fix(hooks): limit Windows hook rebuild workers

graphify/hooks.py

@@ -1,6 +1,7 @@
 # git hook integration - install/uninstall graphify post-commit and post-checkout hooks
 from __future__ import annotations
 import configparser
+import os
 import re
 import sys
 from pathlib import Path
@@ -224,6 +225,13 @@ _HOOK_SCRIPT = """\
 # churn run-to-run. Pinning it makes graphify-out reproducible.
 export PYTHONHASHSEED=0
 
+# Git for Windows/MSYS hooks can inherit fragile pipe handles from GUI clients
+# and agent shells. Keep hook-triggered rebuilds sequential by default there;
+# explicit GRAPHIFY_MAX_WORKERS still wins for users who want parallelism.
+if [ -n "${WINDIR:-}" ] || [ -n "${MSYSTEM:-}" ]; then
+    export GRAPHIFY_MAX_WORKERS="${GRAPHIFY_MAX_WORKERS:-1}"
+fi
+
 # Skip during rebase/merge/cherry-pick to avoid blocking --continue with unstaged changes
 GIT_DIR=$(git rev-parse --git-dir 2>/dev/null)
 [ -d "$GIT_DIR/rebase-merge" ] && exit 0
@@ -269,6 +277,13 @@ _CHECKOUT_SCRIPT = """\
 # churn run-to-run. Pinning it makes graphify-out reproducible.
 export PYTHONHASHSEED=0
 
+# Git for Windows/MSYS hooks can inherit fragile pipe handles from GUI clients
+# and agent shells. Keep hook-triggered rebuilds sequential by default there;
+# explicit GRAPHIFY_MAX_WORKERS still wins for users who want parallelism.
+if [ -n "${WINDIR:-}" ] || [ -n "${MSYSTEM:-}" ]; then
+    export GRAPHIFY_MAX_WORKERS="${GRAPHIFY_MAX_WORKERS:-1}"
+fi
+
 PREV_HEAD=$1
 NEW_HEAD=$2
 BRANCH_SWITCH=$3
@@ -319,6 +334,8 @@ def _reject_windows_path(value: str, source: str) -> None:
     junk directory (backslashes and all), while install reports success and the
     real ``.git/hooks`` gets nothing. Fail loudly instead so the user can fix it.
     """
+    if os.name == "nt":
+        return
     if _WINDOWS_DRIVE_RE.match(value) or "\\" in value:
         raise RuntimeError(
             f"git hooks path from {source} looks like a Windows path: {value!r}. "

tests/test_hooks.py

@@ -254,6 +254,15 @@ def test_hooks_use_cross_platform_detach(name, script):
     assert "0x00000200" in script, f"{name} missing CREATE_NEW_PROCESS_GROUP flag"
 
 
+@pytest.mark.parametrize("name,script", _HOOK_SCRIPTS)
+def test_hooks_limit_windows_workers_by_default(name, script):
+    """Git for Windows/MSYS hooks can expose fragile pipe handles to spawned
+    ProcessPoolExecutor children. Hook-triggered rebuilds should default to one
+    worker there, while still allowing explicit user overrides."""
+    assert '[ -n "${WINDIR:-}" ] || [ -n "${MSYSTEM:-}" ]' in script
+    assert 'export GRAPHIFY_MAX_WORKERS="${GRAPHIFY_MAX_WORKERS:-1}"' in script
+
+
 def _launcher_payload(script: str) -> str:
     """Extract the `python -c "<payload>"` the hook hands to GRAPHIFY_PYTHON.
 
@@ -351,9 +360,10 @@ def _set_hookspath(repo: Path, value: str) -> None:
     r"D:\hooks",
     r"some\back\slashed\path",
 ])
-def test_windows_hookspath_rejected_no_junk_dir(tmp_path, winpath):
+def test_windows_hookspath_rejected_no_junk_dir_on_posix(tmp_path, monkeypatch, winpath):
     """A Windows-style core.hooksPath must raise (loud failure), not silently
-    create a backslash-named junk directory and report success (#1385)."""
+    create a backslash-named junk directory and report success on POSIX/WSL (#1385)."""
+    monkeypatch.setattr("graphify.hooks.os.name", "posix")
     repo = _make_git_repo(tmp_path)
     _set_hookspath(repo, winpath)
     with pytest.raises(RuntimeError, match="Windows path"):