feat: enrich DuplicateTaskError with structured attributes (#66)

docs/usage.md

@@ -103,6 +103,25 @@ except DuplicateTaskError:
     pass  # task is already queued or running
 ```
 
+`DuplicateTaskError` carries structured attributes describing the collision:
+
+```python
+try:
+    await my_task.kiq(user_id=42)
+except DuplicateTaskError as err:
+    logger.info(
+        "Skipped %s; already held by %s (key=%s)",
+        err.task_name,
+        err.holder_task_id,
+        err.key,
+    )
+```
+
+- `task_name` — name of the task that was rejected.
+- `key` — Redis lock key whose owner caused the rejection.
+- `holder_task_id` — `task_id` of the task currently holding the lock, or `None`
+  if it could not be retrieved.
+
 ## Per-task label overrides
 
 Labels can be set at the task level (applied to every call) or at call time.

src/taskiq_deduplication/middleware.py

@@ -31,7 +31,29 @@ _CACHED_KEY_LABEL = "__taskiq_dedup_cached_key"
 
 
 class DuplicateTaskError(Exception):
-    """Raised when a task with identical name and kwargs is already queued or running."""
+    """Raised when a task with identical name and kwargs is already queued or running.
+
+    Attributes:
+        task_name: Name of the task that was rejected.
+        key: Redis lock key whose owner caused the rejection.
+        holder_task_id: ``task_id`` of the task currently holding the lock, or
+            ``None`` if it could not be retrieved (e.g. the lock was released
+            between the failed acquisition and the lookup).
+    """
+
+    def __init__(
+        self,
+        task_name: str,
+        key: str,
+        holder_task_id: str | None = None,
+    ) -> None:
+        self.task_name = task_name
+        self.key = key
+        self.holder_task_id = holder_task_id
+        super().__init__(
+            f"Task {task_name!r} with the same arguments is already queued or "
+            f"running (key={key!r}, holder_task_id={holder_task_id!r})."
+        )
 
 
 class RedisDeduplicationMiddleware(TaskiqMiddleware):
@@ -202,13 +224,19 @@ class RedisDeduplicationMiddleware(TaskiqMiddleware):
         logger.debug("Acquiring lock %s for task %s", key, message.task_name)
         acquired = await self._redis.set(key, message.task_id, ex=ttl, nx=True)
         if not acquired:
+            holder_task_id = await self._redis.get(key)
+            if isinstance(holder_task_id, bytes):
+                holder_task_id = holder_task_id.decode()
             logger.warning(
-                "Duplicate task %s dropped (key=%s).",
+                "Duplicate task %s dropped (key=%s, holder_task_id=%s).",
                 message.task_name,
                 key,
+                holder_task_id,
             )
             raise DuplicateTaskError(
-                f"Task {message.task_name!r} with the same arguments is already queued or running."
+                task_name=message.task_name,
+                key=key,
+                holder_task_id=holder_task_id,
             )
 
         logger.debug("Lock %s acquired for task %s", key, message.task_name)

tests/test_middleware.py

@@ -146,6 +146,34 @@ class TestPreSend:
         with pytest.raises(DuplicateTaskError):
             await middleware.pre_send(make_message())
 
+    async def test_duplicate_error_carries_structured_attributes(
+        self, middleware, make_message
+    ):
+        holder = make_message(task_id="holder-task")
+        await middleware.pre_send(holder)
+        key = middleware._build_deduplication_key(holder)
+        with pytest.raises(DuplicateTaskError) as exc_info:
+            await middleware.pre_send(make_message(task_id="loser-task"))
+        err = exc_info.value
+        assert err.task_name == "my_task"
+        assert err.key == key
+        assert err.holder_task_id == "holder-task"
+        assert key in str(err)
+
+    async def test_duplicate_error_holder_none_when_lock_released_in_race(
+        self, make_message
+    ):
+        # The lock is released between the failed SET NX and the GET lookup, so
+        # GET returns None and holder_task_id is left unset.
+        redis = AsyncMock()
+        redis.set.return_value = False
+        redis.get.return_value = None
+        mw = RedisDeduplicationMiddleware(redis_url="redis://localhost")
+        mw._redis = redis
+        with pytest.raises(DuplicateTaskError) as exc_info:
+            await mw.pre_send(make_message())
+        assert exc_info.value.holder_task_id is None
+
     async def test_deduplication_disabled_label(self, middleware, make_message):
         msg1 = make_message(labels={DEDUP_LABEL: False})
         msg2 = make_message(labels={DEDUP_LABEL: False})