OSS test fix

[pbhandar2]

Summary:
Fix NvmCacheTest.EvictToNvmGet failing in OSS CI.

/home/runner/work/CacheLib/CacheLib/cachelib/allocator/nvmcache/tests/NvmCacheTests.cpp:252: Failure
Expected equality of these values:
  0
  nvm.getNumActiveHandles()
    Which is: 1

I0318 19:45:43.067236 184036 Driver.cpp:84] Driver: finish scheduler
I0318 19:45:43.067307 184036 Driver.cpp:86] Driver: finish scheduler successful
I0318 19:45:43.067613 184036 ThreadPoolJobScheduler.cpp:86] JobScheduler: join threads for reader_pool
I0318 19:45:43.069042 184036 ThreadPoolJobScheduler.cpp:90] JobScheduler: join threads for reader_pool successful
I0318 19:45:43.072589 184036 ThreadPoolJobScheduler.cpp:86] JobScheduler: join threads for writer_pool
I0318 19:45:43.074025 184036 ThreadPoolJobScheduler.cpp:90] JobScheduler: join threads for writer_pool successful
[  FAILED  ] NvmCacheTest.EvictToNvmGet (433 ms)
[----------] 1 test from NvmCacheTest (433 ms total)

[----------] Global test environment tear-down
[==========] 1 test from 1 test suite ran. (433 ms total)
[  PASSED  ] 0 tests.
[  FAILED  ] 1 test, listed below:
[  FAILED  ] NvmCacheTest.EvictToNvmGet

 1 FAILED TEST

Two bugs:

1. Unsigned underflow in loop index. The loop uses the i-- > 0 idiom which already decrements i before the body runs (values nKeys+99 down to 0). The extra index = i - 1 double-decremented, so on the last iteration (i=0), index = (unsigned)0 - 1 = UINT_MAX. This created a spurious lookup for a nonsensical key and skipped key1123.

• Fix: index = i.

2. Race between async NVM callback and handle count check. When a key is fetched from NVM, the async callback runs onGetComplete →removeFromFillMap → ~GetCtx → wakeUpWaiters → baton_.post(). The baton wakes the test thread, but GetCtx::it (the fill handle) is destroyed after the baton post, still inside the same callback. Until that destruction completes, the async thread's handle count is +1. If the test thread reaches getNumActiveHandles() before the callback fully returns, it observes the stale +1.

• Fix: call flushNvmCache() before checking handle counts. This calls scheduler_->finish() which busy-waits until all navy jobs (including the callback that destroys GetCtx::it) have fully completed.

Reviewed By: byahn0996

Differential Revision: D97184057

[meta-codesync]

@pbhandar2 has exported this pull request. If you are a Meta employee, you can view the originating Diff in D97184057.