fix(cdk): prevent deadlock when main thread puts on full queue

The main thread is the sole consumer of the shared Queue(maxsize=10000).
When it also produces into the queue via emit_message() or log_message()
and the queue is full, queue.put() blocks indefinitely — deadlock.

Fix: capture the consumer thread ID at construction. On the consumer
thread use non-blocking put; overflow is buffered in a deque and drained
via consume_queue(), which the main thread already calls after every
queue item. Worker threads still use blocking put for back-pressure.

Co-Authored-By: bot_apk <apk@cognition.ai>

Author: devin-ai-integration[bot]

airbyte_cdk/sources/message/concurrent_repository.py

@@ -1,7 +1,8 @@
 # Copyright (c) 2025 Airbyte, Inc., all rights reserved.
 import logging
-import os
-from queue import Queue
+import threading
+from collections import deque
+from queue import Full, Queue
 from typing import Callable, Iterable
 
 from airbyte_cdk.models import AirbyteMessage, Level
@@ -13,35 +14,61 @@
 
 
 class ConcurrentMessageRepository(MessageRepository):
-    """
-    Message repository that immediately loads messages onto the queue processed on the
-    main thread. This ensures that messages are processed in the correct order they are
-    received. The InMemoryMessageRepository implementation does not have guaranteed
-    ordering since whether to process the main thread vs. partitions is non-deterministic
-    and there can be a lag between reading the main-thread and consuming messages on the
-    MessageRepository.
-
-    This is particularly important for the connector builder which relies on grouping
-    of messages to organize request/response, pages, and partitions.
+    """Message repository that loads messages onto the shared concurrent queue.
+
+    Messages are placed directly onto the queue consumed by the main thread.
+    This ensures correct ordering, which is especially important for the
+    connector builder's grouping of request/response, pages, and partitions.
+
+    Deadlock prevention: the main thread is the sole consumer of the queue.
+    If it also calls ``emit_message`` or ``log_message`` (e.g. when emitting
+    a final state via ``ensure_at_least_one_state_emitted``), a blocking
+    ``put`` on a full queue would deadlock because no other thread can drain
+    it.  To avoid this, the repository captures the consumer (main) thread
+    ID at construction time.  Calls from the main thread use non-blocking
+    ``put``; overflow is buffered in ``_pending`` and drained on the next
+    ``consume_queue`` call, which the main thread already invokes after
+    processing every queue item.  Worker threads continue to use blocking
+    ``put`` for normal back-pressure.
     """
 
     def __init__(self, queue: Queue[QueueItem], message_repository: MessageRepository):
         self._queue = queue
         self._decorated_message_repository = message_repository
+        self._consumer_thread_id: int = threading.get_ident()
+        self._pending: deque[AirbyteMessage] = deque()
 
     def emit_message(self, message: AirbyteMessage) -> None:
         self._decorated_message_repository.emit_message(message)
-        for message in self._decorated_message_repository.consume_queue():
-            self._queue.put(message)
+        for queued_message in self._decorated_message_repository.consume_queue():
+            self._put(queued_message)
 
     def log_message(self, level: Level, message_provider: Callable[[], LogMessage]) -> None:
         self._decorated_message_repository.log_message(level, message_provider)
-        for message in self._decorated_message_repository.consume_queue():
-            self._queue.put(message)
+        for queued_message in self._decorated_message_repository.consume_queue():
+            self._put(queued_message)
 
     def consume_queue(self) -> Iterable[AirbyteMessage]:
+        """Drain any messages buffered because the queue was full.
+
+        The main thread calls this after processing every queue item, so
+        buffered messages are delivered promptly without risking deadlock.
         """
-        This method shouldn't need to be called because as part of emit_message() we are already
-        loading messages onto the queue processed on the main thread.
+        while self._pending:
+            yield self._pending.popleft()
+
+    def _put(self, message: AirbyteMessage) -> None:
+        """Place a message on the shared queue.
+
+        On the consumer (main) thread a non-blocking put is used; if the
+        queue is full the message is appended to ``_pending`` instead of
+        blocking.  Worker threads use a normal blocking put so that
+        back-pressure is preserved.
         """
-        yield from []
+        if threading.get_ident() == self._consumer_thread_id:
+            try:
+                self._queue.put(message, block=False)
+            except Full:
+                self._pending.append(message)
+        else:
+            self._queue.put(message)

