bpo-32528: Make asyncio.CancelledError a BaseException. (GH-13528)

This will address the common mistake many asyncio users make:
an "except Exception" clause breaking Tasks cancellation.

In addition to this change, we stop inheriting asyncio.TimeoutError
and asyncio.InvalidStateError from their concurrent.futures.*
counterparts.  There's no point for these exceptions to share the
inheritance chain.

In 3.9 we'll focus on implementing supervisors and cancel scopes,
which should allow better handling of all exceptions, including
SystemExit and KeyboardInterrupt

12 files changed, 130 insertions, 53 deletions

diff --git a/Lib/asyncio/base_events.py b/Lib/asyncio/base_events.py
index de9fa4f..63b072b 100644
--- a/Lib/asyncio/base_events.py
+++ b/Lib/asyncio/base_events.py
@@ -186,7 +186,7 @@ def _interleave_addrinfos(addrinfos, first_address_family_count=1):
 def _run_until_complete_cb(fut):
     if not fut.cancelled():
         exc = fut.exception()
-        if isinstance(exc, BaseException) and not isinstance(exc, Exception):
+        if isinstance(exc, (SystemExit, KeyboardInterrupt)):
             # Issue #22429: run_forever() already finished, no need to
             # stop it.
             return
@@ -1196,7 +1196,7 @@ class BaseEventLoop(events.AbstractEventLoop):
 
         try:
             await waiter
-        except Exception:
+        except BaseException:
             transport.close()
             conmade_cb.cancel()
             resume_cb.cancel()
@@ -1710,7 +1710,9 @@ class BaseEventLoop(events.AbstractEventLoop):
         if self._exception_handler is None:
             try:
                 self.default_exception_handler(context)
-            except Exception:
+            except (SystemExit, KeyboardInterrupt):
+                raise
+            except BaseException:
                 # Second protection layer for unexpected errors
                 # in the default implementation, as well as for subclassed
                 # event loops with overloaded "default_exception_handler".
@@ -1719,7 +1721,9 @@ class BaseEventLoop(events.AbstractEventLoop):
         else:
             try:
                 self._exception_handler(self, context)
-            except Exception as exc:
+            except (SystemExit, KeyboardInterrupt):
+                raise
+            except BaseException as exc:
                 # Exception in the user set custom exception handler.
                 try:
                     # Let's try default handler.
@@ -1728,7 +1732,9 @@ class BaseEventLoop(events.AbstractEventLoop):
                         'exception': exc,
                         'context': context,
                     })
-                except Exception:
+                except (SystemExit, KeyboardInterrupt):
+                    raise
+                except BaseException:
                     # Guard 'default_exception_handler' in case it is
                     # overloaded.
                     logger.error('Exception in default exception handler '
diff --git a/Lib/asyncio/base_subprocess.py b/Lib/asyncio/base_subprocess.py
index f503f78..14d5051 100644
--- a/Lib/asyncio/base_subprocess.py
+++ b/Lib/asyncio/base_subprocess.py
@@ -182,7 +182,9 @@ class BaseSubprocessTransport(transports.SubprocessTransport):
             for callback, data in self._pending_calls:
                 loop.call_soon(callback, *data)
             self._pending_calls = None
-        except Exception as exc:
+        except (SystemExit, KeyboardInterrupt):
+            raise
+        except BaseException as exc:
             if waiter is not None and not waiter.cancelled():
                 waiter.set_exception(exc)
         else:
diff --git a/Lib/asyncio/events.py b/Lib/asyncio/events.py
index 9a92351..d381b1c 100644
--- a/Lib/asyncio/events.py
+++ b/Lib/asyncio/events.py
@@ -79,7 +79,9 @@ class Handle:
     def _run(self):
         try:
             self._context.run(self._callback, *self._args)
-        except Exception as exc:
+        except (SystemExit, KeyboardInterrupt):
+            raise
+        except BaseException as exc:
             cb = format_helpers._format_callback_source(
                 self._callback, self._args)
             msg = f'Exception in callback {cb}'
diff --git a/Lib/asyncio/exceptions.py b/Lib/asyncio/exceptions.py
index cac31a5..e03602e 100644
--- a/Lib/asyncio/exceptions.py
+++ b/Lib/asyncio/exceptions.py
@@ -5,19 +5,16 @@ __all__ = ('CancelledError', 'InvalidStateError', 'TimeoutError',
            'IncompleteReadError', 'LimitOverrunError',
            'SendfileNotAvailableError')
 
-import concurrent.futures
-from . import base_futures
 
-
-class CancelledError(concurrent.futures.CancelledError):
+class CancelledError(BaseException):
     """The Future or Task was cancelled."""
 
