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The `disconnect.stop_on_client_after` feature is implemented as a loop on the client that's intended to wait on the shortest timeout of all the allocations on the node and then check whether the interval since the last heartbeat has been longer than the timeout. It uses a buffered channel of allocations written and read from the same goroutine to push "stops" from the timeout expiring to the next pass through the loop. Unfortunately if there are multiple allocations that need to be stopped in the same timeout event, or even if a previous event has not yet been dequeued, then sending on the channel will block and the entire goroutine deadlocks itself. While fixing this, I also discovered that the `stop_on_client_after` and heartbeat loops can synchronize in a pathological way that extends the `stop_on_client_after` window. If a heartbeat fails close to the beginning of the shortest `stop_on_client_after` window, the loop will end up waiting until almost 2x the intended wait period. While fixing both of those issues, I discovered that the existing tests had a bug such that we were asserting that an allocrunner was being destroyed when it had already exited. This commit includes the following: * Rework the watch loop so that we handle the stops in the same case as the timer expiration, rather than using a channel in the method scope. * Remove the alloc intervals map field from the struct and keep it in the method scope, in order to discourage writing racy tests that read its value. * Reset the timer whenever we receive a heartbeat, which forces the two intervals to synchronize correctly. * Minor refactoring of the disconnect timeout lookup to improve brevity. Fixes: https://github.com/hashicorp/nomad/issues/24679 Ref: https://hashicorp.atlassian.net/browse/NMD-407