simplify(pairing): address review findings — constants, TOCTOU, nesting

- Remove TOCTOU exists() check in _load(); rely on FileNotFoundError
- Define PAIRING_CODE_META_KEY and PAIRING_COMMAND_META_KEY constants
  in nanobot.pairing, replacing magic strings across base.py, slack.py,
  and builtin.py
- Flatten nested revoke logic in handle_pairing_command()
- Trim redundant docstring/comment noise in is_allowed() and generate_code()

nanobot/channels/base.py

@@ -11,6 +11,7 @@ from loguru import logger
 from nanobot.bus.events import InboundMessage, OutboundMessage
 from nanobot.bus.queue import MessageBus
 from nanobot.pairing import (
+    PAIRING_CODE_META_KEY,
     format_pairing_reply,
     generate_code,
     is_approved,
@@ -181,18 +182,7 @@ class BaseChannel(ABC):
         return bool(streaming) and type(self).send_delta is not BaseChannel.send_delta
 
     def is_allowed(self, sender_id: str) -> bool:
-        """Check if *sender_id* is permitted.
+        """Check sender permission: star > allowlist > pairing store > deny."""
-
-        Priority:
-        1. ``allowFrom: ["*"]`` → allow all.
-        2. ``allowFrom`` list → allow if sender_id is present.
-        3. Pairing store approved list → allow if previously approved.
-        4. Otherwise deny.
-
-        An empty ``allowFrom`` list does not cause a hard exit; instead it
-        defers to the pairing store so that unknown DM senders can request
-        access via a pairing code.
-        """
         if isinstance(self.config, dict):
             if "allow_from" in self.config:
                 allow_list = self.config.get("allow_from") or []
@@ -228,7 +218,7 @@ class BaseChannel(ABC):
                         channel=self.name,
                         chat_id=str(chat_id),
                         content=format_pairing_reply(code),
-                        metadata={"_pairing_code": code},
+                        metadata={PAIRING_CODE_META_KEY: code},
                     )
                 )
                 self.logger.info(

nanobot/channels/slack.py

@@ -18,7 +18,7 @@ from nanobot.bus.queue import MessageBus
 from nanobot.channels.base import BaseChannel
 from nanobot.config.paths import get_media_dir
 from nanobot.config.schema import Base
-from nanobot.pairing import format_pairing_reply, generate_code, is_approved
+from nanobot.pairing import PAIRING_CODE_META_KEY, format_pairing_reply, generate_code, is_approved
 from nanobot.utils.helpers import safe_filename, split_message
 
 
@@ -351,7 +351,7 @@ class SlackChannel(BaseChannel):
                         channel=self.name,
                         chat_id=chat_id,
                         content=reply,
-                        metadata={"_pairing_code": code},
+                        metadata={PAIRING_CODE_META_KEY: code},
                     )
                 )
             return

nanobot/command/builtin.py

@@ -548,14 +548,14 @@ async def cmd_history(ctx: CommandContext) -> OutboundMessage:
 
 async def cmd_pairing(ctx: CommandContext) -> OutboundMessage:
     """List, approve, deny or revoke pairing requests."""
-    from nanobot.pairing import handle_pairing_command
+    from nanobot.pairing import PAIRING_COMMAND_META_KEY, handle_pairing_command
 
     reply = handle_pairing_command(ctx.msg.channel, ctx.args)
     return OutboundMessage(
         channel=ctx.msg.channel,
         chat_id=ctx.msg.chat_id,
         content=reply,
-        metadata={"_pairing_command": True},
+        metadata={PAIRING_COMMAND_META_KEY: True},
     )
 
 

nanobot/pairing/__init__.py

@@ -13,6 +13,10 @@ from nanobot.pairing.store import (
     revoke,
 )
 
+# Metadata keys used by channels and commands to tag pairing-related messages.
+PAIRING_CODE_META_KEY = "_pairing_code"
+PAIRING_COMMAND_META_KEY = "_pairing_command"
+
 __all__ = [
     "approve_code",
     "deny_code",
@@ -24,4 +28,6 @@ __all__ = [
     "is_approved",
     "list_pending",
     "revoke",
+    "PAIRING_CODE_META_KEY",
+    "PAIRING_COMMAND_META_KEY",
 ]

nanobot/pairing/store.py

@@ -35,11 +35,11 @@ def _store_path() -> Path:
 
 def _load() -> dict[str, Any]:
     path = _store_path()
-    if not path.exists():
-        return {"approved": {}, "pending": {}}
     try:
         with open(path, encoding="utf-8") as f:
             data = json.load(f)
+    except FileNotFoundError:
+        return {"approved": {}, "pending": {}}
     except (json.JSONDecodeError, OSError):
         logger.warning("Corrupted pairing store, resetting")
         return {"approved": {}, "pending": {}}
@@ -82,8 +82,6 @@ def generate_code(
     with _LOCK:
         data = _load()
         _gc_pending(data)
-        # Collision probability is negligible (~1e-12 with 20 pending codes),
-        # so we skip an existence check for simplicity.
         raw = "".join(secrets.choice(_ALPHABET) for _ in range(_CODE_LENGTH))
         code = f"{raw[:4]}-{raw[4:]}"
 
@@ -236,21 +234,18 @@ def handle_pairing_command(channel: str, subcommand_text: str) -> str:
         return f"Pairing code `{arg}` not found or already expired"
 
     elif sub == "revoke":
-        if arg is None:
+        if len(parts) == 2:
-            return "Usage: `/pairing revoke <user_id>` or `/pairing revoke <channel> <user_id>`"
-        elif len(parts) == 2:
             return (
                 f"Revoked {arg} from {channel}"
                 if revoke(channel, arg)
                 else f"{arg} was not in the approved list for {channel}"
             )
-        elif len(parts) == 3:
+        if len(parts) == 3:
             return (
                 f"Revoked {parts[2]} from {arg}"
                 if revoke(arg, parts[2])
                 else f"{parts[2]} was not in the approved list for {arg}"
             )
-        else:
         return "Usage: `/pairing revoke <user_id>` or `/pairing revoke <channel> <user_id>`"
 
     return (