Executor QoS - #4967
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QoS: Tasks are categorised so that their relative arrival/service rate can be tracked by 7-bit decaying counters. Work may be assigned by a mixture of highest priority task or least-serviced queue by a deficit round-robin scheme. This prevents e.g. cache flushes from dominating executor threads. Incremental Execution: Implementation fully implements enriched ASYNC/INCR ExecutionContext semantics, so that tasks may be decomposed into multiple steps with optional ATOMIC isolation. This permits sync points in particular to be processed in smaller batches, allowing smaller non-conflicting tasks to interleave. Also Improve: * Continuation tasks allow declaring work that logically continues prior work, and that must execute before treating the initiating task as complete (distinct from notifying any callback); Continuation or partial incremental tasks that fail with ATOMIC visibility mark a key as unavailable to prevent reading inconsistent state. * Consequences of tasks are submitted as part of the task's completion * Consequences of durable tasks that fail are cancelled before they are submitted * Handle partial task failure: propagate failure to counting results instead of silently decrementing, and abandon dependent continuations. * ExclusiveExecutor releases its lock eagerly, opening up tryExecuteImmediately to more executions * Memtables blocking an Accord durability status update will be flushed periodically * NoSpamLogger.NoDuplicateSpamLogStatement: rate-limit per distinct exception * Claude-generated formal specification of execution semantics under formalise/accord/execution * AutoRepair shutdown executors simultaneously, and terminate running sessions * Shutdown UDF and SASI executors * FailureDetector: pass Double.MAX_VALUE on forceConvict to ensure listeners process an explicit shutdown Also Fix: * Data races updating Task.info (between executing thread and executor lock owner) * AccordExecutor/ThreadLocalTaskRunner lock accounting could be corrupted on shutdown * SignalLock incorrectly limited signals to MAX_THREADS not MAX_SIGNAL_COUNT (throwing an exception) * SignalLock invisible bug in tryAcquireAsyncInAwaitLoop * Reject negative histogram values (LogLinearHistogram, LogLinearDecayingHistograms), and ensure callers do not submit * RepairOption.permitNoQuorum() returned repairAccord instead of permitNoQuorum. * FailureDetector: do not convict endpoints already marked down, since Accord may become active while marked !isAlive; patch by Benedict; reviewed by Ariel Weisberg for CASSANDRA-21662
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