Pubsub remove dictionary direct access in _get_in_topic_gossipsub_peers_from_minus in favor of get()

[hayotensor]

What was wrong?

Issue

#1124

There is a race condition in gossipsub.py. When a peer disconnects, it is removed from peer_protocol immediately. However, the heartbeat loop (which runs every 0.5s or so) might still have that peer in its list of candidates. The code is trying to access self.peer_protocol[peer_id] blindly, assuming that if a peer is known it must have protocol info, which leads to a crash when it’s missing.

Logs

Traceback (most recent call last):
  File "/home/bob/py-libp2p-subnet-gossip/.venv/lib/python3.10/site-packages/libp2p/tools/async_service/base.py", line 334, in _run_and_manage_task
    await task.run()
  File "/home/bob/py-libp2p-subnet-gossip/.venv/lib/python3.10/site-packages/libp2p/tools/async_service/trio_service.py", line 118, in run
    await self._async_fn(*self._async_fn_args)
  File "/home/bob/py-libp2p-subnet-gossip/.venv/lib/python3.10/site-packages/libp2p/pubsub/gossipsub.py", line 573, in heartbeat
    peers_to_graft, peers_to_prune = self.mesh_heartbeat()
  File "/home/bob/py-libp2p-subnet-gossip/.venv/lib/python3.10/site-packages/libp2p/pubsub/gossipsub.py", line 630, in mesh_heartbeat
    selected_peers = self._get_in_topic_gossipsub_peers_from_minus(
  File "/home/bob/py-libp2p-subnet-gossip/.venv/lib/python3.10/site-packages/libp2p/pubsub/gossipsub.py", line 825, in _get_in_topic_gossipsub_peers_from_minus
    gossipsub_peers_in_topic = {
  File "/home/bob/py-libp2p-subnet-gossip/.venv/lib/python3.10/site-packages/libp2p/pubsub/gossipsub.py", line 828, in <setcomp>
    if self.peer_protocol[peer_id]
KeyError: <libp2p.peer.id.ID (12D3KooWF963f4jiFX26xDKu7BrqtVYTx4Jk8rUQQUxwiJQjVFWH)>
2026-01-02 08:55:00,460 [ERROR] [__main__] Fatal error: Exceptions from Trio nursery (1 sub-exception)
  + Exception Group Traceback (most recent call last):
  |   File "/home/bob/py-libp2p-subnet-gossip/subnet/cli/run_node.py", line 304, in main
  |     trio.run(server.run)
  |   File "/home/bob/py-libp2p-subnet-gossip/.venv/lib/python3.10/site-packages/trio/_core/_run.py", line 2549, in run
  |     raise runner.main_task_outcome.error
  |   File "/home/bob/py-libp2p-subnet-gossip/subnet/server/server_v2.py", line 315, in run
  |     # Start the peer-store cleanup task, TTL
  |   File "/usr/lib/python3.10/contextlib.py", line 217, in __aexit__
  |     await self.gen.athrow(typ, value, traceback)
  |   File "/home/bob/py-libp2p-subnet-gossip/.venv/lib/python3.10/site-packages/libp2p/host/basic_host.py", line 357, in _run
  |     async with background_trio_service(network):
  |   File "/usr/lib/python3.10/contextlib.py", line 217, in __aexit__
  |     await self.gen.athrow(typ, value, traceback)
  |   File "/home/bob/py-libp2p-subnet-gossip/.venv/lib/python3.10/site-packages/libp2p/tools/async_service/trio_service.py", line 456, in background_trio_service
  |     async with trio.open_nursery() as nursery:
  |   File "/home/bob/py-libp2p-subnet-gossip/.venv/lib/python3.10/site-packages/trio/_core/_run.py", line 1125, in __aexit__
  |     raise combined_error_from_nursery
  | exceptiongroup.ExceptionGroup: Exceptions from Trio nursery (1 sub-exception)
  +-+---------------- 1 ----------------
    | Exception Group Traceback (most recent call last):
    |   File "/home/bob/py-libp2p-subnet-gossip/.venv/lib/python3.10/site-packages/libp2p/tools/async_service/trio_service.py", line 461, in background_trio_service
    |     yield manager
    |   File "/home/bob/py-libp2p-subnet-gossip/.venv/lib/python3.10/site-packages/libp2p/host/basic_host.py", line 374, in _run
    |     yield
    |   File "/home/bob/py-libp2p-subnet-gossip/subnet/server/server_v2.py", line 315, in run
    |     # Start the peer-store cleanup task, TTL
    |   File "/home/bob/py-libp2p-subnet-gossip/.venv/lib/python3.10/site-packages/trio/_core/_run.py", line 1125, in __aexit__
    |     raise combined_error_from_nursery
    | exceptiongroup.ExceptionGroup: Exceptions from Trio nursery (1 sub-exception)
    +-+---------------- 1 ----------------
      | Exception Group Traceback (most recent call last):
      |   File "/home/bob/py-libp2p-subnet-gossip/subnet/server/server_v2.py", line 342, in run
      |     subnet_info_tracker = SubnetInfoTracker(
      |   File "/usr/lib/python3.10/contextlib.py", line 217, in __aexit__
      |     await self.gen.athrow(typ, value, traceback)
      |   File "/home/bob/py-libp2p-subnet-gossip/.venv/lib/python3.10/site-packages/libp2p/tools/async_service/trio_service.py", line 456, in background_trio_service
      |     async with trio.open_nursery() as nursery:
      |   File "/home/bob/py-libp2p-subnet-gossip/.venv/lib/python3.10/site-packages/trio/_core/_run.py", line 1125, in __aexit__
      |     raise combined_error_from_nursery
      | exceptiongroup.ExceptionGroup: Exceptions from Trio nursery (1 sub-exception)
      +-+---------------- 1 ----------------
        | Exception Group Traceback (most recent call last):
