Solve first-recv-cancelled by recursive `.receive()` on wake

tractor/_broadcast.py

@@ -202,13 +202,20 @@ class BroadcastReceiver(ReceiveChannel):
                     state.subs[sub_key] += 1
 
                 # NOTE: this should ONLY be set if the above task was *NOT*
-                # cancelled on the `._recv()` call otherwise sibling
-                # consumers will be awoken with a sequence of -1
+                # cancelled on the `._recv()` call.
                 event.set()
-
                 return value
 
+            except trio.Cancelled:
+                # handle cancelled specially otherwise sibling
+                # consumers will be awoken with a sequence of -1
+                # state.recv_ready = trio.Cancelled
+                if event.statistics().tasks_waiting:
+                    event.set()
+                raise
+
             finally:
+
                 # Reset receiver waiter task event for next blocking condition.
                 # this MUST be reset even if the above ``.recv()`` call
                 # was cancelled to avoid the next consumer from blocking on
@@ -223,29 +230,29 @@ class BroadcastReceiver(ReceiveChannel):
             _, ev = state.recv_ready
             await ev.wait()
 
-            seq = state.subs[key]
-            assert seq > -1, f'Invalid sequence {seq}!?'
-
-            value = state.queue[seq]
-            state.subs[key] -= 1
-            return value
-
             # NOTE: if we ever would like the behaviour where if the
             # first task to recv on the underlying is cancelled but it
             # still DOES trigger the ``.recv_ready``, event we'll likely need
             # this logic:
 
-            # if seq > -1:
-            #     # stuff from above..
-            # elif seq == -1:
-            #     # XXX: In the case where the first task to allocate the
-            #     # ``.recv_ready`` event is cancelled we will be woken with
-            #     # a non-incremented sequence number and thus will read the
-            #     # oldest value if we use that. Instead we need to detect if
-            #     # we have not been incremented and then receive again.
-            #     return await self.receive()
-            # else:
-            #     raise ValueError(f'Invalid sequence {seq}!?')
+            if seq > -1:
+                # stuff from above..
+                seq = state.subs[key]
+
+                value = state.queue[seq]
+                state.subs[key] -= 1
+                return value
+
+            elif seq == -1:
+                # XXX: In the case where the first task to allocate the
+                # ``.recv_ready`` event is cancelled we will be woken with
+                # a non-incremented sequence number and thus will read the
+                # oldest value if we use that. Instead we need to detect if
+                # we have not been incremented and then receive again.
+                return await self.receive()
+
+            else:
+                raise ValueError(f'Invalid sequence {seq}!?')
 
     @asynccontextmanager
     async def subscribe(