PYTHON-4292 Improve TLS read performance (#2020)

Author: ShaneHarvey

pymongo/network_layer.py

@@ -28,7 +28,7 @@
     Union,
 )
 
-from pymongo import _csot, ssl_support
+from pymongo import ssl_support
 from pymongo._asyncio_task import create_task
 from pymongo.errors import _OperationCancelled
 from pymongo.socket_checker import _errno_from_exception
@@ -316,62 +316,47 @@ async def _async_receive(conn: socket.socket, length: int, loop: AbstractEventLo
     return mv
 
 
-# Sync version:
-def wait_for_read(conn: Connection, deadline: Optional[float]) -> None:
-    """Block until at least one byte is read, or a timeout, or a cancel."""
-    sock = conn.conn
-    timed_out = False
-    # Check if the connection's socket has been manually closed
-    if sock.fileno() == -1:
-        return
-    while True:
-        # SSLSocket can have buffered data which won't be caught by select.
-        if hasattr(sock, "pending") and sock.pending() > 0:
-            readable = True
-        else:
-            # Wait up to 500ms for the socket to become readable and then
-            # check for cancellation.
-            if deadline:
-                remaining = deadline - time.monotonic()
-                # When the timeout has expired perform one final check to
-                # see if the socket is readable. This helps avoid spurious
-                # timeouts on AWS Lambda and other FaaS environments.
-                if remaining <= 0:
-                    timed_out = True
-                timeout = max(min(remaining, _POLL_TIMEOUT), 0)
-            else:
-                timeout = _POLL_TIMEOUT
-            readable = conn.socket_checker.select(sock, read=True, timeout=timeout)
-        if conn.cancel_context.cancelled:
-            raise _OperationCancelled("operation cancelled")
-        if readable:
-            return
-        if timed_out:
-            raise socket.timeout("timed out")
-
-
 def receive_data(conn: Connection, length: int, deadline: Optional[float]) -> memoryview:
     buf = bytearray(length)
     mv = memoryview(buf)
     bytes_read = 0
-    while bytes_read < length:
-        try:
-            wait_for_read(conn, deadline)
-            # CSOT: Update timeout. When the timeout has expired perform one
-            # final non-blocking recv. This helps avoid spurious timeouts when
-            # the response is actually already buffered on the client.
-            if _csot.get_timeout() and deadline is not None:
-                conn.set_conn_timeout(max(deadline - time.monotonic(), 0))
-            chunk_length = conn.conn.recv_into(mv[bytes_read:])
-        except BLOCKING_IO_ERRORS:
-            raise socket.timeout("timed out") from None
-        except OSError as exc:
-            if _errno_from_exception(exc) == errno.EINTR:
+    # To support cancelling a network read, we shorten the socket timeout and
+    # check for the cancellation signal after each timeout. Alternatively we
+    # could close the socket but that does not reliably cancel recv() calls
+    # on all OSes.
+    orig_timeout = conn.conn.gettimeout()
+    try:
+        while bytes_read < length:
+            if deadline is not None:
+                # CSOT: Update timeout. When the timeout has expired perform one
+                # final non-blocking recv. This helps avoid spurious timeouts when
+                # the response is actually already buffered on the client.
+                short_timeout = min(max(deadline - time.monotonic(), 0), _POLL_TIMEOUT)
+            else:
+                short_timeout = _POLL_TIMEOUT
+            conn.set_conn_timeout(short_timeout)
+            try:
+                chunk_length = conn.conn.recv_into(mv[bytes_read:])
+            except BLOCKING_IO_ERRORS:
+                if conn.cancel_context.cancelled:
+                    raise _OperationCancelled("operation cancelled") from None
+                # We reached the true deadline.
+                raise socket.timeout("timed out") from None
+            except socket.timeout:
+                if conn.cancel_context.cancelled:
+                    raise _OperationCancelled("operation cancelled") from None
                 continue
-            raise
-        if chunk_length == 0:
-            raise OSError("connection closed")
-
-        bytes_read += chunk_length
+            except OSError as exc:
+                if conn.cancel_context.cancelled:
+                    raise _OperationCancelled("operation cancelled") from None
+                if _errno_from_exception(exc) == errno.EINTR:
+                    continue
+                raise
+            if chunk_length == 0:
+                raise OSError("connection closed")
+
+            bytes_read += chunk_length
+    finally:
+        conn.set_conn_timeout(orig_timeout)
 
     return mv