Summary

MirroringSampler::threadSafeRandom() uses a synchronized_value<PseudoRandom> (mutex-protected) to generate a random number on every operation for mirror sampling decisions. With the default mirroring rate of 1%, 99% of calls determine "don't mirror" — but every call still acquires and releases a mutex. 

This multipatch using the below proposed fix showed a .34% improvement in our 100 read workload. 

SERVER-75834 identified that PseudoRandom is not thread-safe and noted two valid solutions: a synchronization primitive, or thread-local instances. The original fix chose the mutex approach. The thread-local approach is cheaper and equally correct.

Proposed Fix: 

See proposed change in this patch. 

Replace the synchronized_value<PseudoRandom> with a thread_local PseudoRandom:

  static int threadSafeRandom() {
      static thread_local PseudoRandom random{SecureRandom{}.nextInt64()};
      return random.nextInt32(defaultRandomMax());
  }

Each service executor thread gets its own independently-seeded PRNG instance. No shared state means no data race and no synchronization needed.

Precedent

The codebase already uses this exact pattern in multiple places:
  - src/mongo/db/query/query_stats/rate_limiting.cpp:80 — thread_local PseudoRandom for query stats sampling
  - src/mongo/db/query/random_utils.cpp:35 — thread_local PseudoRandom for query random utilities