PYTHON-5517 Simplify pool backpressure behavior

pymongo/asynchronous/pool.py

@@ -37,7 +37,7 @@
 from bson import DEFAULT_CODEC_OPTIONS
 from pymongo import _csot, helpers_shared
 from pymongo.asynchronous.client_session import _validate_session_write_concern
-from pymongo.asynchronous.helpers import _backoff, _handle_reauth
+from pymongo.asynchronous.helpers import _handle_reauth
 from pymongo.asynchronous.network import command
 from pymongo.common import (
     MAX_BSON_SIZE,
@@ -52,6 +52,7 @@
     DocumentTooLarge,
     ExecutionTimeout,
     InvalidOperation,
+    NetworkTimeout,
     NotPrimaryError,
     OperationFailure,
     PyMongoError,
@@ -789,8 +790,8 @@ def __init__(
         # Also used for: clearing the wait queue
         self._max_connecting_cond = _async_create_condition(self.lock)
         self._pending = 0
+        self._max_connecting = self.opts.max_connecting
         self._client_id = client_id
-        self._backoff = 0
         if self.enabled_for_cmap:
             assert self.opts._event_listeners is not None
             self.opts._event_listeners.publish_pool_created(
@@ -846,8 +847,6 @@ async def _reset(
         async with self.size_cond:
             if self.closed:
                 return
-            # Clear the backoff state.
-            self._backoff = 0
             if self.opts.pause_enabled and pause and not self.opts.load_balanced:
                 old_state, self.state = self.state, PoolState.PAUSED
             self.gen.inc(service_id)
@@ -930,11 +929,6 @@ async def _reset(
                 for conn in sockets:
                     await conn.close_conn(ConnectionClosedReason.STALE)
 
-    @property
-    def max_connecting(self) -> int:
-        """The current max connecting limit for the pool."""
-        return 1 if self._backoff else self.opts.max_connecting
-
     async def update_is_writable(self, is_writable: Optional[bool]) -> None:
         """Updates the is_writable attribute on all sockets currently in the
         Pool.
@@ -1001,7 +995,7 @@ async def remove_stale_sockets(self, reference_generation: int) -> None:
                 async with self._max_connecting_cond:
                     # If maxConnecting connections are already being created
                     # by this pool then try again later instead of waiting.
-                    if self._pending >= self.max_connecting:
+                    if self._pending >= self._max_connecting:
                         return
                     self._pending += 1
                     incremented = True
@@ -1029,29 +1023,14 @@ async def remove_stale_sockets(self, reference_generation: int) -> None:
                     self.requests -= 1
                     self.size_cond.notify()
 
-    def _handle_connection_error(self, error: BaseException, phase: str, conn_id: int) -> None:
+    def _handle_connection_error(self, error: BaseException) -> None:
         # Handle system overload condition for non-sdam pools.
-        # Look for an AutoReconnect error raised from a ConnectionResetError with
-        # errno == errno.ECONNRESET or raised from an OSError that we've created due to
-        # a closed connection.
-        # If found, set backoff and add error labels.
-        if self.is_sdam or type(error) != AutoReconnect:
+        # Look for errors of type AutoReconnect and add error labels if appropriate.
+        if self.is_sdam or type(error) not in (AutoReconnect, NetworkTimeout):
             return
-        self._backoff += 1
+        assert isinstance(error, AutoReconnect)  # Appease type checker.
         error._add_error_label("SystemOverloadedError")
         error._add_error_label("RetryableError")
-        # Log the pool backoff message.
-        if self.enabled_for_logging and _CONNECTION_LOGGER.isEnabledFor(logging.DEBUG):
-            _debug_log(
-                _CONNECTION_LOGGER,
-                message=_ConnectionStatusMessage.POOL_BACKOFF,
-                clientId=self._client_id,
-                serverHost=self.address[0],
-                serverPort=self.address[1],
-                driverConnectionId=conn_id,
-                reason=_verbose_connection_error_reason(ConnectionClosedReason.POOL_BACKOFF),
-                error=ConnectionClosedReason.POOL_BACKOFF,
-            )
 
     async def connect(self, handler: Optional[_MongoClientErrorHandler] = None) -> AsyncConnection:
         """Connect to Mongo and return a new AsyncConnection.
@@ -1082,17 +1061,8 @@ async def connect(self, handler: Optional[_MongoClientErrorHandler] = None) -> A
                 driverConnectionId=conn_id,
             )
 
