Keep append-only session revisions

src/session_store.py

@@ -217,6 +217,29 @@ def _write_agent_session_db_payload(
     updated_at = time.time()
     payload_json = json.dumps(payload, ensure_ascii=False, sort_keys=True)
     with _connect_agent_session_db(directory) as conn:
+        current = conn.execute(
+            """
+            select updated_at, snapshot_path, payload_json
+            from agent_sessions
+            where session_id = ?
+            """,
+            (session_id,),
+        ).fetchone()
+        if current is not None and current['payload_json'] != payload_json:
+            _write_agent_session_revision(
+                conn,
+                session_id=session_id,
+                updated_at=float(current['updated_at'] or 0.0),
+                snapshot_path=str(current['snapshot_path'] or ''),
+                payload_json=str(current['payload_json'] or ''),
+            )
+        _write_agent_session_revision(
+            conn,
+            session_id=session_id,
+            updated_at=updated_at,
+            snapshot_path=str(snapshot_path),
+            payload_json=payload_json,
+        )
         conn.execute(
             """
             insert into agent_sessions (
@@ -232,6 +255,39 @@ def _write_agent_session_db_payload(
         )
 
 
+def _write_agent_session_revision(
+    conn: sqlite3.Connection,
+    *,
+    session_id: str,
+    updated_at: float,
+    snapshot_path: str,
+    payload_json: str,
+) -> None:
+    if not payload_json:
+        return
+    latest = conn.execute(
+        """
+        select payload_json
+        from agent_session_revisions
+        where session_id = ?
+        order by revision_id desc
+        limit 1
+        """,
+        (session_id,),
+    ).fetchone()
+    if latest is not None and latest['payload_json'] == payload_json:
+        return
+    conn.execute(
+        """
+        insert into agent_session_revisions (
+            session_id, updated_at, snapshot_path, payload_json
+        )
+        values (?, ?, ?, ?)
+        """,
+        (session_id, updated_at, snapshot_path, payload_json),
+    )
+
+
 def _read_agent_session_db_payload(
     directory: Path,
     session_id: str,
@@ -292,6 +348,23 @@ def _connect_agent_session_db(directory: Path) -> sqlite3.Connection:
         on agent_sessions(updated_at)
         """
     )
+    conn.execute(
+        """
+        create table if not exists agent_session_revisions (
+            revision_id integer primary key autoincrement,
+            session_id text not null,
+            updated_at real not null,
+            snapshot_path text default '',
+            payload_json text not null
+        )
+        """
+    )
+    conn.execute(
+        """
+        create index if not exists idx_agent_session_revisions_session
+        on agent_session_revisions(session_id, revision_id)
+        """
+    )
     return conn