Fix errors on table close with ongoing merges

The merging run finalise was closing input runs before aborting the
ongoing merge. When nothing else was holding the input runs open (like
the levels cache) then this was resulting in trying to delete the run
files while the run readers (in the merge) still had their own open file
handles for the same files. This resulted in test failures with the
MockFS and would have also failed on Windows filesystems (which do not
allow deleting files where there are still open file handles).

Author: dcoutts

src/Database/LSMTree/Internal/MergingRun.hs

@@ -218,8 +218,13 @@ unsafeNew mergeNumRuns mergeNumEntries knownCompleted state = do
         CompletedMerge r ->
           releaseRef r
         OngoingMerge rs m -> do
-          V.forM_ rs releaseRef
+          -- The RunReaders in the Merge keep their own file handles to the
+          -- run kopsFile open. We must close these handles *before* we release
+          -- the runs themselves, which will close and delete the files.
+          -- Otherwise we would be removing files that still have open handles
+          -- (which does not work on Windows, and is caught by the MockFS).
           Merge.abort m
+          V.forM_ rs releaseRef
 
 -- | Create references to the runs that should be queried for lookups.
 -- In particular, if the merge is not complete, these are the input runs.