* test(storage): ensure multiple engines can run concurrently
* feat(storage): expose control over retention run
Fixes#15134.
This commit adds the ability to inject a functional option into a
storage.Engine for controlling when the retention enforcer can run.
Previously, retention enforcers ran on an interval; if you ran multiple
storage engines (as we do in some environments) then it was not possible
to coordinate when engines ran retention. Often they would synchronise
because they started at the same time.
This change will let you specify a blocking function to control when the
retention enforcer can run.
A simple function for serialising retention enforcement across multiple
storage engines could look like:
```go
var mu sync.Mutex
func f() (done func()) {
mu.Lock()
return func() { mu.Unlock() }
}
```
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