Fix `SharedMemory` under `subint_forkserver`

Implements the resolution described in c99d475d's
`subint_forkserver_mp_shared_memory_issue.md` (now
updated with the resolution post-mortem). Two-part
fix that side-steps `mp.resource_tracker` entirely
rather than try to make it fork-safe — turns out
that's both simpler AND more correct given tractor
already SC-manages allocation lifetimes.

Deats,
- `tractor/ipc/_mp_bs.py::disable_mantracker()`: drop the
  `platform.python_version_tuple()[:-1] >= ('3', '13')` branch — patches
  now run unconditionally:
  * monkey-patch `mp.resource_tracker. _resource_tracker` to a no-op
    `ManTracker` subclass (empty `register` / `unregister`
    / `ensure_running`).
  * return `partial(SharedMemory, track=False)` for the per-allocation
    opt-out.
  * belt + suspenders: even if something dodges the wrapper, the
    singleton can't talk to the inherited (broken) parent fd.

- `tractor/ipc/_shm.py::open_shm_list()`: drop the 3.13+ conditional
  skip of the unlink-callback; install a `try_unlink()` wrapper that
  swallows `FileNotFoundError` (sibling-already-cleaned race in
  shared-key setups). Without `mp.resource_tracker` doing it for us, we
  own the unlink — `actor. lifetime_stack` is the right place since
  tractor already controls actor lifecycle.

- `tests/test_shm.py`: uncomment-out `subint_forkserver` from the
  module-level skip- list (tests pass now). Inline comment cross-refs
  the two `_mp_bs` / `_shm` workarounds.

- `ai/conc-anal/subint_forkserver_mp_shared_memory_ issue.md`: heavy
  rewrite — flips status from "open / unresolvable in tractor" to
  "resolved, kept as decision record". Adds Resolution section, "Why
  this is the right call" rationale (mp tracker is widely criticized;
  tractor already owns lifecycle), trade-offs (crash-leaked segments,
  lost mp leak warning), verification (7 passed under both
  `subint_forkserver` and `trio` backends), and upstream issue links

(this patch was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code

(cherry picked from commit aa3e230926)
(factored: dropped subint_forkserver conc-anal doc update)

tests/test_shm.py

@@ -17,6 +17,10 @@ from tractor.ipc._shm import (
 pytestmark = pytest.mark.skipon_spawn_backend(
     'subint',
     # 'subint_forkserver',
+    # XXX we hack around this stdlib limitation by both,
+    # - setting `ShareMemory(track=False)`
+    # - overriding the `mp.ResourceTracker` nonsense in
+    # `.ipc._mp_bs`.
     reason=(
         'subint: GIL-contention hanging class.\n'
         'subint_forkserver: `multiprocessing.SharedMemory` '

tractor/ipc/_mp_bs.py

@@ -17,7 +17,7 @@
 Utils to tame mp non-SC madeness
 
 '''
-import platform
+from functools import partial
 
 
 def disable_mantracker():
@@ -27,49 +27,37 @@ def disable_mantracker():
 
     '''
     from multiprocessing.shared_memory import SharedMemory
+    from multiprocessing import (
+        resource_tracker as mantracker,
+    )
 
+    # XXX ALWAYS disable the stdlib's "resource tracker"; it prevents
+    # fork backends and never was useful to us since we're SC
+    # lifetime managing all allocations.
+    class ManTracker(mantracker.ResourceTracker):
+        def register(self, name, rtype):
+            pass
+
+        def unregister(self, name, rtype):
+            pass
+
+        def ensure_running(self):
+            pass
+
+    # "know your land and know your prey"
+    # https://www.dailymotion.com/video/x6ozzco
+    mantracker._resource_tracker = ManTracker()
+    mantracker.register = mantracker._resource_tracker.register
+    mantracker.ensure_running = mantracker._resource_tracker.ensure_running
+    mantracker.unregister = mantracker._resource_tracker.unregister
+    mantracker.getfd = mantracker._resource_tracker.getfd
 
     # 3.13+ only.. can pass `track=False` to disable
     # all the resource tracker bs.
     # https://docs.python.org/3/library/multiprocessing.shared_memory.html
-    if (_py_313 := (
+    shmT = partial(
-            platform.python_version_tuple()[:-1]
+        SharedMemory,
-            >=
+        track=False,
-            ('3', '13')
+    )
-        )
-    ):
-        from functools import partial
-        return partial(
-            SharedMemory,
-            track=False,
-        )
-
-    # !TODO, once we drop 3.12- we can obvi remove all this!
-    else:
-        from multiprocessing import (
-            resource_tracker as mantracker,
-        )
-
-        # Tell the "resource tracker" thing to fuck off.
-        class ManTracker(mantracker.ResourceTracker):
-            def register(self, name, rtype):
-                pass
-
-            def unregister(self, name, rtype):
-                pass
-
-            def ensure_running(self):
-                pass
-
-        # "know your land and know your prey"
-        # https://www.dailymotion.com/video/x6ozzco
-        mantracker._resource_tracker = ManTracker()
-        mantracker.register = mantracker._resource_tracker.register
-        mantracker.ensure_running = mantracker._resource_tracker.ensure_running
-        mantracker.unregister = mantracker._resource_tracker.unregister
-        mantracker.getfd = mantracker._resource_tracker.getfd
-
-        # use std type verbatim
-        shmT = SharedMemory
 
     return shmT

tractor/ipc/_shm.py

@@ -929,15 +929,26 @@ def open_shm_list(
     # "close" attached shm on actor teardown
     try:
         actor = tractor.current_actor()
-
         actor.lifetime_stack.callback(shml.shm.close)
 
-        # XXX on 3.13+ we don't need to call this?
+        # >XXX NOTE< on 3.13+ we need to call this AS WELL AS pass
-        # -> bc we pass `track=False` for `SharedMemeory` orr?
+        # `track=False` for `mp.SharedMemeory` otherwise fork based
-        if (
+        # backends will error out due to long lived stdlib
-            platform.python_version_tuple()[:-1] < ('3', '13')
+        # limitations,
-        ):
+        # - https://bugs.python.org/issue38119
-            actor.lifetime_stack.callback(shml.shm.unlink)
+        # - https://bugs.python.org/issue45209
+        #
+        def try_unlink():
+            try:
+                shml.shm.unlink()
+            except FileNotFoundError as fne:
+                log.debug(
+                    f'ShmList already deallocated pre-actor-shutdown.\n'
+                    f'{fne!r}\n'
+                )
+
+        actor.lifetime_stack.callback(try_unlink)
+
     except RuntimeError:
         log.warning('tractor runtime not active, skipping teardown steps')