Add notes around py3.10 stdlib bug from `pdb++`

There's a bug that's triggered in the stdlib without latest `pdb++`
installed; add a note for that.

Further inside `wait_for_parent_stdin_hijack()` don't `.started()` until
the interactor stream has been opened to avoid races when debugging this
`._debug.py` module (at the least) since we usually don't want the
spawning (parent) task to resume until we know for sure the tty lock has
been acquired. Also, drop the random checkpoint we had inside
`_breakpoint()`, not sure it was actually adding anything useful since
we're (mostly) carefully shielded throughout this func.

tractor/_debug.py

@@ -291,8 +291,7 @@ async def _hijack_stdin_for_child(
                 # trio.ClosedResourceError,  # by self._rx_chan
                 # ContextCancelled,
                 # ConnectionResetError,
-            ) as err:
+            ):
-
                 # XXX: there may be a race with the portal teardown
                 # with the calling actor which we can safely ignore.
                 # The alternative would be sending an ack message
@@ -355,10 +354,10 @@ async def wait_for_parent_stdin_hijack(
 
                     async with ctx.open_stream() as stream:
                         # unblock local caller
-                        task_status.started(cs)
 
                         try:
                             assert _local_pdb_complete
+                            task_status.started(cs)
                             await _local_pdb_complete.wait()
 
                         finally:
@@ -418,12 +417,6 @@ async def _breakpoint(
     '''
     __tracebackhide__ = True
 
-    # TODO: is it possible to debug a trio.Cancelled except block?
-    # right now it seems like we can kinda do with by shielding
-    # around ``tractor.breakpoint()`` but not if we move the shielded
-    # scope here???
-    # with trio.CancelScope(shield=shield):
-
     pdb, undo_sigint = mk_mpdb()
     actor = tractor.current_actor()
     task_name = trio.lowlevel.current_task().name
@@ -431,7 +424,12 @@ async def _breakpoint(
     global _local_pdb_complete, _pdb_release_hook
     global _local_task_in_debug, _global_actor_in_debug
 
-    await trio.lowlevel.checkpoint()
+    # TODO: is it possible to debug a trio.Cancelled except block?
+    # right now it seems like we can kinda do with by shielding
+    # around ``tractor.breakpoint()`` but not if we move the shielded
+    # scope here???
+    # with trio.CancelScope(shield=shield):
+    #     await trio.lowlevel.checkpoint()
 
     if not _local_pdb_complete or _local_pdb_complete.is_set():
         _local_pdb_complete = trio.Event()
@@ -441,12 +439,9 @@ async def _breakpoint(
 
         if _local_task_in_debug:
             if _local_task_in_debug == task_name:
-                print("LOCAL TASK ALREADY IN DEBUG")
                 # this task already has the lock and is
                 # likely recurrently entering a breakpoint
                 return
-            else:
-                print("NOT LOCAL TASK ALREADY IN DEBUG")
 
             # if **this** actor is already in debug mode block here
             # waiting for the control to be released - this allows
@@ -713,6 +708,29 @@ def _post_mortem(
 
     '''
     log.pdb(f"\nAttaching to pdb in crashed actor: {actor.uid}\n")
+
+    # XXX: on py3.10 if you don't have latest ``pdbpp`` installed.
+    # The exception looks something like:
+    # Traceback (most recent call last):
+    # File ".../tractor/_debug.py", line 729, in _post_mortem
+    #   for _ in range(100):
+    # File "../site-packages/pdb.py", line 1227, in xpm
+    #   post_mortem(info[2], Pdb)
+    # File "../site-packages/pdb.py", line 1175, in post_mortem
+    #   p.interaction(None, t)
+    # File "../site-packages/pdb.py", line 216, in interaction
+    #   ret = self.setup(frame, traceback)
+    # File "../site-packages/pdb.py", line 259, in setup
+    #   ret = super(Pdb, self).setup(frame, tb)
+    # File "/usr/lib/python3.10/pdb.py", line 217, in setup
+    #   self.curframe = self.stack[self.curindex][0]
+    # IndexError: list index out of range
+
+    # NOTE: you need ``pdbpp`` master (at least this commit
+    # https://github.com/pdbpp/pdbpp/commit/b757794857f98d53e3ebbe70879663d7d843a6c2)
+    # to fix this and avoid the hang it causes XD.
+    # see also: https://github.com/pdbpp/pdbpp/issues/480
+
     pdbpp.xpm(Pdb=lambda: pdb)