$ cat >> $HGRCPATH << EOF

  > [storage]

  > dirstate-v2.slow-path=allow

  > EOF

Set up a v1 repo

  $ hg init repo

  $ cd repo

  $ echo a > a

  $ hg add a

  $ hg commit -m a

  $ hg debugrequires | grep dirstate

  [1]

  $ ls -1 .hg/dirstate*

  .hg/dirstate

Copy v1 dirstate

  $ cp .hg/dirstate $TESTTMP/dirstate-v1-backup

Upgrade it to v2

  $ hg debugupgraderepo -q --config format.use-dirstate-v2=1 --run | egrep 'added:|removed:'

     added: dirstate-v2

  $ hg debugrequires | grep dirstate

  dirstate-v2

  $ ls -1 .hg/dirstate*

  .hg/dirstate

  .hg/dirstate.* (glob)

Manually reset to dirstate v1 to simulate an incomplete dirstate-v2 upgrade

  $ rm .hg/dirstate*

  $ cp $TESTTMP/dirstate-v1-backup .hg/dirstate

There should be no errors, but a v2 dirstate should be written back to disk

  $ hg st

  $ ls -1 .hg/dirstate*

  .hg/dirstate

  .hg/dirstate.* (glob)

Corrupt the dirstate to see how the errors show up to the user

  $ echo "I ate your data" > .hg/dirstate

  $ hg st

  abort: working directory state appears damaged! (no-rhg !)

  (falling back to dirstate-v1 from v2 also failed) (no-rhg !)

  abort: Too little data for dirstate. (rhg !)

  [255]