Fix frozen delta watermark and add error stats, lazy source, concurrent disk reads, and per-engine config

tests/test_cache.py

@@ -1,4 +1,5 @@
 import sqlite3
+import threading
 
 import pytest
 
@@ -96,6 +97,68 @@ def test_disk_mode_reload_in_new_instance(tmp_path, source_conn):
     c2.close()
 
 
+def test_quoted_reserved_and_spaced_identifiers(tmp_path):
+    """Table/column names that are reserved words or contain spaces must work."""
+    src = sqlite3.connect(":memory:")
+    src.execute('CREATE TABLE "weird tbl" ("order" TEXT, "group by" TEXT)')
+    src.executemany('INSERT INTO "weird tbl" VALUES (?, ?)', [("1", "a"), ("2", "b")])
+    src.commit()
+
+    c = CacheManager(db_path=tmp_path / "c.db", backup_interval=9999)
+    c.load_table("weird tbl", ["order", "group by"], src)
+    assert c.is_table_cached("weird tbl") is True
+    _, rows = c.execute_in_memory('SELECT "order", "group by" FROM "weird tbl"')
+    assert ("1", "a") in rows
+    c.close()
+    src.close()
+
+
+def test_disk_mode_uses_separate_read_connection(tmp_path, source_conn):
+    """Disk-mode reads go through a per-thread read connection, not the writer."""
+    c = CacheManager(db_path=tmp_path / "c.db", backup_interval=9999, in_memory=False)
+    c.load_table("users", ["name", "email"], source_conn)
+
+    _, rows = c.execute_in_memory("SELECT name FROM users ORDER BY name")
+    assert [r[0] for r in rows] == ["alice", "bob"]
+    assert len(c._read_conns) == 1
+    assert c._read_conns[0] is not c.connection  # dedicated read conn
+    c.close()
+
+
+def test_disk_mode_concurrent_reads(tmp_path, source_conn):
+    """Several reader threads each get their own connection and correct results."""
+    c = CacheManager(db_path=tmp_path / "c.db", backup_interval=9999, in_memory=False)
+    c.load_table("users", ["name"], source_conn)
+
+    results: list[int] = []
+    errors: list[Exception] = []
+
+    def reader() -> None:
+        try:
+            _, rows = c.execute_in_memory("SELECT name FROM users")
+            results.append(len(rows))
+        except Exception as e:  # noqa: BLE001
+            errors.append(e)
+
+    threads = [threading.Thread(target=reader) for _ in range(5)]
+    for t in threads:
+        t.start()
+    for t in threads:
+        t.join(5)
+
+    assert not errors
+    assert results == [2] * 5
+    assert len(c._read_conns) == 5  # one read connection per reader thread
+    c.close()
+
+
+def test_memory_mode_uses_shared_connection(cache, source_conn):
+    """In-memory mode can't share :memory: across connections — no read conns."""
+    cache.load_table("users", ["name"], source_conn)
+    cache.execute_in_memory("SELECT name FROM users")
+    assert cache._read_conns == []
+
+
 def test_disk_mode_reset_keeps_file(tmp_path, source_conn):
     db_path = tmp_path / "cache.db"
     c = CacheManager(db_path=db_path, backup_interval=9999, in_memory=False)