[PATCH 2 of 2] requires: write requires file on commit

[Matt Mackall]

On Mon, 2015-05-04 at 13:27 -0500, Drew Gottlieb wrote:
> # HG changeset patch
> # User Drew Gottlieb <drgott at google.com>
> # Date 1429641493 25200
> #      Tue Apr 21 11:38:13 2015 -0700
> # Node ID 85c369520dd97bad369a0c4b924597f9e720077b
> # Parent  3f86e3b2548def52732e1f782e89297db8075e8b
> requires: write requires file on commit
> 
> Writing the requires file on commit makes it possible for changes to a
> repo's config to enable features that affect the repo's requirements. An
> example of this would be enabling manifestv2 in the future, which would then
> have future commits be written with the new manifest format, making it
> necessary for the requires file to indicate this.
> 
> This change also modifies test-inherit-mode to expect the requires file's
> mode to inherit from the store's mode after a commit. Previously, it only
> inherited from the store's mode on repo init, and the test assumed it would
> never be touched after this point.

I can't say I'm excited about this. Today, changing the requirements of
a repository is a big deal, and is intentionally done only at creation
time[1]. Among other things, it means people can create a repo with an
old Mercurial and be assured of continuing compatibility with that
version. Very important for repositories on shared disk.

Writing it unconditionally (and non-atomically!) on every commit is just
inviting people/extensions to break this. I think it should instead
continue to be an extension-specific headache.

[1] Largefiles breaks this rule; largefiles breaks everything and should
not be looked to as a role model.

-- 
Mathematics is the supreme nostalgia of our time.