[PATCH 1 of 3] dirstate: change placeholder hash length to 20 bytes

[Augie Fackler]

> On Nov 10, 2016, at 14:29, Durham Goode <durham at fb.com> wrote:
> 
> On Nov 10, 2016 6:08 PM, Sean Farley <sean at farley.io> wrote:
> >
> > Durham Goode <durham at fb.com> writes:
> >
> > > # HG changeset patch
> > > # User Durham Goode <durham at fb.com>
> > > # Date 1478773042 28800
> > > #      Thu Nov 10 02:17:22 2016 -0800
> > > # Node ID 84d7edbc0712c8c71b5e32c9cc1ee04032b9eb2d
> > > # Parent  3fd53cc1aad882ac9191d7388885acdbbc2d7103
> > > dirstate: change placeholder hash length to 20 bytes
> > >
> > > Previously the new-node placeholder hash for manifests generated from the
> > > dirstate was a 21byte long string of "!" characters. Normal hashes are only 20
> > > bytes long.  This makes it complicated to implement more efficient manifest
> > > implementations which rely on the hashes being fixed length.
> > >
> > > Let's change this hash to just be 20 bytes long, and rely on the astronomical
> > > improbability of an actual hash being 20 "!" bytes in a row (just like we rely
> > > on no hash ever being the nullid).
> >
> > Will this cause a problem for a user that has a 21-length dirstate entry
> > then upgrades Mercurial?
> I don't think the dirstate file includes hashes, so I don't think there's any backwards compatibility concerns.

The 21-byte hashes never made it onto disk as of the last time I looked.