tree 16670cb1920a7ed854ac978e0c9e1a7fbb471871
parent 21ec281f3e53e72edd77ea3ef142c953ebe0c465
author Dave Borowitz <dborowitz@google.com> 1499278776 -0400
committer Dave Borowitz <dborowitz@google.com> 1500306995 -0400

RefDirectory: Throw exception if CAS of packed ref list fails

The contents of the packedRefList AtomicReference should never differ
from what we expect prior to writing, because this segment of the code
is protected by the packed-refs lock file on disk. If it does happen,
whether due to programmer error or a rogue process not respecting the
locking protocol, it's better to let the caller know than to silently
drop the whole commit operation on the floor.

The existing concurrentOnlyOneWritesPackedRefs test is inherently
nondeterministic as written, and was already about 6% flaky as measured
by bazel:

  $ bazel test --runs_per_test=200 //org.eclipse.jgit.test:org_eclipse_jgit_internal_storage_file_GcPackRefsTest
  ...
  INFO: Elapsed time: 42.608s, Critical Path: 10.35s
  //org.eclipse.jgit.test:org_eclipse_jgit_internal_storage_file_GcPackRefsTest FAILED in 12 out of 200 in 1.6s
    Stats over 200 runs: max = 1.6s, min = 1.1s, avg = 1.3s, dev = 0.1s

This flakiness was caused by the assumption that exactly one of the 2
threads would fail, when both might actually succeed in practice due to
racing on the compare-and-swap.

For whatever reason, this change affected the interleaving behavior in
such a way that the flakiness jumped to around 50%. Making the
interleaving of the test fully deterministic is beyond the scope of this
change, but a simple tweak to the assertion is enough to make it pass
consistently 200+ times both before and after this change.

Change-Id: I5ff4dc39ee05bda88d47909acb70118f3d0c8f74