stats: fix race condition when refreshing cache #51805
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First commit is #51780 |
RaduBerinde
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Jul 22, 2020
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This commit fixes a race condition that was introduced in cockroachdb#51616. The race condition existed because it was possible for two refreshes to be triggered in close succession, and there was no mechanism to prevent the first refresh from overwriting the second if it was delayed. We fix the race condition by introducing two boolean flags into each cache entry: refreshing and mustRefreshAgain. If refreshing is true, that indicates that the current statistics for this table are stale, and we are in the process of fetching the updated stats from the database. In the mean time, other callers can use the stale stats and do not need to wait. If a goroutine tries to perform a refresh when a refresh is already in progress, it will see that refreshing=true and will set the mustRefreshAgain flag to true before returning. When the original goroutine that was performing the refresh returns from the database and sees that mustRefreshAgain=true, it will trigger another refresh. Informs cockroachdb#50863 Release note: None
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This was referenced Jul 31, 2020
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This commit fixes a race condition that was introduced in #51616.
The race condition existed because it was possible for two refreshes
to be triggered in close succession, and there was no mechanism to
prevent the first refresh from overwriting the second if it was
delayed.
We fix the race condition by introducing two boolean flags into each
cache entry:
refreshingandmustRefreshAgain. Ifrefreshingis true,that indicates that the current statistics for this table are stale,
and we are in the process of fetching the updated stats from the database.
In the mean time, other callers can use the stale stats and do not need to
wait.
If a goroutine tries to perform a refresh when a refresh is already
in progress, it will see that
refreshing=trueand will set themustRefreshAgainflag totruebefore returning. When the originalgoroutine that was performing the refresh returns from the database and
sees that
mustRefreshAgain=true, it will trigger another refresh.Informs #50863
Release note: None