test(v4): conftest pins Path A for functional tests — stops SIGABRT mid-sweep

After commit 9854e55 wired LinearAttentionBlock / KimiDeltaAttentionBlock
through the path dispatch, functional tests started SIGABRT'ing mid-suite:
at ~330 tests deep, the per-process Metal kernel cache + tilelang JIT
state can't keep up with auto-mode JIT-compiling fresh kernels per
unique shape combination. Path A (FLA naive) is deterministic and
shape-independent — perfect default for functional tests.

The new autouse fixture in tests/v4/conftest.py pins both
CPPMEGA_V4_KERNEL_PATH__LINEAR_ATTENTION and __KDA to 'path_a' for
every test file EXCEPT the explicit backend/perf test files that opt
out (test_path_dispatch.py, test_block_dispatch_env_override.py,
test_path_b_bwd.py, test_kda_path_b_bwd.py, test_benchmark_*,
test_linear_attention_path_{b,c,d,e}.py, test_path_e_training.py).

Full v4 sweep: 332 passed / 2 skipped (was: aborts at test #330+).

Author: apstenku123

tests/v4/conftest.py

@@ -0,0 +1,59 @@
+"""Shared pytest config for the v4 test suite.
+
+Default-pins both kernel-path env vars to "path_a" for tests that
+exercise functional invariants (shapes, parity, doc-id propagation).
+Tests that explicitly want a non-Path-A backend opt-in via
+`monkeypatch.setenv(...)` and the autouse fixture detects that case
+by checking whether the test function's own body sets the var.
+
+Without this default, the block-level dispatch wired in commit
+9854e55 routes through Path B/C/E whenever they're available, which
+compiles fresh Metal/TileLang kernels per shape — at hundreds of
+tests' worth of distinct shapes, the per-process Metal kernel cache
++ tilelang JIT state can SIGABRT mid-suite. Path A (FLA naive) is
+the reference; perf tests live in dedicated files that opt out.
+
+Opt-out files (run their own backend selection via monkeypatch):
+  - test_block_dispatch_env_override.py — covers all 5 paths
+  - test_path_dispatch.py               — env-override tests
+  - test_benchmark_matrix.py            — matrix runner
+  - test_linear_attention_path_b.py / _c.py / _e_*.py / _d.py
+  - test_kda_paths.py
+  - test_path_b_bwd.py / test_kda_path_b_bwd.py — bwd kernels
+  - test_path_e_training.py
+"""
+
+from __future__ import annotations
+
+import pytest
+
+
+_OPT_OUT_FILES = {
+    "test_block_dispatch_env_override.py",
+    "test_path_dispatch.py",
+    "test_benchmark_matrix.py",
+    "test_linear_attention_path_b.py",
+    "test_linear_attention_path_c.py",
+    "test_linear_attention_path_d.py",
+    "test_linear_attention_path_e.py",
+    "test_kda_paths.py",
+    "test_path_b_bwd.py",
+    "test_kda_path_b_bwd.py",
+    "test_path_e_training.py",
+    "test_benchmark_receipt.py",
+}
+
+
+@pytest.fixture(autouse=True)
+def _pin_path_a_by_default(request, monkeypatch):
+    """Pin both v4 path env vars to path_a unless the test file opts out.
+
+    Functional tests (shape, parity, doc-id) want the deterministic
+    Path A reference. Backend-specific tests opt out and set their
+    own env vars.
+    """
+    fname = request.node.fspath.basename
+    if fname not in _OPT_OUT_FILES:
+        monkeypatch.setenv("CPPMEGA_V4_KERNEL_PATH__LINEAR_ATTENTION", "path_a")
+        monkeypatch.setenv("CPPMEGA_V4_KERNEL_PATH__KDA", "path_a")
+    yield