feat: append-only event log with projector skeleton

chat/db/migrations/0003_event_log.sql

@@ -0,0 +1,10 @@
+CREATE TABLE event_log (
+    id INTEGER PRIMARY KEY,
+    branch_id INTEGER NOT NULL DEFAULT 1,
+    ts TEXT NOT NULL DEFAULT (datetime('now')),
+    kind TEXT NOT NULL,
+    payload_json TEXT NOT NULL,
+    superseded_by INTEGER REFERENCES event_log(id),
+    hidden INTEGER NOT NULL DEFAULT 0
+);
+CREATE INDEX idx_event_log_branch_kind ON event_log(branch_id, kind);

chat/eventlog/log.py

@@ -0,0 +1,38 @@
+from __future__ import annotations
+import json
+from dataclasses import dataclass
+from typing import Any, Iterator
+from sqlite3 import Connection
+
+
+@dataclass
+class Event:
+    id: int
+    branch_id: int
+    ts: str
+    kind: str
+    payload: dict[str, Any]
+    superseded_by: int | None
+    hidden: bool
+
+
+def append_event(conn: Connection, *, kind: str, payload: dict[str, Any], branch_id: int = 1) -> int:
+    cur = conn.execute(
+        "INSERT INTO event_log (branch_id, kind, payload_json) VALUES (?, ?, ?)",
+        (branch_id, kind, json.dumps(payload)),
+    )
+    return cur.lastrowid
+
+
+def read_events(conn: Connection, branch_id: int = 1, after_id: int = 0) -> Iterator[Event]:
+    cur = conn.execute(
+        "SELECT id, branch_id, ts, kind, payload_json, superseded_by, hidden "
+        "FROM event_log WHERE branch_id = ? AND id > ? AND hidden = 0 "
+        "AND superseded_by IS NULL ORDER BY id",
+        (branch_id, after_id),
+    )
+    for row in cur:
+        yield Event(
+            id=row[0], branch_id=row[1], ts=row[2], kind=row[3],
+            payload=json.loads(row[4]), superseded_by=row[5], hidden=bool(row[6]),
+        )

chat/eventlog/projector.py

@@ -0,0 +1,27 @@
+from __future__ import annotations
+from collections.abc import Callable
+from sqlite3 import Connection
+from .log import Event, read_events
+
+Handler = Callable[[Connection, Event], None]
+_REGISTRY: dict[str, Handler] = {}
+
+
+def on(kind: str):
+    def deco(fn: Handler) -> Handler:
+        _REGISTRY[kind] = fn
+        return fn
+    return deco
+
+
+def project(conn: Connection, branch_id: int = 1) -> None:
+    for event in read_events(conn, branch_id=branch_id):
+        h = _REGISTRY.get(event.kind)
+        if h:
+            h(conn, event)
+
+
+def apply_event(conn: Connection, event: Event) -> None:
+    h = _REGISTRY.get(event.kind)
+    if h:
+        h(conn, event)

tests/test_eventlog.py

@@ -0,0 +1,15 @@
+from chat.db.migrate import apply_migrations
+from chat.db.connection import open_db
+from chat.eventlog.log import append_event, read_events
+
+
+def test_append_and_read(tmp_path):
+    db = tmp_path / "t.db"
+    apply_migrations(db)
+    with open_db(db) as conn:
+        eid = append_event(conn, kind="test_kind", payload={"a": 1})
+        assert eid > 0
+        rows = list(read_events(conn))
+        assert len(rows) == 1
+        assert rows[0].kind == "test_kind"
+        assert rows[0].payload["a"] == 1