unit_tests/sources/message/test_concurrent_repository.py

@@ -0,0 +1,178 @@
+#
+# Copyright (c) 2025 Airbyte, Inc., all rights reserved.
+#
+
+import threading
+from queue import Queue
+
+import pytest
+
+from airbyte_cdk.models import (
+    AirbyteControlConnectorConfigMessage,
+    AirbyteControlMessage,
+    AirbyteMessage,
+    AirbyteStateMessage,
+    AirbyteStateType,
+    Level,
+    OrchestratorType,
+    Type,
+)
+from airbyte_cdk.sources.message.concurrent_repository import ConcurrentMessageRepository
+from airbyte_cdk.sources.message.repository import InMemoryMessageRepository
+
+
+def _make_state_message(stream_name: str = "test_stream") -> AirbyteMessage:
+    return AirbyteMessage(
+        type=Type.STATE,
+        state=AirbyteStateMessage(type=AirbyteStateType.STREAM, data={"stream_name": stream_name}),
+    )
+
+
+def _make_control_message() -> AirbyteMessage:
+    return AirbyteMessage(
+        type=Type.CONTROL,
+        control=AirbyteControlMessage(
+            type=OrchestratorType.CONNECTOR_CONFIG,
+            emitted_at=0,
+            connectorConfig=AirbyteControlConnectorConfigMessage(config={"key": "value"}),
+        ),
+    )
+
+
+@pytest.fixture()
+def small_queue() -> Queue:
+    return Queue(maxsize=2)
+
+
+@pytest.fixture()
+def repo(small_queue: Queue) -> ConcurrentMessageRepository:
+    return ConcurrentMessageRepository(small_queue, InMemoryMessageRepository())
+
+
+def test_emit_message_puts_on_queue_when_space_available() -> None:
+    """When the queue has space, emit_message places the message directly on it."""
+    queue: Queue = Queue(maxsize=100)
+    repo = ConcurrentMessageRepository(queue, InMemoryMessageRepository())
+
+    msg = _make_control_message()
+    repo.emit_message(msg)
+
+    assert not queue.empty()
+    assert queue.get_nowait() == msg
+
+
+def test_emit_message_buffers_when_queue_full_on_consumer_thread(
+    small_queue: Queue, repo: ConcurrentMessageRepository
+) -> None:
+    """When called from the consumer thread with a full queue, the message goes to _pending."""
+    small_queue.put("filler_1")
+    small_queue.put("filler_2")
+    assert small_queue.full()
+
+    msg = _make_state_message()
+    repo.emit_message(msg)
+
+    assert len(repo._pending) == 1
+    assert repo._pending[0] == msg
+
+
+def test_consume_queue_drains_pending_buffer(
+    small_queue: Queue, repo: ConcurrentMessageRepository
+) -> None:
+    """consume_queue yields messages that were buffered due to a full queue."""
+    small_queue.put("filler_1")
+    small_queue.put("filler_2")
+
+    msg1 = _make_state_message("stream_1")
+    msg2 = _make_state_message("stream_2")
+    repo.emit_message(msg1)
+    repo.emit_message(msg2)
+
+    drained = list(repo.consume_queue())
+    assert drained == [msg1, msg2]
+    assert len(repo._pending) == 0
+
+
+def test_consume_queue_empty_when_no_pending(repo: ConcurrentMessageRepository) -> None:
+    """consume_queue yields nothing when there are no pending messages."""
+    assert list(repo.consume_queue()) == []
+
+
+def test_log_message_buffers_when_queue_full_on_consumer_thread(
+    small_queue: Queue, repo: ConcurrentMessageRepository
+) -> None:
+    """log_message also uses non-blocking put on the consumer thread."""
+    small_queue.put("filler_1")
+    small_queue.put("filler_2")
+
+    repo.log_message(Level.INFO, lambda: {"message": "test log"})
+
+    assert len(repo._pending) == 1
+
+
+def test_worker_thread_uses_blocking_put() -> None:
+    """Worker threads (non-consumer) should use blocking put for back-pressure."""
+    queue: Queue = Queue(maxsize=1)
+    repo = ConcurrentMessageRepository(queue, InMemoryMessageRepository())
+
+    queue.put("filler")
+
+    worker_started = threading.Event()
+    worker_done = threading.Event()
+
+    def worker_emit() -> None:
+        worker_started.set()
+        repo.emit_message(_make_state_message())
+        worker_done.set()
+
+    t = threading.Thread(target=worker_emit, daemon=True)
+    t.start()
+
+    worker_started.wait(timeout=1.0)
+    assert not worker_done.wait(timeout=0.5), "Worker should be blocked on full queue"
+
+    queue.get()
+    assert worker_done.wait(timeout=2.0), "Worker should complete after queue space freed"
+    t.join(timeout=2.0)
+
+
+def test_main_thread_does_not_deadlock_on_full_queue(
+    small_queue: Queue, repo: ConcurrentMessageRepository
+) -> None:
+    """Simulate the deadlock: main thread emits on a full queue.
+
+    Without the fix this would hang forever because the main thread
+    (sole consumer) blocks on queue.put() and nobody drains the queue.
+    """
+    small_queue.put("record_1")
+    small_queue.put("record_2")
+    assert small_queue.full()
+
+    state_msg = _make_state_message("contact_lists")
+    repo.emit_message(state_msg)  # Must return immediately
+
+    pending = list(repo.consume_queue())
+    assert len(pending) == 1
+    assert pending[0] == state_msg
+
+
+def test_ordering_preserved_across_queue_and_pending(
+    small_queue: Queue, repo: ConcurrentMessageRepository
+) -> None:
+    """Messages maintain order: first queued directly, overflow to pending."""
+    msg1 = _make_state_message("stream_1")
+    msg2 = _make_state_message("stream_2")
+    msg3 = _make_state_message("stream_3")
+
+    repo.emit_message(msg1)
+    repo.emit_message(msg2)
+    assert small_queue.qsize() == 2
+
+    repo.emit_message(msg3)
+    assert len(repo._pending) == 1
+
+    from_queue = [small_queue.get_nowait(), small_queue.get_nowait()]
+    from_pending = list(repo.consume_queue())
+    all_messages = from_queue + from_pending
+
+    assert all_messages == [msg1, msg2, msg3]