/florian/proj/qutebrowser/git/.tox/py39-pyqt515/lib/python3.9/site-packages/PyQt5/Qt5/lib/libQt5Core.so.5
  #2  0x00007fffeffccb54 in QAbstractSocketPrivate::canReadNotification() () from /home/florian/proj/qutebrowser/git/.tox/py39-pyqt515/lib/python3.9/site-packages/PyQt5/Qt5/lib/libQt5Network.so.5
  #3  0x00007fffeffdf061 in QReadNotifier::event(QEvent*) () from /home/florian/proj/qutebrowser/git/.tox/py39-pyqt515/lib/python3.9/site-packages/PyQt5/Qt5/lib/libQt5Network.so.5
  #4  0x00007ffff269e43c in QApplicationPrivate::notify_helper(QObject*, QEvent*) () from /home/florian/proj/qutebrowser/git/.tox/py39-pyqt515/lib/python3.9/site-packages/PyQt5/Qt5/lib/libQt5Widgets.so.5
  #5  0x00007ffff26a4f20 in QApplication::notify(QObject*, QEvent*) () from /home/florian/proj/qutebrowser/git/.tox/py39-pyqt515/lib/python3.9/site-packages/PyQt5/Qt5/lib/libQt5Widgets.so.5
  #6  0x00007ffff318d0d6 in sipQApplication::notify(QObject*, QEvent*) () from /home/florian/proj/qutebrowser/git/.tox/py39-pyqt515/lib/python3.9/site-packages/PyQt5/QtWidgets.abi3.so
  #7  0x00007ffff5cb4808 in QCoreApplication::notifyInternal2(QObject*, QEvent*) () from /home/florian/proj/qutebrowser/git/.tox/py39-pyqt515/lib/python3.9/site-packages/PyQt5/Qt5/lib/libQt5Core.so.5
  #8  0x00007ffff5d10d98 in socketNotifierSourceDispatch(_GSource*, int (*)(void*), void*) () from /home/florian/proj/qutebrowser/git/.tox/py39-pyqt515/lib/python3.9/site-packages/PyQt5/Qt5/lib/libQt5Core.so.5
  #9  0x00007ffff691df9c in g_main_context_dispatch () from /usr/lib/libglib-2.0.so.0
  #10 0x00007ffff6971a49 in ?? () from /usr/lib/libglib-2.0.so.0
  #11 0x00007ffff691b6f1 in g_main_context_iteration () from /usr/lib/libglib-2.0.so.0
  #12 0x00007ffff5d101cc in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () from /home/florian/proj/qutebrowser/git/.tox/py39-pyqt515/lib/python3.9/site-packages/PyQt5/Qt5/lib/libQt5Core.so.5
  #13 0x00007ffff5cb321a in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) () from /home/florian/proj/qutebrowser/git/.tox/py39-pyqt515/lib/python3.9/site-packages/PyQt5/Qt5/lib/libQt5Core.so.5
  #14 0x00007ffff5cbc1d3 in QCoreApplication::exec() () from /home/florian/proj/qutebrowser/git/.tox/py39-pyqt515/lib/python3.9/site-packages/PyQt5/Qt5/lib/libQt5Core.so.5

qutebrowser/misc/ipc.py

@@ -195,6 +195,7 @@ def __init__(self, socketname, parent=None):
             self.handle_connection)
 
         self._socket = None
+        self._old_socket = None
 
         if utils.is_windows:  # pragma: no cover
             # As a WORKAROUND for a Qt bug, we can't use UserAccessOption on Unix. If we
@@ -288,13 +289,12 @@ def handle_connection(self):
     @pyqtSlot()
     def on_disconnected(self):
         """Clean up socket when the client disconnected."""
-        if self._socket is None:
-            log.ipc.debug("Disconnected with None-socket.")
-            return
         log.ipc.debug("Client disconnected from socket 0x{:x}.".format(
             id(self._socket)))
         self._timer.stop()
-        self._socket.deleteLater()
+        if self._old_socket is not None:
+            self._old_socket.deleteLater()
+        self._old_socket = self._socket
         self._socket = None
         # Maybe another connection is waiting.
         self.handle_connection()
@@ -360,18 +360,27 @@ def _handle_data(self, data):
     def on_ready_read(self):
         """Read json data from the client."""
         if self._socket is None:  # pragma: no cover
-            return
+            # This happens when doing a connection while another one is already
+            # active for some reason.
+            if self._old_socket is None:
+                log.ipc.warning("In on_ready_read with None socket and "
+                                "old_socket!")
+                return
+            log.ipc.debug("In on_ready_read with None socket!")
+            socket = self._old_socket
+        else:
+            socket = self._socket
 
-        if sip.isdeleted(self._socket):  # pragma: no cover
+        if sip.isdeleted(socket):  # pragma: no cover
             log.ipc.warning("Ignoring deleted IPC socket")
             return
 
         self._timer.stop()
-        while self._socket is not None and self._socket.canReadLine():
-            data = bytes(self._socket.readLine())
+        while socket is not None and socket.canReadLine():
+            data = bytes(socket.readLine())
             self.got_raw.emit(data)
             log.ipc.debug("Read from socket 0x{:x}: {!r}".format(
-                id(self._socket), data))
+                id(socket), data))
             self._handle_data(data)
 
         if self._socket is not None:

tests/unit/misc/test_ipc.py

@@ -600,9 +600,13 @@ def test_timeout(qtbot, caplog, qlocalsocket, ipc_server):
     assert caplog.messages[-1].startswith("IPC connection timed out")
 
 
-def test_ipcserver_socket_none_readyread(ipc_server):
+def test_ipcserver_socket_none_readyread(ipc_server, caplog):
     assert ipc_server._socket is None
-    ipc_server.on_ready_read()
+    assert ipc_server._old_socket is None
+    with caplog.at_level(logging.WARNING):
+        ipc_server.on_ready_read()
+    msg = "In on_ready_read with None socket and old_socket!"
+    assert msg in caplog.messages
 
 
 @pytest.mark.posix