 
-class TimeoutError(concurrent.futures.TimeoutError):
+class TimeoutError(Exception):
     """The operation exceeded the given deadline."""
 
 
-class InvalidStateError(concurrent.futures.InvalidStateError):
+class InvalidStateError(Exception):
     """The operation is not allowed in this state."""
 
 
diff --git a/Lib/asyncio/proactor_events.py b/Lib/asyncio/proactor_events.py
index a849be1..7dfe295 100644
--- a/Lib/asyncio/proactor_events.py
+++ b/Lib/asyncio/proactor_events.py
@@ -212,7 +212,9 @@ class _ProactorReadPipeTransport(_ProactorBasePipeTransport,
 
         try:
             keep_open = self._protocol.eof_received()
-        except Exception as exc:
+        except (SystemExit, KeyboardInterrupt):
+            raise
+        except BaseException as exc:
             self._fatal_error(
                 exc, 'Fatal error: protocol.eof_received() call failed.')
             return
@@ -235,7 +237,9 @@ class _ProactorReadPipeTransport(_ProactorBasePipeTransport,
         if isinstance(self._protocol, protocols.BufferedProtocol):
             try:
                 protocols._feed_data_to_buffered_proto(self._protocol, data)
-            except Exception as exc:
+            except (SystemExit, KeyboardInterrupt):
+                raise
+            except BaseException as exc:
                 self._fatal_error(exc,
                                   'Fatal error: protocol.buffer_updated() '
                                   'call failed.')
@@ -625,7 +629,9 @@ class BaseProactorEventLoop(base_events.BaseEventLoop):
         except exceptions.CancelledError:
             # _close_self_pipe() has been called, stop waiting for data
             return
-        except Exception as exc:
+        except (SystemExit, KeyboardInterrupt):
+            raise
+        except BaseException as exc:
             self.call_exception_handler({
                 'message': 'Error on reading from the event loop self pipe',
                 'exception': exc,
diff --git a/Lib/asyncio/selector_events.py b/Lib/asyncio/selector_events.py
index 6461d30..f5f43a9 100644
--- a/Lib/asyncio/selector_events.py
+++ b/Lib/asyncio/selector_events.py
@@ -208,12 +208,14 @@ class BaseSelectorEventLoop(base_events.BaseEventLoop):
 
             try:
                 await waiter
-            except:
+            except BaseException:
                 transport.close()
                 raise
+                # It's now up to the protocol to handle the connection.
 
-            # It's now up to the protocol to handle the connection.
-        except Exception as exc:
+        except (SystemExit, KeyboardInterrupt):
+            raise
+        except BaseException as exc:
             if self._debug:
                 context = {
                     'message':
@@ -370,7 +372,9 @@ class BaseSelectorEventLoop(base_events.BaseEventLoop):
             data = sock.recv(n)
         except (BlockingIOError, InterruptedError):
             return  # try again next time
-        except Exception as exc:
+        except (SystemExit, KeyboardInterrupt):
+            raise
+        except BaseException as exc:
             fut.set_exception(exc)
         else:
             fut.set_result(data)
@@ -404,7 +408,9 @@ class BaseSelectorEventLoop(base_events.BaseEventLoop):
             nbytes = sock.recv_into(buf)
         except (BlockingIOError, InterruptedError):
             return  # try again next time
-        except Exception as exc:
+        except (SystemExit, KeyboardInterrupt):
+            raise
+        except BaseException as exc:
             fut.set_exception(exc)
         else:
             fut.set_result(nbytes)
@@ -447,7 +453,9 @@ class BaseSelectorEventLoop(base_events.BaseEventLoop):
             n = sock.send(view[start:])
         except (BlockingIOError, InterruptedError):
             return
-        except Exception as exc:
+        except (SystemExit, KeyboardInterrupt):
+            raise
+        except BaseException as exc:
             fut.set_exception(exc)
             return
 
