Add a specially handled `ContextCancelled` error

tractor/_exceptions.py

@@ -1,7 +1,7 @@
 """
 Our classy exception set.
 """
-from typing import Dict, Any
+from typing import Dict, Any, Optional
 import importlib
 import builtins
 import traceback
@@ -15,17 +15,16 @@ _this_mod = importlib.import_module(__name__)
 class RemoteActorError(Exception):
     # TODO: local recontruction of remote exception deats
     "Remote actor exception bundled locally"
-    def __init__(self, message, type_str, **msgdata) -> None:
-        super().__init__(message)
-        for ns in [builtins, _this_mod, trio]:
-            try:
-                self.type = getattr(ns, type_str)
-                break
-            except AttributeError:
-                continue
-        else:
-            self.type = Exception
+    def __init__(
+        self,
+        message: str,
+        suberror_type: Optional[Exception] = None,
+        **msgdata
 
+    ) -> None:
+        super().__init__(message)
+
+        self.type = suberror_type
         self.msgdata = msgdata
 
     # TODO: a trio.MultiError.catch like context manager
@@ -42,6 +41,10 @@ class TransportClosed(trio.ClosedResourceError):
     "Underlying channel transport was closed prior to use"
 
 
+class ContextCancelled(RemoteActorError):
+    "Inter-actor task context cancelled itself on the callee side."
+
+
 class NoResult(RuntimeError):
     "No final result is expected for this actor"
 
@@ -77,12 +80,35 @@ def unpack_error(
     into a local ``RemoteActorError``.
 
     """
-    tb_str = msg['error'].get('tb_str', '')
-    return err_type(
-        f"{chan.uid}\n" + tb_str,
+    error = msg['error']
+
+    tb_str = error.get('tb_str', '')
+    message = f"{chan.uid}\n" + tb_str
+    type_name = error['type_str']
+    suberror_type = Exception
+
+    if type_name == 'ContextCancelled':
+        err_type = ContextCancelled
+        suberror_type = trio.Cancelled
+
+    else:  # try to lookup a suitable local error type
+        for ns in [builtins, _this_mod, trio]:
+            try:
+                suberror_type = getattr(ns, type_name)
+                break
+            except AttributeError:
+                continue
+
+    exc = err_type(
+        message,
+        suberror_type=suberror_type,
+
+        # unpack other fields into error type init
         **msg['error'],
     )
 
+    return exc
+
 
 def is_multi_cancelled(exc: BaseException) -> bool:
     """Predicate to determine if a ``trio.MultiError`` contains only