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

pyproject.toml

@@ -217,7 +217,7 @@ addopts = [
   '--show-capture=no',
 
   # sys-level capture. REQUIRED for fork-based spawn
-  # backends (e.g. `subint_forkserver`): default
+  # backends (e.g. `main_thread_forkserver`): default
   # `--capture=fd` redirects fd 1,2 to temp files, and fork
   # children inherit those fds — opaque deadlocks happen in
   # the pytest-capture-machinery ↔ fork-child stdio

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
-    # higher to avoid flaky runs..
+    # XXX for `main_thread_forkserver` this is SUPER SENSITIVE
+    # 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

@@ -452,8 +452,12 @@ async def spawn_and_error(
             await nursery.run_in_actor(*args, **kwargs)
 
 
-# NOTE: subint_forkserver skip handled by file-level `pytestmark`
-# above (same pytest-capture-fd hang class as siblings).
+# NOTE: `main_thread_forkserver` capture-fd hang class is no
+# longer skipped here — `--capture=sys` (the new `pyproject.toml`
+# default) sidesteps the pipe-buffer-fill deadlock for
+# `test_nested_multierrors`. See
+# `ai/conc-anal/subint_forkserver_test_cancellation_leak_issue.md`
+# / #449 for the post-mortem.
 @pytest.mark.timeout(
     10,
     method='thread',

tests/test_infected_asyncio.py

@@ -1113,7 +1113,7 @@ 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

@@ -16,22 +16,16 @@ 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`.
+    # 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/_testing/pytest.py

@@ -303,7 +303,7 @@ def _reap_orphaned_subactors():
     grace window, then SIGKILL survivors.
 
     Rationale: under fork-based spawn backends (notably
-    `subint_forkserver`), a test that times out or bails
+    `main_thread_forkserver`), a test that times out or bails
     mid-teardown can leave subactor forks alive. Without
     this reap, they linger across sessions and compete
     for ports / inherit pytest's capture-pipe fds — which

tractor/runtime/_runtime.py

@@ -1763,7 +1763,7 @@ 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

@@ -122,8 +122,8 @@ 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
-# that needs it).
+# `tractor.spawn._main_thread_forkserver` for the one current
+# caller that needs it).
 _RUNTIME_VARS_DEFAULTS: dict[str, Any] = {
     # root of actor-process tree info
     '_is_root': False,  # bool
@@ -165,7 +165,7 @@ 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))

tractor/spawn/_subint.py

@@ -257,7 +257,8 @@ async def subint_proc(
         # via a `nonlocal err` slot inspected after
         # `subint_exited.wait()` — see anyio's
         # `to_interpreter._interp_call` `(retval, is_exception)`
-        # tuple pattern + `_subint_forkserver.py:480-494`'s
+        # tuple pattern +
+        # `_subint_forkserver.run_subint_in_worker_thread._drive`'s
         # equivalent which already does this. Skipped here for
         # now: re-raise from the parent must coordinate with
         # the existing `trio.Cancelled` paths around the