-        # Apply backoff if applicable.
-        if self._backoff:
-            await asyncio.sleep(_backoff(self._backoff))
-
-        # Pass a context to determine if we successfully create a configured socket.
-        context = dict(has_created_socket=False)
-
         try:
-            networking_interface = await _configured_protocol_interface(
-                self.address, self.opts, context=context
-            )
+            networking_interface = await _configured_protocol_interface(self.address, self.opts)
         # Catch KeyboardInterrupt, CancelledError, etc. and cleanup.
         except BaseException as error:
             async with self.lock:
@@ -1113,8 +1083,7 @@ async def connect(self, handler: Optional[_MongoClientErrorHandler] = None) -> A
                     reason=_verbose_connection_error_reason(ConnectionClosedReason.ERROR),
                     error=ConnectionClosedReason.ERROR,
                 )
-            if context["has_created_socket"]:
-                self._handle_connection_error(error, "handshake", conn_id)
+            self._handle_connection_error(error)
             if isinstance(error, (IOError, OSError, *SSLErrors)):
                 details = _get_timeout_details(self.opts)
                 _raise_connection_failure(self.address, error, timeout_details=details)
@@ -1126,9 +1095,11 @@ async def connect(self, handler: Optional[_MongoClientErrorHandler] = None) -> A
             self.active_contexts.discard(tmp_context)
         if tmp_context.cancelled:
             conn.cancel_context.cancel()
+        completed_hello = False
         try:
             if not self.is_sdam:
                 await conn.hello()
+                completed_hello = True
                 self.is_writable = conn.is_writable
             if handler:
                 handler.contribute_socket(conn, completed_handshake=False)
@@ -1138,15 +1109,14 @@ async def connect(self, handler: Optional[_MongoClientErrorHandler] = None) -> A
         except BaseException as e:
             async with self.lock:
                 self.active_contexts.discard(conn.cancel_context)
-            self._handle_connection_error(e, "hello", conn_id)
+            if not completed_hello:
+                self._handle_connection_error(e)
             await conn.close_conn(ConnectionClosedReason.ERROR)
             raise
 
         if handler:
             await handler.client._topology.receive_cluster_time(conn._cluster_time)
 
-        # Clear the backoff state.
-        self._backoff = 0
         return conn
 
     @contextlib.asynccontextmanager
@@ -1323,12 +1293,12 @@ async def _get_conn(
                 # to be checked back into the pool.
                 async with self._max_connecting_cond:
                     self._raise_if_not_ready(checkout_started_time, emit_event=False)
-                    while not (self.conns or self._pending < self.max_connecting):
+                    while not (self.conns or self._pending < self._max_connecting):
                         timeout = deadline - time.monotonic() if deadline else None
                         if not await _async_cond_wait(self._max_connecting_cond, timeout):
                             # Timed out, notify the next thread to ensure a
                             # timeout doesn't consume the condition.
-                            if self.conns or self._pending < self.max_connecting:
+                            if self.conns or self._pending < self._max_connecting:
                                 self._max_connecting_cond.notify()
                             emitted_event = True
                             self._raise_wait_queue_timeout(checkout_started_time)
@@ -1469,7 +1439,7 @@ async def _perished(self, conn: AsyncConnection) -> bool:
         :class:`~pymongo.errors.AutoReconnect` exceptions on server
         hiccups, etc. We only check if the socket was closed by an external
         error if it has been > 1 second since the socket was checked into the
-        pool, or we are in backoff mode, to keep performance reasonable -
+        pool to keep performance reasonable -
         we can't avoid AutoReconnects completely anyway.
         """
         idle_time_seconds = conn.idle_time_seconds()
@@ -1482,8 +1452,6 @@ async def _perished(self, conn: AsyncConnection) -> bool:
             return True
 
