test: skip POSIX-only fixtures on Windows instead of failing

[sashankh]

Fixes #2919.

The problem

Five tests construct their fixture out of POSIX-only OS behaviour with no platform guard, so
on Windows they fail before the code under test is reached:

tests/test_non_regular_files.py::test_fifo_is_rejected
    AttributeError: module 'os' has no attribute 'mkfifo'
tests/test_non_regular_files.py::test_unix_socket_is_rejected
    AttributeError: module 'socket' has no attribute 'AF_UNIX'
tests/test_non_regular_files.py::test_symlink_pointing_at_a_fifo_is_rejected
    AttributeError: module 'os' has no attribute 'mkfifo'
tests/test_watch.py::test_rebuild_code_deleted_cwd_without_repo_root_returns_false
    PermissionError: [WinError 32] ... '...\gone'
tests/test_watch.py::test_rebuild_code_deleted_cwd_uses_graphify_repo_root
    PermissionError: [WinError 32] ... '...\gone'

The last two come from

os.chdir(gone)
gone.rmdir()      # POSIX unlinks a live CWD; Windows holds an open handle on it

which is the exact state the test wants (_rebuild_code surviving a detached hook's deleted
CWD) — and which Windows cannot be made to enter at all.

This is the argument #2642 already made for symlinks:

A plain non-elevated Windows shell raises OSError: [WinError 1314], which pytest reports as
a FAILURE — 15 of them, drowning out real defects — when what it means is "unsupported here".

tests/test_watch.py already skips ten tests with fcntl-only (POSIX), so the practice is
settled; these five were simply missed.

The change

Extends the _can_symlink / requires_symlinks pair in tests/conftest.py with siblings for
the three capabilities involved:

fixture	guards
requires_fifo	os.mkfifo
requires_unix_socket	binding an AF_UNIX socket
requires_deletable_cwd	removing a directory that is the process CWD

Each is probed, not inferred from sys.platform — #2642's reasoning applies unchanged:
Windows 10+ does support AF_UNIX and CPython exposes it on some builds, and a POSIX host can
lack FIFOs when its temp dir is on a filesystem that has none. Hosts that can do these things
keep the coverage.

They live in conftest.py beside _can_symlink rather than in the two test files, so the
"probe the capability, don't guess from the platform name" doctrine stays in one place and the
next test needing a FIFO finds it.

Also adds the existing requires_symlinks fixture to the two symlink tests in
test_non_regular_files.py that never took it — test_symlink_to_a_regular_file_is_accepted
and test_broken_symlink_is_rejected_without_raising. They pass on a Windows box with
Developer Mode enabled and raise WinError 1314 on one without, which is precisely the case
that fixture exists for.

Verification

Windows 11 / Python 3.12.10, v8 @ b2cd362.

Targeted:

$ pytest tests/test_non_regular_files.py tests/test_watch.py -q -rs
120 passed, 17 skipped
  SKIPPED tests/test_non_regular_files.py:37: named pipes (os.mkfifo) unavailable on this platform
  SKIPPED tests/test_non_regular_files.py:45: AF_UNIX sockets unavailable on this platform
  SKIPPED tests/test_non_regular_files.py:68: named pipes (os.mkfifo) unavailable on this platform
  SKIPPED tests/test_watch.py:654: cannot remove a directory that is the process CWD on this
                                   platform (Windows holds an open handle: WinError 32)
  SKIPPED tests/test_watch.py:680: (same)

Full suite, before and after:

	failures	passed	skipped
origin/v8	50	4511	216
this branch	44	4512	221

Set-diff of the FAILED ids: no new failures. The six that stopped failing are the five
guarded here plus test_incremental_mtime_collision.py::test_same_size_rewrite_in_one_tick_is_requeued,
which is an unrelated timing-sensitive test that happened to pass on this run — not something
this PR fixes.

Tests only — no change under graphify/. On Linux every probe succeeds, nothing skips, and CI
behaviour is unchanged.

[graphify-labs]

Graphify reviewed this change.

Worth a look — the grounded gate found no coupling regressions or blocking issues, but 2 advisory finding(s) below merit a look before merge.

---

Graphify review — findings

Adds probed requires_fifo, requires_unix_socket, and requires_deletable_cwd skip fixtures to tests/conftest.py, each testing the capability at runtime rather than by platform name. Applies them (plus the existing requires_symlinks) to the FIFO, unix-socket, symlink, and deleted-CWD tests in test_non_regular_files.py and test_watch.py so they skip cleanly on Windows and unsupported filesystems.

Worth a look

• AF_UNIX probe does not handle socket creation failures — tests/conftest.py:80 · Escalate · medium
  • agreed by 2 of 2 members but NOT verified (no proof, no reproducing execution) — consensus is not a verdict; needs human review
• AF_UNIX probe leaks socket-construction OSError instead of reporting unsupported — tests/conftest.py:96 · Escalate · medium
  • agreed by 2 of 2 members but NOT verified (no proof, no reproducing execution) — consensus is not a verdict; needs human review

Analysis details — impact, health, verification

Impact & health

Graphify review

Impact — 310 functions depend on the 310 functions this change touches.

Health — grade A; no new coupling hotspots.

Verification — 310 functions in the blast radius were not formally verified this run (proofs are advisory here).

Gate & verification

graphify gate

PASS — objectively clean (no health regressions, tests not run — proofs not run this pass (advisory)). Grounded, not self-assessed.

Advisory (not blocking):

• verification_scope: 310 function(s) in the blast radius were not formally verified this run