feat: remember target in state.json for bare-dir resume

An archive dir that has been backed up once now records its target in
state.json, so later runs can be just --archive-dir. --target is still
required the first time, and still wins when given.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

1 files changed, 15 insertions, 3 deletions

diff --git a/tg_backup.py b/tg_backup.py
index e038b92..6c77878 100644
--- a/tg_backup.py
+++ b/tg_backup.py
@@ -188,7 +188,10 @@ async def run_backup(target, archive_dir):
                  "chat is in the session's entity cache.")
 
     state = load_state(archive_dir)
+    # Remember the target so later runs can resume with just --archive-dir.
+    state['target'] = target
     last_id = state['last_id']
+    save_state(archive_dir, state)
     print(f"Resuming from message ID: {last_id}")
     
     # reverse=True ensures oldest-to-newest processing. 
@@ -236,10 +239,14 @@ def main():
     args = parser.parse_args()
     if args.list_chats:
         asyncio.run(list_chats(args.archive_dir))
-    elif args.target:
-        asyncio.run(run_backup(args.target, args.archive_dir))
+        return
+    # An archive dir already backed up once knows its own target.
+    target = args.target or load_state(args.archive_dir).get('target')
+    if target:
+        asyncio.run(run_backup(target, args.archive_dir))
     else:
-        parser.error("--target is required (or use --list-chats)")
+        parser.error(f"--target is required the first time ({args.archive_dir}/state.json "
+                     "has no saved target), or use --list-chats")
 
 def _self_check():
     """Run with --self-check. Guards the path-traversal and state-write logic."""
@@ -280,6 +287,11 @@ def _self_check():
     assert not list(archive.glob('*.tmp')), "temp state file left behind"
     assert load_state(Path(tempfile.mkdtemp())) == {'last_id': 0}
 
+    # A saved target survives, and a fresh dir has none to fall back on.
+    save_state(archive, {'last_id': 9, 'target': '-1001234567890'})
+    assert load_state(archive).get('target') == '-1001234567890'
+    assert load_state(Path(tempfile.mkdtemp())).get('target') is None
+
     print("self-check OK")
 
 if __name__ == "__main__":