        |   File "/home/bob/py-libp2p-subnet-gossip/.venv/lib/python3.10/site-packages/libp2p/tools/async_service/trio_service.py", line 461, in background_trio_service
        |     yield manager
        |   File "/home/bob/py-libp2p-subnet-gossip/subnet/server/server_v2.py", line 357, in run
        |     async with background_trio_service(gossipsub):
        |   File "/usr/lib/python3.10/contextlib.py", line 217, in __aexit__
        |     await self.gen.athrow(typ, value, traceback)
        |   File "/home/bob/py-libp2p-subnet-gossip/.venv/lib/python3.10/site-packages/libp2p/tools/async_service/trio_service.py", line 456, in background_trio_service
        |     async with trio.open_nursery() as nursery:
        |   File "/home/bob/py-libp2p-subnet-gossip/.venv/lib/python3.10/site-packages/trio/_core/_run.py", line 1125, in __aexit__
        |     raise combined_error_from_nursery
        | exceptiongroup.ExceptionGroup: Exceptions from Trio nursery (1 sub-exception)
        +-+---------------- 1 ----------------
          | Exception Group Traceback (most recent call last):
          |   File "/home/bob/py-libp2p-subnet-gossip/.venv/lib/python3.10/site-packages/libp2p/tools/async_service/trio_service.py", line 461, in background_trio_service
          |     yield manager
          |   File "/home/bob/py-libp2p-subnet-gossip/subnet/server/server_v2.py", line 358, in run
          |     logger.info("Pubsub and GossipSub services started.")
          |   File "/usr/lib/python3.10/contextlib.py", line 217, in __aexit__
          |     await self.gen.athrow(typ, value, traceback)
          |   File "/home/bob/py-libp2p-subnet-gossip/.venv/lib/python3.10/site-packages/libp2p/tools/async_service/trio_service.py", line 456, in background_trio_service
          |     async with trio.open_nursery() as nursery:
          |   File "/home/bob/py-libp2p-subnet-gossip/.venv/lib/python3.10/site-packages/trio/_core/_run.py", line 1125, in __aexit__
          |     raise combined_error_from_nursery
          | exceptiongroup.ExceptionGroup: Exceptions from Trio nursery (1 sub-exception)
          +-+---------------- 1 ----------------
            | Exception Group Traceback (most recent call last):
            |   File "/home/bob/py-libp2p-subnet-gossip/.venv/lib/python3.10/site-packages/libp2p/tools/async_service/trio_service.py", line 248, in run
            |     raise ExceptionGroup(
            | exceptiongroup.ExceptionGroup: Encountered multiple Exceptions:  (1 sub-exception)
            +-+---------------- 1 ----------------
              | Traceback (most recent call last):
              |   File "/home/bob/py-libp2p-subnet-gossip/.venv/lib/python3.10/site-packages/libp2p/tools/async_service/base.py", line 334, in _run_and_manage_task
              |     await task.run()
              |   File "/home/bob/py-libp2p-subnet-gossip/.venv/lib/python3.10/site-packages/libp2p/tools/async_service/trio_service.py", line 118, in run
              |     await self._async_fn(*self._async_fn_args)
              |   File "/home/bob/py-libp2p-subnet-gossip/.venv/lib/python3.10/site-packages/libp2p/pubsub/gossipsub.py", line 573, in heartbeat
              |     peers_to_graft, peers_to_prune = self.mesh_heartbeat()
              |   File "/home/bob/py-libp2p-subnet-gossip/.venv/lib/python3.10/site-packages/libp2p/pubsub/gossipsub.py", line 630, in mesh_heartbeat
              |     selected_peers = self._get_in_topic_gossipsub_peers_from_minus(
              |   File "/home/bob/py-libp2p-subnet-gossip/.venv/lib/python3.10/site-packages/libp2p/pubsub/gossipsub.py", line 825, in _get_in_topic_gossipsub_peers_from_minus
              |     gossipsub_peers_in_topic = {
              |   File "/home/bob/py-libp2p-subnet-gossip/.venv/lib/python3.10/site-packages/libp2p/pubsub/gossipsub.py", line 828, in <setcomp>
              |     if self.peer_protocol[peer_id]
              | KeyError: <libp2p.peer.id.ID (12D3KooWF963f4jiFX26xDKu7BrqtVYTx4Jk8rUQQUxwiJQjVFWH)>

How was it fixed?

In Pubsub._get_in_topic_gossipsub_peers_from_minus to use self.peer_protocol.get(peer_id) instead of direct dictionary access via self.peer_protocol[peer_id]. This safely ignores peers that are partially disconnected during the heartbeat cycle.

Summary of approach.

Use dict.get() to possibly return None, instead of directly trying to access the element in a dictionary.

To-Do

• Clean up commit history
• Add or update documentation related to these changes
• Add entry to the release notes

Cute Animal Picture

[sumanjeet0012]

@hayotensor
That is a good suggestion. We can either use the get method or a try–catch block when accessing the value directly. For this scenario, using the get method should be sufficient.

@seetadev
Please run the CI/CD pipeline.

[sumanjeet0012]

@hayotensor
Please create an issue for this PR and add the corresponding newsfragment file.

[hayotensor]

@sumanjeet0012 Added the newsfragment.

I kept the get method rather than using a try except due to more concise code