gh-113538: Add asycio.Server.{close,abort}_clients (#114432)

These give applications the option of more forcefully terminating client
connections for asyncio servers. Useful when terminating a service and
there is limited time to wait for clients to finish up their work.

Lib/asyncio/base_events.py

@@ -279,7 +279,9 @@ class Server(events.AbstractServer):
                  ssl_handshake_timeout, ssl_shutdown_timeout=None):
         self._loop = loop
         self._sockets = sockets
-        self._active_count = 0
+        # Weak references so we don't break Transport's ability to
+        # detect abandoned transports
+        self._clients = weakref.WeakSet()
         self._waiters = []
         self._protocol_factory = protocol_factory
         self._backlog = backlog
@@ -292,14 +294,13 @@ class Server(events.AbstractServer):
     def __repr__(self):
         return f'<{self.__class__.__name__} sockets={self.sockets!r}>'
 
-    def _attach(self):
+    def _attach(self, transport):
         assert self._sockets is not None
-        self._active_count += 1
+        self._clients.add(transport)
 
-    def _detach(self):
-        assert self._active_count > 0
-        self._active_count -= 1
-        if self._active_count == 0 and self._sockets is None:
+    def _detach(self, transport):
+        self._clients.discard(transport)
+        if len(self._clients) == 0 and self._sockets is None:
             self._wakeup()
 
     def _wakeup(self):
@@ -348,9 +349,17 @@ class Server(events.AbstractServer):
             self._serving_forever_fut.cancel()
             self._serving_forever_fut = None
 
-        if self._active_count == 0:
+        if len(self._clients) == 0:
             self._wakeup()
 
+    def close_clients(self):
+        for transport in self._clients.copy():
+            transport.close()
+
+    def abort_clients(self):
+        for transport in self._clients.copy():
+            transport.abort()
+
     async def start_serving(self):
         self._start_serving()
         # Skip one loop iteration so that all 'loop.add_reader'

Lib/asyncio/events.py

@@ -175,6 +175,14 @@ class AbstractServer:
         """Stop serving.  This leaves existing connections open."""
         raise NotImplementedError
 
+    def close_clients(self):
+        """Close all active connections."""
+        raise NotImplementedError
+
+    def abort_clients(self):
+        """Close all active connections immediately."""
+        raise NotImplementedError
+
     def get_loop(self):
         """Get the event loop the Server object is attached to."""
         raise NotImplementedError

Lib/asyncio/proactor_events.py

@@ -63,7 +63,7 @@ class _ProactorBasePipeTransport(transports._FlowControlMixin,
         self._called_connection_lost = False
         self._eof_written = False
         if self._server is not None:
-            self._server._attach()
+            self._server._attach(self)
         self._loop.call_soon(self._protocol.connection_made, self)
         if waiter is not None:
             # only wake up the waiter when connection_made() has been called
@@ -167,7 +167,7 @@ class _ProactorBasePipeTransport(transports._FlowControlMixin,
             self._sock = None
             server = self._server
             if server is not None:
-                server._detach()
+                server._detach(self)
                 self._server = None
             self._called_connection_lost = True
 

Lib/asyncio/selector_events.py

@@ -791,7 +791,7 @@ class _SelectorTransport(transports._FlowControlMixin,
         self._paused = False  # Set when pause_reading() called
 
         if self._server is not None:
-            self._server._attach()
+            self._server._attach(self)
         loop._transports[self._sock_fd] = self
 
     def __repr__(self):
@@ -868,6 +868,8 @@ class _SelectorTransport(transports._FlowControlMixin,
         if self._sock is not None:
             _warn(f"unclosed transport {self!r}", ResourceWarning, source=self)
             self._sock.close()
+            if self._server is not None:
+                self._server._detach(self)
 
     def _fatal_error(self, exc, message='Fatal error on transport'):
         # Should be called from exception handler only.
@@ -906,7 +908,7 @@ class _SelectorTransport(transports._FlowControlMixin,
             self._loop = None
             server = self._server
             if server is not None:
-                server._detach()
+                server._detach(self)
                 self._server = None
 
     def get_write_buffer_size(self):