@@ -487,7 +495,9 @@ class BaseSelectorEventLoop(base_events.BaseEventLoop):
             fut.add_done_callback(
                 functools.partial(self._sock_write_done, fd))
             self.add_writer(fd, self._sock_connect_cb, fut, sock, address)
-        except Exception as exc:
+        except (SystemExit, KeyboardInterrupt):
+            raise
+        except BaseException as exc:
             fut.set_exception(exc)
         else:
             fut.set_result(None)
@@ -507,7 +517,9 @@ class BaseSelectorEventLoop(base_events.BaseEventLoop):
         except (BlockingIOError, InterruptedError):
             # socket is still registered, the callback will be retried later
             pass
-        except Exception as exc:
+        except (SystemExit, KeyboardInterrupt):
+            raise
+        except BaseException as exc:
             fut.set_exception(exc)
         else:
             fut.set_result(None)
@@ -537,7 +549,9 @@ class BaseSelectorEventLoop(base_events.BaseEventLoop):
             conn.setblocking(False)
         except (BlockingIOError, InterruptedError):
             self.add_reader(fd, self._sock_accept, fut, True, sock)
-        except Exception as exc:
+        except (SystemExit, KeyboardInterrupt):
+            raise
+        except BaseException as exc:
             fut.set_exception(exc)
         else:
             fut.set_result((conn, address))
@@ -785,7 +799,9 @@ class _SelectorSocketTransport(_SelectorTransport):
             buf = self._protocol.get_buffer(-1)
             if not len(buf):
                 raise RuntimeError('get_buffer() returned an empty buffer')
-        except Exception as exc:
+        except (SystemExit, KeyboardInterrupt):
+            raise
+        except BaseException as exc:
             self._fatal_error(
                 exc, 'Fatal error: protocol.get_buffer() call failed.')
             return
@@ -794,7 +810,9 @@ class _SelectorSocketTransport(_SelectorTransport):
             nbytes = self._sock.recv_into(buf)
         except (BlockingIOError, InterruptedError):
             return
-        except Exception as exc:
+        except (SystemExit, KeyboardInterrupt):
+            raise
+        except BaseException as exc:
             self._fatal_error(exc, 'Fatal read error on socket transport')
             return
 
@@ -804,7 +822,9 @@ class _SelectorSocketTransport(_SelectorTransport):
 
         try:
             self._protocol.buffer_updated(nbytes)
-        except Exception as exc:
+        except (SystemExit, KeyboardInterrupt):
+            raise
+        except BaseException as exc:
             self._fatal_error(
                 exc, 'Fatal error: protocol.buffer_updated() call failed.')
 
@@ -815,7 +835,9 @@ class _SelectorSocketTransport(_SelectorTransport):
             data = self._sock.recv(self.max_size)
         except (BlockingIOError, InterruptedError):
             return
-        except Exception as exc:
+        except (SystemExit, KeyboardInterrupt):
+            raise
+        except BaseException as exc:
             self._fatal_error(exc, 'Fatal read error on socket transport')
             return
 
@@ -825,7 +847,9 @@ class _SelectorSocketTransport(_SelectorTransport):
 
         try:
             self._protocol.data_received(data)
-        except Exception as exc:
+        except (SystemExit, KeyboardInterrupt):
+            raise
+        except BaseException as exc:
             self._fatal_error(
                 exc, 'Fatal error: protocol.data_received() call failed.')
 
@@ -835,7 +859,9 @@ class _SelectorSocketTransport(_SelectorTransport):
 
         try:
             keep_open = self._protocol.eof_received()
-        except Exception as exc:
+        except (SystemExit, KeyboardInterrupt):
+            raise
+        except BaseException as exc:
             self._fatal_error(
                 exc, 'Fatal error: protocol.eof_received() call failed.')
             return
@@ -871,7 +897,9 @@ class _SelectorSocketTransport(_SelectorTransport):
                 n = self._sock.send(data)
             except (BlockingIOError, InterruptedError):
                 pass
-            except Exception as exc:
+            except (SystemExit, KeyboardInterrupt):
+                raise
+            except BaseException as exc:
                 self._fatal_error(exc, 'Fatal write error on socket transport')
                 return
             else:
@@ -894,7 +922,9 @@ class _SelectorSocketTransport(_SelectorTransport):
             n = self._sock.send(self._buffer)
         except (BlockingIOError, InterruptedError):
             pass
-        except Exception as exc:
+        except (SystemExit, KeyboardInterrupt):
+            raise
+        except BaseException as exc:
             self._loop._remove_writer(self._sock_fd)
             self._buffer.clear()
             self._fatal_error(exc, 'Fatal write error on socket transport')
@@ -970,7 +1000,9 @@ class _SelectorDatagramTransport(_SelectorTransport):
             pass
         except OSError as exc:
             self._protocol.error_received(exc)
-        except Exception as exc:
+        except (SystemExit, KeyboardInterrupt):
+            raise
+        except BaseException as exc:
             self._fatal_error(exc, 'Fatal read error on datagram transport')
         else:
             self._protocol.datagram_received(data, addr)
@@ -1007,7 +1039,9 @@ class _SelectorDatagramTransport(_SelectorTransport):
             except OSError as exc:
                 self._protocol.error_received(exc)
                 return
-            except Exception as exc:
+            except (SystemExit, KeyboardInterrupt):
+                raise
+            except BaseException as exc:
                 self._fatal_error(
                     exc, 'Fatal write error on datagram transport')
                 return
@@ -1030,7 +1064,9 @@ class _SelectorDatagramTransport(_SelectorTransport):
             except OSError as exc:
                 self._protocol.error_received(exc)
                 return
-            except Exception as exc:
+            except (SystemExit, KeyboardInterrupt):
+                raise
+            except BaseException as exc:
                 self._fatal_error(
                     exc, 'Fatal write error on datagram transport')
                 return
diff --git a/Lib/asyncio/sslproto.py b/Lib/asyncio/sslproto.py
index 97a6fc6..8546985 100644
--- a/Lib/asyncio/sslproto.py
+++ b/Lib/asyncio/sslproto.py
@@ -527,7 +527,9 @@ class SSLProtocol(protocols.Protocol):
 
