Sweep `subint_forkserver` → `main_thread_forkserver` in code

After the variant-1 / variant-2 backend split, update remaining
string-match refs to the variant-1 backend so user-visible gates
+ skip-marks + comments name the working backend correctly:

- `tractor._root._DEBUG_COMPATIBLE_BACKENDS`: include
  `main_thread_forkserver`, drop the stub-only `subint_forkserver`
  entry.
- `tests/test_spawning.py::test_loglevel_propagated_to_subactor`:
  capfd-skip flips to `main_thread_forkserver`.
- `tests/test_infected_asyncio.py::test_sigint_closes_lifetime_stack`:
  xfail-condition flips to `main_thread_forkserver`.
- `tests/test_shm.py`: drop stale "broken on `main_thread_forkserver`"
  reason-text since the `mp.SharedMemory(track=False)`
  + resource-tracker monkey-patch in `.ipc._mp_bs` makes the tests pass;
  the skip-mark only fires on plain `subint` now.
- Comment / docstring sweep: `runtime._state`, `runtime._runtime`,
  `_testing.pytest`, `_subint.py`, `pyproject.toml`,
  `test_cancellation.py`, `test_registrar.py` — refs to variant-1
  backend updated.

(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 205382a39b)
(factored: dropped spawn-backend-only path: tractor/spawn/_subint.py)

tests/discovery/test_registrar.py

@@ -538,13 +538,13 @@ async def kill_transport(
 @pytest.mark.timeout(
     30,
     # NOTE should be a 2.1s happy path.
-    # XXX for `subint_forkserver` this is SUPER SENSITIVE so keep it
+    # XXX for `main_thread_forkserver` this is SUPER SENSITIVE
-    # higher to avoid flaky runs..
+    # so keep it higher to avoid flaky runs..
     method='thread',
 )
 @pytest.mark.skipon_spawn_backend(
     'subint',
-    # 'subint_forkserver',
+    # 'main_thread_forkserver',
     reason=(
         'XXX SUBINT HANGING TEST XXX\n'
         'See oustanding issue(s)\n'

tests/test_cancellation.py

@@ -452,21 +452,12 @@ async def spawn_and_error(
             await nursery.run_in_actor(*args, **kwargs)
 
 
-@pytest.mark.skipon_spawn_backend(
+# NOTE: `main_thread_forkserver` capture-fd hang class is no
-    'subint_forkserver',
+# longer skipped here — `--capture=sys` (the new `pyproject.toml`
-    reason=(
+# default) sidesteps the pipe-buffer-fill deadlock for
-        'Passes cleanly with `pytest -s` (no stdout capture) '
+# `test_nested_multierrors`. See
-        'but hangs under default `--capture=fd` due to '
+# `ai/conc-anal/subint_forkserver_test_cancellation_leak_issue.md`
-        'pytest-capture-pipe buffer fill from high-volume '
+# / #449 for the post-mortem.
-        'subactor error-log traceback output inherited via fds '
-        '1,2 in fork children. Fix direction: redirect subactor '
-        'stdout/stderr to `/dev/null` in `_child_target` / '
-        '`_actor_child_main` so forkserver children don\'t hold '
-        'pytest\'s capture pipe open. See `ai/conc-anal/'
-        'subint_forkserver_test_cancellation_leak_issue.md` '
-        '"Update — pytest capture pipe is the final gate".'
-    ),
-)
 @pytest.mark.timeout(
     10,
     method='thread',

tests/test_infected_asyncio.py

@@ -1129,7 +1129,7 @@ def test_sigint_closes_lifetime_stack(
                             if (
                                 send_sigint_to == 'child'
                                 and
-                                start_method == 'subint_forkserver'
+                                start_method == 'main_thread_forkserver'
                             ):
                                 pytest.xfail(
                                     reason=(

tests/test_shm.py

@@ -16,18 +16,16 @@ from tractor.ipc._shm import (
 
 pytestmark = pytest.mark.skipon_spawn_backend(
     'subint',
-    # 'subint_forkserver',
+    # NOTE, `main_thread_forkserver` works for these tests
+    # via the `mp.SharedMemory(track=False)` +
+    # `mp.resource_tracker` monkey-patch in `.ipc._mp_bs`.
+    # Without that workaround the fork-inherited
+    # `resource_tracker` fd would EBADF on first shm op +
+    # cascade into `FileExistsError` across parametrize
+    # variants. Tracker doc:
+    # `ai/conc-anal/subint_forkserver_mp_shared_memory_issue.md`.
     reason=(
         'subint: GIL-contention hanging class.\n'
-        'subint_forkserver: `multiprocessing.SharedMemory` '
-        'is fork-without-exec unsafe — child inherits parent\'s '
-        '`resource_tracker` fd → EBADF on first shm op '
-        '(`test_child_attaches_alot`); leaked `/shm_list` from '
-        'a "passing" run cascades into `FileExistsError` across '
-        'parametrize variants (`test_parent_writer_child_reader`). '
-        'Canonical CPython issue class, NOT a tractor bug; full '
-        'tracker doc:\n'
-        'ai/conc-anal/subint_forkserver_mp_shared_memory_issue.md'
     )
 )
 

tests/test_spawning.py

@@ -194,7 +194,7 @@ def test_loglevel_propagated_to_subactor(
     reg_addr: tuple,
     level: str,
 ):
-    if start_method in ('mp_forkserver', 'subint_forkserver'):
+    if start_method in ('mp_forkserver', 'main_thread_forkserver'):
         pytest.skip(
             "a bug with `capfd` seems to make forkserver capture not work? "
             "(same class as the `mp_forkserver` pre-existing skip — fork-"

tractor/_root.py

@@ -79,7 +79,7 @@ _DEBUG_COMPATIBLE_BACKENDS: tuple[str, ...] = (
     'trio',
     # forkserver children run `_trio_main` in their own OS
     # process — same child-side runtime shape as `trio_proc`.
-    'subint_forkserver',
+    'main_thread_forkserver',
 )
 
 

tractor/runtime/_runtime.py

@@ -1778,7 +1778,7 @@ async def async_main(
                     # shielded loop would park on the parent chan
                     # indefinitely waiting for EOF that only arrives
                     # after the PARENT tears down, which under
-                    # fork-based backends (e.g. `subint_forkserver`)
+                    # fork-based backends (e.g. `main_thread_forkserver`)
                     # it waits on THIS actor's exit — deadlock.
                     actor._parent_chan_cs = await root_tn.start(
                         partial(

tractor/runtime/_state.py

@@ -123,8 +123,8 @@ class RuntimeVars(Struct):
 # `open_root_actor()` nor received a parent `SpawnSpec`. Kept
 # as a module-level constant so `get_runtime_vars(clear_values=
 # True)` can reset the live dict back to this baseline (see
-# `tractor.spawn._subint_forkserver` for the one current caller
+# `tractor.spawn._main_thread_forkserver` for the one current
-# that needs it).
+# caller that needs it).
 _RUNTIME_VARS_DEFAULTS: dict[str, Any] = {
     # root of actor-process tree info
     '_is_root': False,  # bool
@@ -167,7 +167,7 @@ def get_runtime_vars(
     defaults (`_RUNTIME_VARS_DEFAULTS`) instead of the live
     dict. Useful in combination with `set_runtime_vars()` to
     reset process-global state back to "cold" — the main caller
-    today is the `subint_forkserver` spawn backend's post-fork
+    today is the `main_thread_forkserver` spawn backend's post-fork
     child prelude:
 
         set_runtime_vars(get_runtime_vars(clear_values=True))