D3845: worker: support more return types in posix worker

[durin42 (Augie Fackler)]

durin42 added a comment.


  I'm not in love with pickle. Could we use json or cbor instead?

REPOSITORY
  rHG Mercurial

REVISION DETAIL
  https://phab.mercurial-scm.org/D3845

To: hooper, #hg-reviewers
Cc: durin42, mercurial-devel