Compaction and clustering has a static instant time. So, when retried it may not have a new instant time, but the same.
So, lets walk through the scenario of what happens when compaction fails after synced to metadata table.
c1, c2, cc3( compaction commit), c4.
c1, c2, c4 on completion is applied to metadata table.
cc3 also gets synced to metadata table, but before committing to data table, it failed(process crashed). Its is a small window, but still a possibility.
So, from a timeline standpoint this is what looks like
c1 complete, c2 complete, cc3 inflight. c4 complete.
dc_c1, dc_c2, dc_cc3, dc_c4
Lets say there are few more commits went in.
c1 complete, c2 complete, cc3 inflight. c4 complete, c5 complete. c6 complete.
dc_c1, dc_c2, dc_cc3, dc_c4, dc_c5, dc_c6
Now, compaction in datatable is being re-attempted. So, first we rollback pending compaction in data table. So, this will trigger an upsert to metadata table. even thought this is a rollback, all updates to metadata table is an upsert which would result in a delta table.
And then, the compaction will be retried in datatable. when this is nearing completion, we try to upsert to metadata table. which will fail. because already we have a completed dc_cc3 in metadata table.
when a commit is being retried, we delete the completed instant and then proceed with upsert. So, when log blocks/files are merged together, final state will be intact and will ensure only those files added in 2nd attempt is returned and those added during 1st attempt is not returned (since there will be complimenting log block corresponding to a rollback).