Merge pull request #29 from JPHutchins/feature/matrix-fs-isolation

PoC: isolate camas matrix cells in git worktrees (fixes pyrefly fs race)

Author: JPHutchins

tasks.py

@@ -26,10 +26,37 @@
 all = Sequential(fix, Parallel(typecheck, coverage))
 check = Parallel(format_check, lint, typecheck, test)
 
+# Per-cell filesystem isolation. This is a rare workaround, NOT something camas
+# requires — the matrix / cwd / env primitives below are general; we just
+# compose them here to dodge one flaky type-checker.
+#
+# `check` runs five type-checkers, and the matrix fans it across six Python
+# versions in parallel: ~6 `uv sync`s and ~30 checkers pound ONE working tree at
+# once. pyrefly globs `**/*.py*` over the whole tree and intermittently reads a
+# file that a sibling cell's `uv sync` is mid-way through deleting under its
+# `.venv`, dying with "No such file or directory (os error 2)" and flaking CI.
+# That race is pyrefly's, not ours; the fix is simply to stop sharing the tree.
+#
+# So each cell runs in a throwaway `git worktree` — a sibling dir holding only
+# tracked files, with its own `.venv` — and no two cells ever touch the same
+# path. `rm -rf` then `add --force` is idempotent and safe to run concurrently
+# across cells (no `git worktree prune`, which would race a sibling mid-rebuild).
+# Worktrees check out HEAD, so commit locally before running the matrix if you
+# want it to cover your working changes.
+#
+# The dir name has no leading dot on purpose: pyrefly skips hidden directories,
+# so a `.`-prefixed worktree silently matches zero files and "passes" vacuously.
+_worktree = "../camas-matrix/{PY}"
+
 matrix = Sequential(
-	Task("uv sync"),
-	check,
-	env={"UV_PROJECT_ENVIRONMENT": ".venv-{PY}", "UV_PYTHON": "{PY}"},
+	Task(f"rm -rf {_worktree}"),
+	Task(f"git worktree add --force --detach {_worktree} HEAD"),
+	Sequential(
+		Task("uv sync"),
+		check,
+		cwd=_worktree,
+		env={"UV_PYTHON": "{PY}"},
+	),
 	matrix={
 		"PY": tuple(
 			stripped