test: harden scheduling review boundaries

tests/artifacts/test_store.py

@@ -1,7 +1,10 @@
 from __future__ import annotations
 
 import json
+import threading
 from concurrent.futures import ThreadPoolExecutor
+from pathlib import Path
+from typing import Any
 
 import pytest
 
@@ -69,7 +72,7 @@ def test_file_store_round_trips_deployment(tmp_path) -> None:
 
 
 def test_concurrent_deployment_saves_advance_revision_without_lost_updates(
-    tmp_path,
+    tmp_path, monkeypatch
 ) -> None:
     store = FileWorkflowArtifactStore(tmp_path)
     store.save_deployment(
@@ -88,9 +91,45 @@ def test_concurrent_deployment_saves_advance_revision_without_lost_updates(
         for version in range(2, 10)
     ]
 
-    with ThreadPoolExecutor(max_workers=len(updates)) as executor:
-        list(executor.map(store.save_deployment, updates))
+    start_gate = threading.Barrier(len(updates))
+    read_gate = threading.Barrier(len(updates))
+    read_state_lock = threading.Lock()
+    active_reads = 0
+    max_active_reads = 0
+    deployment_path = (store.deployments_dir / "concurrent.personal.json").resolve()
+    original_read_text = Path.read_text
 
+    def coordinated_read(path: Path, *args: Any, **kwargs: Any) -> str:
+        nonlocal active_reads, max_active_reads
+        if path.resolve() != deployment_path:
+            return original_read_text(path, *args, **kwargs)
+        with read_state_lock:
+            active_reads += 1
+            max_active_reads = max(max_active_reads, active_reads)
+        try:
+            # A broken implementation reaches this gate from every worker
+            # after reading the same revision. The real store's lock lets
+            # only one worker enter the read/modify/write window, so the
+            # gate times out once and later workers proceed immediately.
+            try:
+                read_gate.wait(timeout=0.5)
+            except threading.BrokenBarrierError:
+                pass
+            return original_read_text(path, *args, **kwargs)
+        finally:
+            with read_state_lock:
+                active_reads -= 1
+
+    monkeypatch.setattr(Path, "read_text", coordinated_read)
+
+    def save(deployment: WorkflowDeployment) -> None:
+        start_gate.wait(timeout=10)
+        store.save_deployment(deployment)
+
+    with ThreadPoolExecutor(max_workers=len(updates)) as executor:
+        list(executor.map(save, updates))
+
+    assert max_active_reads == 1
     assert store.get_deployment("concurrent.personal").revision == 9