_lastPublishedByRef was a plain Dictionary<string, object> mutated inside
ShouldPublish, which runs on the PollGroupEngine onChange callback. The engine
runs one background Task per subscription, so a driver with two or more
subscriptions invokes ShouldPublish concurrently on separate threads. Concurrent
TryGetValue/indexer writes on a non-thread-safe Dictionary can corrupt internal
state, drop entries, or throw, crashing the poll loop.
Switch _lastPublishedByRef to ConcurrentDictionary<string, object>; its
TryGetValue and indexer-set operations are individually thread-safe, so the
deadband cache is now correct under concurrent multi-subscription publishing,
consistent with the lock-guarded sibling cache _lastWrittenByRef.
Add an xUnit + Shouldly regression test that runs 24 deadband-configured
single-tag subscriptions concurrently and asserts the poll loop survives without
faulting.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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