Revert usage of asyncio cancellation mechanism (#142)

* Revert usage of asyncio cancellation mechanism (#132)
* Improvements to exception, expose SdkInternalBaseException and is_internal_exception

Author: slinkydeveloper

python/restate/__init__.py

@@ -22,7 +22,7 @@
 from .retry_policy import InvocationRetryPolicy
 # pylint: disable=line-too-long
 from .context import DurablePromise, RestateDurableFuture, RestateDurableCallFuture, RestateDurableSleepFuture, SendHandle, RunOptions
-from .exceptions import TerminalError
+from .exceptions import TerminalError, SdkInternalBaseException, is_internal_exception
 from .asyncio import as_completed, gather, wait_completed, select
 
 from .endpoint import app
@@ -67,5 +67,7 @@ def test_harness(app, # type: ignore
     "select",
     "logging",
     "RestateLoggingFilter",
-    "InvocationRetryPolicy"
+    "InvocationRetryPolicy",
+    "SdkInternalBaseException",
+    "is_internal_exception"
 ]

python/restate/exceptions.py

@@ -10,10 +10,53 @@
 #
 """This module contains the restate exceptions"""
 
-class TerminalError(Exception):
-    """This exception is raised to indicate a termination of the execution"""
+# pylint: disable=C0301
 
+class TerminalError(Exception):
+    """This exception is thrown to indicate that Restate should not retry."""
     def __init__(self, message: str, status_code: int = 500) -> None:
         super().__init__(message)
         self.message = message
         self.status_code = status_code
+
+
+class SdkInternalBaseException(Exception):
+    """This exception is internal, and you should not catch it.
+    If you need to distinguish with other exceptions, use is_internal_exception."""
+    def __init__(self, message: str) -> None:
+        super().__init__(
+            message +
+"""
+This exception is safe to ignore. If you see it, you might be using a try/catch all statement.
+
+Don't do:
+try:
+  # Code
+except:
+  # This catches all exceptions, including the SdkInternalBaseException!
+
+Do instead:
+try:
+  # Code
+except TerminalError:
+  # In Restate handlers you typically want to catch TerminalError only
+
+Or remove the try/except altogether if you don't need it.
+For further info on error handling, refer to https://docs.restate.dev/develop/python/error-handling
+""")
+
+class SuspendedException(SdkInternalBaseException):
+    """This exception is raised to indicate that the execution is suspended"""
+    def __init__(self) -> None:
+        super().__init__("Invocation got suspended, Restate will resume this invocation when progress can be made.")
+
+class SdkInternalException(SdkInternalBaseException):
+    """This exception is raised to indicate that the execution raised a retryable error."""
+    def __init__(self) -> None:
+        super().__init__("Invocation attempt raised a retryable error.\n"
+                         "Restate will retry executing this invocation from the point where it left off.")
+
+
+def is_internal_exception(e) -> bool:
+    """Returns true if the exception is an internal Restate exception"""
+    return isinstance(e, SdkInternalBaseException)

python/restate/server_context.py

@@ -30,7 +30,7 @@
 import time
 
 from restate.context import DurablePromise, AttemptFinishedEvent, HandlerType, ObjectContext, Request, RestateDurableCallFuture, RestateDurableFuture, RunAction, SendHandle, RestateDurableSleepFuture, RunOptions, P
-from restate.exceptions import TerminalError
+from restate.exceptions import TerminalError, SdkInternalBaseException, SdkInternalException, SuspendedException
 from restate.handler import Handler, handler_from_callable, invoke_handler
 from restate.serde import BytesSerde, DefaultSerde, JsonSerde, Serde
 from restate.server_types import ReceiveChannel, Send
@@ -273,19 +273,6 @@ def update_restate_context_is_replaying(vm: VMWrapper):
     """Update the context var 'restate_context_is_replaying'. This should be called after each vm.sys_*"""
     restate_context_is_replaying.set(vm.is_replaying())
 
-async def cancel_current_task():
-    """Cancel the current task"""
-    current_task = asyncio.current_task()
-    if current_task is not None:
-        # Cancel through asyncio API
-        current_task.cancel(
-            "Cancelled by Restate SDK, you should not call any Context method after this exception is thrown."
-        )
-        # Sleep 0 will pop up the cancellation
-        await asyncio.sleep(0)
-    else:
-        raise asyncio.CancelledError("Cancelled by Restate SDK, you should not call any Context method after this exception is thrown.")
-
 # pylint: disable=R0902
 class ServerInvocationContext(ObjectContext):
     """This class implements the context for the restate framework based on the server."""
@@ -327,6 +314,8 @@ async def enter(self):
         # pylint: disable=W0718
         except asyncio.CancelledError:
             pass
+        except SdkInternalBaseException:
+            pass
         except DisconnectedException:
             raise
         except Exception as e:
@@ -393,11 +382,11 @@ async def must_take_notification(self, handle):
             await self.take_and_send_output()
             # Print this exception, might be relevant for the user
             traceback.print_exception(res)
-            await cancel_current_task()
+            raise SdkInternalException() from res
         if isinstance(res, Suspended):
             # We might need to write out something at this point.
             await self.take_and_send_output()
-            await cancel_current_task()
+            raise SuspendedException()
         if isinstance(res, NotReady):
             raise ValueError(f"Unexpected value error: {handle}")
         if res is None:
@@ -414,9 +403,9 @@ async def create_poll_or_cancel_coroutine(self, handles: typing.List[int]) -> No
             if isinstance(do_progress_response, BaseException):
                 # Print this exception, might be relevant for the user
                 traceback.print_exception(do_progress_response)
-                await cancel_current_task()
+                raise SdkInternalException() from do_progress_response
             if isinstance(do_progress_response, Suspended):
-                await cancel_current_task()
+                raise SuspendedException()
             if isinstance(do_progress_response, DoProgressAnyCompleted):
                 # One of the handles completed
                 return
@@ -565,6 +554,8 @@ async def create_run_coroutine(self,
             self.vm.propose_run_completion_failure(handle, failure)
         except asyncio.CancelledError as e:
             raise e from None
+        except SdkInternalBaseException as e:
+            raise e from None
         # pylint: disable=W0718
         except Exception as e:
             end = time.time()

src/lib.rs

@@ -784,23 +784,22 @@ impl ErrorFormatter for PythonErrorFormatter {
     fn display_closed_error(&self, f: &mut fmt::Formatter<'_>, event: &str) -> fmt::Result {
         write!(f, "Execution is suspended, but the handler is still attempting to make progress (calling '{event}'). This can happen:
 
-* If you don't need to handle task cancellation, just avoid catch all statements. Don't do:
+* If you use try/catch all statements.
+Don't do:
 try:
   # Code
 except:
-  # This catches all exceptions, including the asyncio.CancelledError!
+  # This catches all exceptions, including the SdkInternalBaseException!
   # '{event}' <- This operation prints this exception
 
 Do instead:
 try:
   # Code
-except TerminalException:
-  # In Restate handlers you typically want to catch TerminalException only
+except TerminalError:
+  # In Restate handlers you typically want to catch TerminalError only
 
-* To catch ctx.run/ctx.run_typed errors, check https://docs.restate.dev/develop/python/durable-steps#run for more details.
-
-* If the asyncio.CancelledError is caught, you must not run any Context operation in the except arm.
-  Check https://docs.python.org/3/library/asyncio-task.html#task-cancellation for more details on task cancellation.
+Or remove the try/except altogether if you don't need it.
+For further info on error handling, refer to https://docs.restate.dev/develop/python/error-handling
 
 * If you use the context after the handler completed, e.g. moving the context to another thread.
   Check https://docs.restate.dev/develop/python/concurrent-tasks for more details on how to create durable concurrent tasks in Python.")