Fix abstract named socket used for GUI IPC on linux not being shutdown till process exit, thereby preventing restart from working. Fixes #1298756 [error restarting](https://bugs.launchpad.net/calibre/+bug/1298756)

src/calibre/gui2/main.py

@@ -418,13 +418,11 @@ def communicate(opts, args):
     raise SystemExit(0)
 
 def create_listener():
-    from multiprocessing.connection import Listener
-    listener = Listener(address=gui_socket_address())
-    if islinux and hasattr(listener._listener._unlink, 'cancel'):
-        # multiprocessing tries to call unlink even on abstract
-        # named sockets, prevent it from doing so.
-        listener._listener._unlink.cancel()
-    return listener
+    if islinux:
+        from calibre.utils.ipc.server import LinuxListener as Listener
+    else:
+        from multiprocessing.connection import Listener
+    return Listener(address=gui_socket_address())
 
 def main(args=sys.argv):
     gui_debug = None

src/calibre/utils/ipc/server.py

@@ -87,6 +87,22 @@ class CriticalError(Exception):
 _name_counter = 0
 
 if islinux:
+    class LinuxListener(Listener):
+
+        def __init__(self, *args, **kwargs):
+            Listener.__init__(self, *args, **kwargs)
+            # multiprocessing tries to call unlink even on abstract
+            # named sockets, prevent it from doing so.
+            self._listener._unlink.cancel()
+
+        def close(self):
+            # To ensure that the socket is released, we have to call
+            # shutdown() not close(). This is needed to allow calibre to
+            # restart using the same socket address.
+            import socket
+            self._listener._socket.shutdown(socket.SHUT_RDWR)
+            self._listener._socket.close()
+
     def create_listener(authkey, backlog=4):
         # Use abstract named sockets on linux to avoid creating unnecessary temp files
         global _name_counter
@@ -95,11 +111,7 @@ if islinux:
             _name_counter += 1
             address = (prefix % _name_counter).encode('ascii')
             try:
-                l = Listener(address=address, authkey=authkey, backlog=backlog)
-                if hasattr(l._listener._unlink, 'cancel'):
-                    # multiprocessing tries to call unlink even on abstract
-                    # named sockets, prevent it from doing so.
-                    l._listener._unlink.cancel()
+                l = LinuxListener(address=address, authkey=authkey, backlog=backlog)
                 return address, l
             except EnvironmentError as err:
                 if err.errno == errno.EADDRINUSE: