Unit test errors with 9p filesystem

[Ngoguey42]

I encountered fails and a stuck test when running the unit tests on WSL from a 9p drive mounted as follow:

Y:\ on /mnt/y type 9p (rw,noatime,dirsync,aname=drvfs;path=Y:\;uid=1000;gid=1000;symlinkroot=/mnt/,mmap,access=client,msize=65536,trans=fd,rfd=8,wfd=8)

  [OK]          io_array                0   fresh.
  [OK]          io_array                1   prefetch.
> [FAIL]        merge                   0   readonly in sequence.
  [FAIL]        merge                   1   readonly interleaved.
  [FAIL]        merge                   2   interleaved merge.
  [FAIL]        merge                   3   write at the end of merge.
  [OK]          merge                   4   write in log_async.
  ...           merge                   5   find while merging.  <----------------------- stuck
  [OK]          merge                   6   find in async without log.
  [OK]          merge                   7   find in async with log.
  [FAIL]        merge                   8   sync and find after log cleared.
  [FAIL]        merge                   9   merge during ro sync.
  [OK]          merge                  10   is_merging.
  [OK]          merge                  11   clear is not blocking.
  [OK]          merge                  12   `clear` aborts merge.
  [OK]          merge                  13   `close ~immediately` aborts merge.
  [OK]          live                    0   find (present).
  [OK]          live                    1   find (absent).
  [OK]          live                    2   replace.
  [OK]          live                    3   fail add (key).
  [OK]          live                    4   fail add (value).
  [OK]          live                    5   membership.
  [OK]          live                    6   clear and iter.
  [OK]          live                    7   clear and find.
  [OK]          live                    8   open after clear.
  [OK]          on restart              0   find (present).
  [OK]          on restart              1   find (absent).
  [OK]          on restart              2   replace.
  [OK]          on restart              3   membership.
  [OK]          on restart              4   fail restart readonly fresh.
  [OK]          on restart              5   in sync.
  [OK]          readonly                0   add.
  [OK]          readonly                1   read after clear.
  [FAIL]        readonly                2   snapshot isolation.
  [OK]          readonly                3   Readonly v after replace.
  [OK]          readonly                4   add not allowed.
  [OK]          readonly                5   fail read if no flush.
  [OK]          readonly                6   readonly v is in sync.
  [OK]          readonly                7   read values added in log before clear.
  [FAIL]        readonly                8   read values added in index before clear.
  [OK]          readonly                9   read old values in log after clear.
  [FAIL]        readonly               10   read old values in index after clear.
  [OK]          readonly               11   readonly open after clear.
  [OK]          readonly               12   race between sync and merge.
  [OK]          close                   0   close and reopen.
  [OK]          close                   1   find (absent).
  [OK]          close                   2   replace.
  [FAIL]        close                   3   open two instances, close one.
  [OK]          close                   4   close and reopen on readonly.
  [OK]          close                   5   non-close operations fail after close.
  [OK]          close                   6   double close.
  [OK]          close                   7   double restart.
  [OK]          close                   8   aborted merge.
  [OK]          filter                  0   filter none.
  [OK]          filter                  1   filter all.
  [OK]          filter                  2   filter one.
  [OK]          filter                  3   clone then filter.
  [OK]          filter                  4   filter then clone.
  [OK]          filter                  5   empty after filter+fresh.
  [OK]          flush_callback          0   close.
  [OK]          flush_callback          1   flush.
  [FAIL]        flush_callback          2   replace.
  [OK]          throttle                0   force merge.
  [OK]          throttle                1   implicit merge.

The same tests work when running from an ext4 filesystem.