feat(db): WAL-safe seismo_relay.db snapshot helper

sfm/db_snapshot.py

@@ -0,0 +1,24 @@
+"""
+db_snapshot.py — WAL-safe snapshot / restore helpers for seismo_relay.db and
+the waveform store, backing the /db/snapshot, /db/waveforms/recent.zip and
+/db/restore endpoints in server.py. Kept out of server.py so the logic is
+unit-testable without HTTP.
+"""
+from __future__ import annotations
+import sqlite3
+from pathlib import Path
+
+
+def snapshot_db(src_path: Path, dest_path: Path) -> Path:
+    """WAL-safe copy of a live SQLite DB via the SQLite online backup API.
+    Consistent even while the source is being written under WAL, and never
+    blocks the source's writers."""
+    src = sqlite3.connect(str(src_path))
+    dest = sqlite3.connect(str(dest_path))
+    try:
+        with dest:
+            src.backup(dest)
+    finally:
+        src.close()
+        dest.close()
+    return Path(dest_path)

tests/test_db_snapshot.py

@@ -0,0 +1,47 @@
+"""
+test_db_snapshot.py — unit tests for sfm/db_snapshot.py (snapshot/restore
+helpers behind the /db/snapshot, /db/waveforms/recent.zip, /db/restore routes).
+
+Run:
+    python -m pytest tests/test_db_snapshot.py -v
+"""
+from __future__ import annotations
+import os
+import sys
+import sqlite3
+from pathlib import Path
+
+sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
+
+
+def _make_db(path: Path, rows: int = 3) -> None:
+    conn = sqlite3.connect(str(path))
+    conn.execute("CREATE TABLE t (id INTEGER PRIMARY KEY, v TEXT)")
+    conn.executemany("INSERT INTO t (v) VALUES (?)", [(f"row{i}",) for i in range(rows)])
+    conn.commit()
+    conn.close()
+
+
+def _seed_event(db_path: Path, serial: str, fn: str, ts: str) -> None:
+    conn = sqlite3.connect(str(db_path))
+    conn.execute(
+        "INSERT INTO events (id, serial, waveform_key, timestamp, "
+        "blastware_filename, a5_pickle_filename, sidecar_filename, false_trigger) "
+        "VALUES (?, ?, ?, ?, ?, ?, ?, 0)",
+        (f"id-{fn}", serial, fn[:8], ts, fn, f"{fn}.a5.pkl", f"{fn}.sfm.json"),
+    )
+    conn.commit()
+    conn.close()
+
+
+def test_snapshot_db_produces_queryable_copy(tmp_path):
+    from sfm.db_snapshot import snapshot_db
+    src = tmp_path / "src.db"
+    _make_db(src, rows=5)
+    dest = tmp_path / "snap.db"
+    out = snapshot_db(src, dest)
+    assert out == dest and dest.exists()
+    conn = sqlite3.connect(str(dest))
+    n = conn.execute("SELECT COUNT(*) FROM t").fetchone()[0]
+    conn.close()
+    assert n == 5