refactor(store): install_meta_belief → INSERT OR IGNORE for atomic idempotence (#772)

src/aelfrice/store.py

@@ -3287,21 +3287,16 @@ class is unknown.
         for cls in signal_weights:
             if not is_valid_signal_class(cls):
                 raise ValueError(f"unknown signal class: {cls!r}")
-        cur = self._conn.execute(
-            "SELECT 1 FROM meta_beliefs WHERE key = ?", (key,)
-        )
-        if cur.fetchone() is not None:
-            return False
         encoded = encode_signal_weights(signal_weights)
-        self._conn.execute(
-            "INSERT INTO meta_beliefs "
+        cur = self._conn.execute(
+            "INSERT OR IGNORE INTO meta_beliefs "
             "(key, static_default, half_life_seconds, last_updated_ts, signal_classes) "
             "VALUES (?, ?, ?, ?, ?)",
             (key, float(static_default), int(half_life_seconds),
              int(now_ts), encoded),
         )
         self._conn.commit()
-        return True
+        return cur.rowcount > 0
 
     def update_meta_belief(
         self,

tests/test_meta_beliefs.py

@@ -392,6 +392,38 @@ def test_doctor_meta_beliefs_json_is_parseable(tmp_path, monkeypatch):
     assert any(p["signal_class"] == SIGNAL_RELEVANCE for p in row["posteriors"])
 
 
+def test_install_concurrent_race_returns_false_not_raises(tmp_path):
+    """INSERT OR IGNORE path: second install returns False without raising.
+
+    Simulates the outcome of two concurrent writers where the first write
+    wins.  Single-threaded; exercises the rowcount > 0 return path rather
+    than actual concurrency.
+    """
+    import sqlite3
+    db_path = tmp_path / "race.sqlite"
+    s = MemoryStore(str(db_path))
+    try:
+        first = s.install_meta_belief(
+            "meta:race", static_default=0.5, half_life_seconds=3600,
+            signal_weights={SIGNAL_RELEVANCE: 1.0}, now_ts=1,
+        )
+        assert first is True
+
+        # Second call: row already present — must return False, NOT raise.
+        try:
+            second = s.install_meta_belief(
+                "meta:race", static_default=0.5, half_life_seconds=3600,
+                signal_weights={SIGNAL_RELEVANCE: 1.0}, now_ts=2,
+            )
+        except sqlite3.IntegrityError as exc:
+            raise AssertionError(
+                f"install_meta_belief raised IntegrityError on duplicate key: {exc}"
+            ) from exc
+        assert second is False
+    finally:
+        s.close()
+
+
 def test_doctor_meta_beliefs_empty_store_reports_none(tmp_path, monkeypatch):
     import os
     db_path = tmp_path / "empty.sqlite"