         try:
             ssldata, appdata = self._sslpipe.feed_ssldata(data)
-        except Exception as e:
+        except (SystemExit, KeyboardInterrupt):
+            raise
+        except BaseException as e:
             self._fatal_error(e, 'SSL error in data received')
             return
 
@@ -542,7 +544,9 @@ class SSLProtocol(protocols.Protocol):
                             self._app_protocol, chunk)
                     else:
                         self._app_protocol.data_received(chunk)
-                except Exception as ex:
+                except (SystemExit, KeyboardInterrupt):
+                    raise
+                except BaseException as ex:
                     self._fatal_error(
                         ex, 'application protocol failed to receive SSL data')
                     return
@@ -628,7 +632,9 @@ class SSLProtocol(protocols.Protocol):
                 raise handshake_exc
 
             peercert = sslobj.getpeercert()
-        except Exception as exc:
+        except (SystemExit, KeyboardInterrupt):
+            raise
+        except BaseException as exc:
             if isinstance(exc, ssl.CertificateError):
                 msg = 'SSL handshake failed on verifying the certificate'
             else:
@@ -691,7 +697,9 @@ class SSLProtocol(protocols.Protocol):
                 # delete it and reduce the outstanding buffer size.
                 del self._write_backlog[0]
                 self._write_buffer_size -= len(data)
-        except Exception as exc:
+        except (SystemExit, KeyboardInterrupt):
+            raise
+        except BaseException as exc:
             if self._in_handshake:
                 # Exceptions will be re-raised in _on_handshake_complete.
                 self._on_handshake_complete(exc)
diff --git a/Lib/asyncio/staggered.py b/Lib/asyncio/staggered.py
index feec681..27c665a 100644
--- a/Lib/asyncio/staggered.py
+++ b/Lib/asyncio/staggered.py
@@ -105,7 +105,9 @@ async def staggered_race(
 