         check_interval_seconds = self._check_interval_seconds
-        if self._backoff:
-            check_interval_seconds = 0
         if check_interval_seconds is not None and (
             check_interval_seconds == 0 or idle_time_seconds > check_interval_seconds
         ):

pymongo/asynchronous/topology.py

@@ -890,8 +890,8 @@ async def _handle_error(self, address: _Address, err_ctx: _ErrorContext) -> None
                 # Clear the pool.
                 await server.reset(service_id)
         elif isinstance(error, ConnectionFailure):
-            if isinstance(error, WaitQueueTimeoutError) or error.has_error_label(
-                "SystemOverloadedError"
+            if isinstance(error, WaitQueueTimeoutError) or (
+                error.has_error_label("SystemOverloadedError")
             ):
                 return
             # "Client MUST replace the server's description with type Unknown

pymongo/network_layer.py

@@ -256,7 +256,6 @@ def __init__(self, timeout: Optional[float] = None):
         self._timeout = timeout
         self._closed = asyncio.get_running_loop().create_future()
         self._connection_lost = False
-        self._closing_exception = None
 
     def settimeout(self, timeout: float | None) -> None:
         self._timeout = timeout
@@ -270,11 +269,9 @@ def close(self, exc: Optional[Exception] = None) -> None:
         self.transport.abort()
         self._resolve_pending(exc)
         self._connection_lost = True
-        self._closing_exception = exc  # type:ignore[assignment]
 
     def connection_lost(self, exc: Optional[Exception] = None) -> None:
         self._resolve_pending(exc)
-        self._closing_exception = exc  # type:ignore[assignment]
         if not self._closed.done():
             self._closed.set_result(None)
 
@@ -338,11 +335,8 @@ async def read(self, request_id: Optional[int], max_message_size: int) -> tuple[
         if self._done_messages:
             message = await self._done_messages.popleft()
         else:
-            if self._closed.done():
-                if self._closing_exception:
-                    raise self._closing_exception
-                else:
-                    raise OSError("connection closed")
+            if self.transport and self.transport.is_closing():
+                raise OSError("connection is already closed")
             read_waiter = asyncio.get_running_loop().create_future()
             self._pending_messages.append(read_waiter)
             try:
@@ -480,7 +474,6 @@ def _resolve_pending(self, exc: Optional[Exception] = None) -> None:
                 else:
                     msg.set_exception(exc)
             self._done_messages.append(msg)
-        self._pending_messages.clear()
 
 
 class PyMongoKMSProtocol(PyMongoBaseProtocol):

pymongo/pool_shared.py

@@ -250,7 +250,6 @@ async def _configured_protocol_interface(
     address: _Address,
     options: PoolOptions,
     protocol_kls: type[PyMongoBaseProtocol] = PyMongoProtocol,
-    context: dict[str, bool] | None = None,
 ) -> AsyncNetworkingInterface:
     """Given (host, port) and PoolOptions, return a configured AsyncNetworkingInterface.
 
@@ -262,10 +261,6 @@ async def _configured_protocol_interface(
     ssl_context = options._ssl_context
     timeout = options.socket_timeout
 
-    # Signal that we have created the socket successfully.
-    if context:
-        context["has_created_socket"] = True
-
     if ssl_context is None:
         return AsyncNetworkingInterface(
             await asyncio.get_running_loop().create_connection(
@@ -379,7 +374,7 @@ def _create_connection(address: _Address, options: PoolOptions) -> socket.socket
 
 
 def _configured_socket_interface(
-    address: _Address, options: PoolOptions, *args: Any, context: dict[str, bool] | None = None
+    address: _Address, options: PoolOptions, *args: Any
 ) -> NetworkingInterface:
     """Given (host, port) and PoolOptions, return a NetworkingInterface wrapping a configured socket.
 
@@ -390,10 +385,6 @@ def _configured_socket_interface(
     sock = _create_connection(address, options)
     ssl_context = options._ssl_context
 
-    # Signal that we have created the socket successfully.
-    if context:
-        context["has_created_socket"] = True
-
     if ssl_context is None:
         sock.settimeout(options.socket_timeout)
         return NetworkingInterface(sock)