A bug in rolling back to a degraded object

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Hi Sage and all,

Another bug discovered during the proxy write teuthology testing is when rolling back to a degraded object. This doesn't seem specific to proxy write. I see a scenario like below from the log file:

- A rollback op comes in, and is enqueued.
- Several other ops on the same object come in, and are enqueued.
- The rollback op dispatches, and finds the object which it rollbacks to is degraded, then this op is pushbacked into a list to wait for the degraded object to recover.
- The later ops are handled and responded back to client.
- The degraded object recovers. The rollback op is enqueued again and finally responded to client.

This breaks the op order. A fix for this is to maintain a map to track the <source, destination> pair. And when an op on the source dispatches, if such a pair exists, queue the op in the destination's degraded waiting list. A drawback of this approach is that some entries in the ' waiting_for_degraded_object' list of the destination object may not be actually accessing the destination, but the source. Does this make sense?
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