         try:
             result = await coro_fn()
-        except Exception as e:
+        except (SystemExit, KeyboardInterrupt):
+            raise
+        except BaseException as e:
             exceptions[this_index] = e
             this_failed.set()  # Kickstart the next coroutine
         else:
diff --git a/Lib/asyncio/tasks.py b/Lib/asyncio/tasks.py
index 1dc5952..78e7600 100644
--- a/Lib/asyncio/tasks.py
+++ b/Lib/asyncio/tasks.py
@@ -260,11 +260,11 @@ class Task(futures._PyFuture):  # Inherit Python Task implementation
                 super().set_result(exc.value)
         except exceptions.CancelledError:
             super().cancel()  # I.e., Future.cancel(self).
-        except Exception as exc:
+        except (KeyboardInterrupt, SystemExit) as exc:
             super().set_exception(exc)
+            raise
         except BaseException as exc:
             super().set_exception(exc)
-            raise
         else:
             blocking = getattr(result, '_asyncio_future_blocking', None)
             if blocking is not None:
@@ -318,7 +318,7 @@ class Task(futures._PyFuture):  # Inherit Python Task implementation
     def __wakeup(self, future):
         try:
             future.result()
-        except Exception as exc:
+        except BaseException as exc:
             # This may also be a cancellation.
             self.__step(exc)
         else:
@@ -858,7 +858,9 @@ def run_coroutine_threadsafe(coro, loop):
     def callback():
         try:
             futures._chain_future(ensure_future(coro, loop=loop), future)
-        except Exception as exc:
+        except (SystemExit, KeyboardInterrupt):
+            raise
+        except BaseException as exc:
             if future.set_running_or_notify_cancel():
                 future.set_exception(exc)
             raise
diff --git a/Lib/asyncio/transports.py b/Lib/asyncio/transports.py
index 233bbb5..47b37fa 100644
--- a/Lib/asyncio/transports.py
+++ b/Lib/asyncio/transports.py
@@ -262,7 +262,9 @@ class _FlowControlMixin(Transport):
             self._protocol_paused = True
             try:
                 self._protocol.pause_writing()
-            except Exception as exc:
+            except (SystemExit, KeyboardInterrupt):
+                raise
+            except BaseException as exc:
                 self._loop.call_exception_handler({
                     'message': 'protocol.pause_writing() failed',
                     'exception': exc,
@@ -276,7 +278,9 @@ class _FlowControlMixin(Transport):
             self._protocol_paused = False
             try:
                 self._protocol.resume_writing()
-            except Exception as exc:
+            except (SystemExit, KeyboardInterrupt):
+                raise
+            except BaseException as exc:
                 self._loop.call_exception_handler({
                     'message': 'protocol.resume_writing() failed',
                     'exception': exc,
diff --git a/Lib/asyncio/unix_events.py b/Lib/asyncio/unix_events.py
index 1aa3b39..81d10b1 100644
--- a/Lib/asyncio/unix_events.py
+++ b/Lib/asyncio/unix_events.py
@@ -194,7 +194,9 @@ class _UnixSelectorEventLoop(selector_events.BaseSelectorEventLoop):
                                       self._child_watcher_callback, transp)
             try:
                 await waiter
-            except Exception:
+            except (SystemExit, KeyboardInterrupt):
+                raise
+            except BaseException:
                 transp.close()
                 await transp._wait()
                 raise
@@ -390,7 +392,9 @@ class _UnixSelectorEventLoop(selector_events.BaseSelectorEventLoop):
             else:
                 self._sock_sendfile_update_filepos(fileno, offset, total_sent)
                 fut.set_exception(exc)
-        except Exception as exc:
+        except (SystemExit, KeyboardInterrupt):
+            raise
+        except BaseException as exc:
             self._sock_sendfile_update_filepos(fileno, offset, total_sent)
             fut.set_exception(exc)
         else:
@@ -641,7 +645,9 @@ class _UnixWritePipeTransport(transports._FlowControlMixin,
                 n = os.write(self._fileno, data)
             except (BlockingIOError, InterruptedError):
                 n = 0
-            except Exception as exc:
+            except (SystemExit, KeyboardInterrupt):
+                raise
+            except BaseException as exc:
                 self._conn_lost += 1
                 self._fatal_error(exc, 'Fatal write error on pipe transport')
                 return
@@ -661,7 +667,9 @@ class _UnixWritePipeTransport(transports._FlowControlMixin,
             n = os.write(self._fileno, self._buffer)
         except (BlockingIOError, InterruptedError):
             pass
-        except Exception as exc:
+        except (SystemExit, KeyboardInterrupt):
+            raise
+        except BaseException as exc:
             self._buffer.clear()
             self._conn_lost += 1
             # Remove writer here, _fatal_error() doesn't it
@@ -879,7 +887,9 @@ class BaseChildWatcher(AbstractChildWatcher):
     def _sig_chld(self):
         try:
             self._do_waitpid_all()
-        except Exception as exc:
+        except (SystemExit, KeyboardInterrupt):
+            raise
+        except BaseException as exc:
             # self._loop should always be available here
             # as '_sig_chld' is added as a signal handler
             # in 'attach_loop'
diff --git a/Lib/asyncio/windows_events.py b/Lib/asyncio/windows_events.py
index 29750f1..b5b2e24 100644
--- a/Lib/asyncio/windows_events.py
+++ b/Lib/asyncio/windows_events.py
@@ -388,7 +388,9 @@ class ProactorEventLoop(proactor_events.BaseProactorEventLoop):
                                              **kwargs)
         try:
             await waiter
-        except Exception:
+        except (SystemExit, KeyboardInterrupt):
+            raise
+        except BaseException:
             transp.close()
             await transp._wait()
             raise