Fix race in SLM master/cluster state listeners #59897
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This change fixes two possible race conditions in SLM related to how local master changes and cluster state events are observed. When implementing the `LocalNodeMasterListener` interface, it is only recommended to execute on a separate threadpool if the operations are heavy and would block the cluster state thread. SLM specified that the listeners should run in the Snapshot thread pool, but the operations in the listener were lightweight. This had the side effect of causing master changes to be delayed if the Snapshot threads were all busy and could also potentially cause the `onMaster` and `offMaster` calls to race if both were queued and then executed concurrently. Additionally, the `SnapshotLifecycleService` is also a `ClusterStateListener` and there is currently no order of operations guarantee between `LocalNodeMasterListeners` and `ClusterStateListeners` so this could lead to incorrect behavior. The resolution for these two issues is that the SnapshotRetentionService now specifies the `SAME` executor for its implementation of the `LocalNodeMasterListener` interface. The `SnapshotLifecycleService` is no longer a `LocalNodeMasterListener` and instead tracks local master changes in its `ClusterStateListner`.
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This change fixes two possible race conditions in SLM related to
how local master changes and cluster state events are observed. When
implementing the
LocalNodeMasterListenerinterface, it is onlyrecommended to execute on a separate threadpool if the operations are
heavy and would block the cluster state thread. SLM specified that the
listeners should run in the Snapshot thread pool, but the operations
in the listener were lightweight. This had the side effect of causing
master changes to be delayed if the Snapshot threads were all busy and
could also potentially cause the
onMasterandoffMastercalls torace if both were queued and then executed concurrently. Additionally,
the
SnapshotLifecycleServiceis also aClusterStateListenerandthere is currently no order of operations guarantee between
LocalNodeMasterListenersandClusterStateListenersso this couldlead to incorrect behavior.
The resolution for these two issues is that the
SnapshotRetentionService now specifies the
SAMEexecutor for itsimplementation of the
LocalNodeMasterListenerinterface. TheSnapshotLifecycleServiceis no longer aLocalNodeMasterListenerandinstead tracks local master changes in its
ClusterStateListner.Backport of #59801