synchronizer: fix race during concurrent status updates - #10870
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We would incorrectly disconnect from a server when this race occurs:
1. We receive S1 (e.g. tx in mempool)
2. We request history H1 for S1 -> req1
3. We receive S2 (e.g. tx got mined) concurrently
4. We request history H2 for S2 -> req2
5. We receive H2 for S2 (before req1 got satisfied)
6. req1 receives H2 for current status (S2) from the server as well
7. _on_address_status schedules disconnect because H2 mismatches S1
8. S2 got already handled before, we never cancel the disconnect
```
DEBUG electrum.tests.toyserver.toyserver.ToyServerSession:toyserver.py:465 127.0.0.1:42170 disconnected
ERROR electrum.interface.Interface.[127.0.0.1:46059]:util.py:1231 Exception in run: SynchronizerFailure('timeout reached waiting for addr bcrt1qyt2w5jw5ktkr0ta96zuque93l4p86u3k2tj36cxcvs6gzl6wyj5q04p7y3: history still stale')
Traceback (most recent call last):
File "/home/runner/work/electrum/electrum/electrum/util.py", line 1225, in wrapper
return await func(*args, **kwargs)
File "/home/runner/work/electrum/electrum/electrum/interface.py", line 756, in wrapper_func
return await func(self, *args, **kwargs)
File "/home/runner/work/electrum/electrum/electrum/interface.py", line 782, in run
await self.open_session(ssl_context=ssl_context)
File "/home/runner/work/electrum/electrum/electrum/interface.py", line 1072, in open_session
async with self.taskgroup as group:
File "/opt/hostedtoolcache/Python/3.10.20/x64/lib/python3.10/site-packages/aiorpcx/curio.py", line 304, in __aexit__
await self.join()
File "/home/runner/work/electrum/electrum/electrum/util.py", line 1430, in join
task.result()
File "/home/runner/work/electrum/electrum/electrum/util.py", line 1564, in run_tasks_wrapper
await self._run_tasks(taskgroup=taskgroup)
File "/home/runner/work/electrum/electrum/electrum/synchronizer.py", line 79, in _run_tasks
async with taskgroup as group:
File "/opt/hostedtoolcache/Python/3.10.20/x64/lib/python3.10/site-packages/aiorpcx/curio.py", line 304, in __aexit__
await self.join()
File "/home/runner/work/electrum/electrum/electrum/util.py", line 1430, in join
task.result()
File "/home/runner/work/electrum/electrum/electrum/synchronizer.py", line 216, in disconnect_if_still_stale
raise SynchronizerFailure(f"timeout reached waiting for addr {addr}: history still stale")
electrum.synchronizer.SynchronizerFailure: timeout reached waiting for addr bcrt1qyt2w5jw5ktkr0ta96zuque93l4p86u3k2tj36cxcvs6gzl6wyj5q04p7y3: history still stale
```
If the status reverts to the one our stored history already has, _on_address_status returns early and the task disconnect stays active, so we drop a server that behaved correctly. Also cancel previous disconnect task before scheduling to prevent the following (rather unlikely) scenario: 1. We receive S1 -> req1 2. We receive S2 -> req2 3. req2 answers mismatching S2 -> disconnect task (_stale_histories[addr] = DT2) 4. We receive S1 again, req1 is still in flight, so (addr, S1) is still in requested_histories -> _on_address_status returns early and does not cancel DT2 5. req1 answers mismatching S1, S1 is the current status, so this is not obsolete -> we arm DT1 and assign _stale_histories[addr] = DT1, dropping DT2 6. A later status with matching history cancels _stale_histories[addr], which is DT1. DT2 is unreachable, fires, and disconnects a server that behaved correctly.
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August 19, 2026 08:22
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We would incorrectly disconnect from a server when this race occurs:
This causes the failing unittest in #10852: