Jason Tedor ac94253dce Clarify acquiring index shard permit
In previous work, we refactored the delay mechanism in index shard
operation permits to allow for async delaying of acquisition. This
refactoring made explicit when permit acquisition is disabled whereas
previously we were relying on an implicit condition, namely that all
permits were acquired by the thread trying to delay acquisition. When
using the implicit mechanism, we tried to acquire a permit and if this
failed, we returned a null releasable as an indication that our
operation should be queued. Yet, now we know when we are delayed and we
should not even try to acquire a permit. If we try to acquire a permit
and one is not available, we know that we are not delayed, and so
acquisition should be successful. If it is not successful, something is
deeply wrong. This commit takes advantage of this refactoring to
simplify the internal implementation.

Relates #24971
2017-05-30 16:22:17 -04:00
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2017-05-30 16:22:17 -04:00
2017-05-30 16:22:17 -04:00