fix: only codes 10015/50027 mark a webhook dead; serialize cold creates

T1-1: _is_dead_webhook treated every NotFound/401/404 as dead, so a 404 for
another resource (10003 Unknown Channel from a deleted thread=) deleted the
picked webhook, possibly a third party's. key on discord's error code only.

T2-5: concurrent cold get_or_create calls each created a webhook (8 sends ->
8 webhooks, 7 orphaned against the cap). a cache miss now takes a lock and
re-checks the cache before restore-or-create.

Signed-off-by: disqualifier <dev@disqualifier.me>

src/dpy_webhooks/dpy_webhooks.py

@@ -60,6 +60,7 @@ through :meth:`clear` as usual.
 """
 from __future__ import annotations
 
+import asyncio
 import io
 import logging
 import random
@@ -71,7 +72,6 @@ log = logging.getLogger(__name__)
 
 MAX_WEBHOOKS_PER_CHANNEL = 10
 
-_DEAD_WEBHOOK_STATUSES = (401, 404)
 _UNKNOWN_WEBHOOK_CODE = 10015
 _INVALID_TOKEN_CODE = 50027
 
@@ -132,11 +132,8 @@ class InMemoryWebhookStore:
 
 def _is_dead_webhook(exc: discord.HTTPException) -> bool:
     """true when an HTTPException signals a webhook that no longer exists or has an invalid
-    token (the recover-and-recreate trigger)"""
-    if isinstance(exc, discord.NotFound):
-        return True
-    if exc.status in _DEAD_WEBHOOK_STATUSES:
-        return True
+    token (the recover-and-recreate trigger); keyed on discord's error code only - a bare
+    404/401 can be another resource (e.g. 10003 Unknown Channel for a deleted thread=)"""
     return exc.code in (_UNKNOWN_WEBHOOK_CODE, _INVALID_TOKEN_CODE)
 
 
@@ -203,6 +200,7 @@ class DPYWebhooks:
         self.default_name = default_name
         self._cache: dict[int, discord.Webhook] = {}
         self._rr: dict[int, dict] = {}
+        self._create_lock = asyncio.Lock()
 
     async def get_or_create(
         self,
@@ -212,15 +210,18 @@ class DPYWebhooks:
     ) -> discord.Webhook:
         """return the channel's managed webhook, restoring from store or creating if absent;
         raises WebhookCapacityError at the cap unless evict_oldest, and propagates Forbidden
-        when Manage Webhooks is missing"""
+        when Manage Webhooks is missing; concurrent cold calls share one create"""
         cached = self._cache.get(channel.id)
         if cached is not None:
             return cached
 
+        async with self._create_lock:
+            cached = self._cache.get(channel.id)
+            if cached is not None:
+                return cached
             restored = await self._restore_from_store(channel)
             if restored is not None:
                 return restored
-
             return await self._create(channel, evict_oldest=evict_oldest)
 
     async def get(self, channel: discord.TextChannel) -> "Optional[discord.Webhook]":