Add a @pub kwarg to allow specifying a "startup response message"

The std lib's `pdb` internals override SIGINT handling whenever one
enters the debugger repl. Force a handler that kills the tree if SIGINT
is triggered from the root actor, otherwise igore it since supervised
children should be managed already. This resolves an issue with guest
mode where `pdb` causes SIGINTs to be swallowed resulting in the host
loop never terminating the process tree.

tractor/_debug.py

@@ -248,6 +248,7 @@ def _breakpoint(debug_func) -> Awaitable[None]:
             # may have the tty locked prior
             if _debug_lock.locked():  # root process already has it; ignore
                 return
+
             await _debug_lock.acquire()
             _pdb_release_hook = _debug_lock.release
 

tractor/msg.py

@@ -97,6 +97,7 @@ def pub(
     wrapped: typing.Callable = None,
     *,
     tasks: Set[str] = set(),
+    send_on_connect: Any = None,
 ):
     """Publisher async generator decorator.
 
@@ -182,7 +183,7 @@ def pub(
 
     # handle the decorator not called with () case
     if wrapped is None:
-        return partial(pub, tasks=tasks)
+        return partial(pub, tasks=tasks, send_on_connect=send_on_connect)
 
     task2lock: Dict[str, trio.StrictFIFOLock] = {}
 
@@ -225,6 +226,11 @@ def pub(
 
             try:
                 modify_subs(topics2ctxs, topics, ctx)
+
+                # if specified send the startup message back to consumer
+                if send_on_connect is not None:
+                    await ctx.send_yield(send_on_connect)
+
                 # block and let existing feed task deliver
                 # stream data until it is cancelled in which case
                 # the next waiting task will take over and spawn it again