Make `maybe_open_context()` re-entrant safe, use per factory locks

tractor/trionics/_mngrs.py

@@ -81,7 +81,7 @@ async def gather_contexts(
     This function is somewhat similar to common usage of
     ``contextlib.AsyncExitStack.enter_async_context()`` (in a loop) in
     combo with ``asyncio.gather()`` except the managers are concurrently
-    entered and exited cancellation just works.
+    entered and exited, and cancellation just works.
 
     '''
     unwrapped: dict[int, Optional[T]] = {}.fromkeys(id(mngr) for mngr in mngrs)
@@ -119,7 +119,7 @@ class _Cache:
     a kept-alive-while-in-use async resource.
 
     '''
-    lock = trio.Lock()
+    locks: dict[Hashable, trio.Lock] = {}
     users: int = 0
     values: dict[Any,  Any] = {}
     resources: dict[
@@ -165,13 +165,18 @@ async def maybe_open_context(
     _Cached instance on a _Cache hit.
 
     '''
-    # lock resource acquisition around task racing  / ``trio``'s
-    # scheduler protocol
-    await _Cache.lock.acquire()
-
-    ctx_key = (id(acm_func), key or tuple(kwargs.items()))
+    fid = id(acm_func)
+    ctx_key = (fid, key or tuple(kwargs.items()))
     value = None
 
+    # Lock resource acquisition around task racing  / ``trio``'s
+    # scheduler protocol.
+    # NOTE: the lock is target context manager func specific in order
+    # to allow re-entrant use cases where one `maybe_open_context()`
+    # wrapped factor may want to call into another.
+    lock = _Cache.locks.setdefault(fid, trio.Lock())
+    await lock.acquire()
+
     try:
         # **critical section** that should prevent other tasks from
         # checking the _Cache until complete otherwise the scheduler
@@ -189,21 +194,21 @@ async def maybe_open_context(
         # TODO: does this need to be a tractor "root nursery"?
         resources = _Cache.resources
         assert not resources.get(ctx_key), f'Resource exists? {ctx_key}'
-        ln, _ = resources[ctx_key] = (service_n, trio.Event())
+        resources[ctx_key] = (service_n, trio.Event())
 
-        value = await ln.start(
+        value = await service_n.start(
             _Cache.run_ctx,
             mngr,
             ctx_key,
         )
         _Cache.users += 1
-        _Cache.lock.release()
+        lock.release()
         yield False, value
 
     else:
         log.info(f'Reusing _Cached resource for {ctx_key}')
         _Cache.users += 1
-        _Cache.lock.release()
+        lock.release()
         yield True, value
 
     finally:
@@ -221,3 +226,5 @@ async def maybe_open_context(
                 if entry:
                     _, no_more_users = entry
                     no_more_users.set()
+
+                _Cache.locks.pop(fid)