Checkpoint — and the 20 GB WAL

1def checkpoint_passive(wal, db, readers):
2    floor = min((r.snapshot_end for r in readers), default=None)
3    for frame in wal.frames:
4        db.write_page(frame.page_number, frame.bytes)
5    if floor is None:
6        wal.rewind_to_start()  # next commit overwrites from f1
7    # else: cannot rewind past floor; WAL keeps growing
8    return

1def checkpoint_truncate(wal, db, readers):
2    wait_for_writers_to_finish()  # FULL semantics
3    for frame in wal.frames:      # transfer EVERY frame
4        db.write_page(frame.page_number, frame.bytes)
5    if any(r.snapshot_end is not None for r in readers):
6        return  # pinned reader → cannot rewind, file stays full
7    wal.rewind_to_start()
8    wal.truncate_file_to_zero_bytes()  # physical shrink

1def on_commit(wal, shm, threshold = 1000):
2    wal.append_commit_frame()         # page image + commit marker
3    if synchronous in (FULL, EXTRA):
4        wal.fsync()                   # durability point
5    shm.publish_new_end_mark(len(wal.frames))
6    if len(wal.frames) >= threshold:
7        # auto-checkpoint runs PASSIVE on this connection
8        checkpoint_passive(wal, db, readers)