[PATCH 1 of 2 "V3] compression: introduce an official `zstd-revlog` requirement

[Augie Fackler]

Greg: ping on these? I'm aware you had some unease on this topic, and
I'd like to figure out if we should ship this. Thanks!

On Tue, Apr 09, 2019 at 12:06:00PM +0200, Pierre-Yves David wrote:
> # HG changeset patch
> # User Pierre-Yves David <pierre-yves.david at octobus.net>
> # Date 1553707623 -3600
> #      Wed Mar 27 18:27:03 2019 +0100
> # Node ID 8ce788576265eb8b84cb697ea2e09b2246fb7b81
> # Parent  456c37433c433ee59d321315da24eb944e495f08
> # EXP-Topic zstd-revlog
> # Available At https://bitbucket.org/octobus/mercurial-devel/
> #              hg pull https://bitbucket.org/octobus/mercurial-devel/ -r 8ce788576265
> compression: introduce an official `zstd-revlog` requirement
>
> This requirement supersedes `exp-compression-zstd`. However, we keep support for
> the old requirement.
>
> Strictly speaking, we do not need to add a new requirement, there are no logic
> change making "new" repo incompatible with mercurial client using a version
> that support `exp-compression-zstd`.
>
> The choice to introduce a new requirement is motivated by the following:
>
> * The previous requirement was explicitly "experimental". Using it by default
>   could confuse users.
>
> * adding support for a hypothetical third compression engine will requires new
>   code, and will comes with its own requirement tool.
>
> * We won't use it as the default for a while since I do not think we support
>   zstd on all platform. I can imagine we'll gain another (unrelated but on my
>   default) requirement by the time we turn this zstd by default.
>
> diff --git a/mercurial/localrepo.py b/mercurial/localrepo.py
> --- a/mercurial/localrepo.py
> +++ b/mercurial/localrepo.py
> @@ -645,6 +645,8 @@ def gathersupportedrequirements(ui):
>          engine = util.compengines[name]
>          if engine.revlogheader():
>              supported.add(b'exp-compression-%s' % name)
> +            if engine.name() == 'zstd':
> +                supported.add(b'revlog-compression-zstd')
>
>      return supported
>
> @@ -794,8 +796,13 @@ def resolverevlogstorevfsoptions(ui, req
>          options[b'maxchainlen'] = maxchainlen
>
>      for r in requirements:
> -        if r.startswith(b'exp-compression-'):
> -            options[b'compengine'] = r[len(b'exp-compression-'):]
> +        # we allow multiple compression engine requirement to co-exist because
> +        # strickly speaking, revlog seems to support mixed compression style.
> +        #
> +        # The compression used for new entries will be "the last one"
> +        prefix = r.startswith
> +        if prefix('revlog-compression-') or prefix('exp-compression-'):
> +            options[b'compengine'] = r.split('-', 2)[2]
>
>      options[b'zlib.level'] = ui.configint(b'storage', b'revlog.zlib.level')
>      if options[b'zlib.level'] is not None:
> @@ -2929,7 +2936,9 @@ def newreporequirements(ui, createopts):
>                                   'compression engines'))
>
>      # zlib is the historical default and doesn't need an explicit requirement.
> -    if compengine != 'zlib':
> +    elif compengine == 'zstd':
> +        requirements.add('revlog-compression-zstd')
> +    elif compengine != 'zlib':
>          requirements.add('exp-compression-%s' % compengine)
>
>      if scmutil.gdinitconfig(ui):
> diff --git a/mercurial/upgrade.py b/mercurial/upgrade.py
> --- a/mercurial/upgrade.py
> +++ b/mercurial/upgrade.py
> @@ -325,10 +325,16 @@ class compressionengine(formatvariant):
>
>      @classmethod
>      def fromrepo(cls, repo):
> +        # we allow multiple compression engine requirement to co-exist because
> +        # strickly speaking, revlog seems to support mixed compression style.
> +        #
> +        # The compression used for new entries will be "the last one"
> +        compression = 'zlib'
>          for req in repo.requirements:
> -            if req.startswith('exp-compression-'):
> -                return req.split('-', 2)[2]
> -        return 'zlib'
> +            prefix = req.startswith
> +            if prefix('revlog-compression-') or prefix('exp-compression-'):
> +                compression = req.split('-', 2)[2]
> +        return compression
>
>      @classmethod
>      def fromconfig(cls, repo):
> diff --git a/tests/test-repo-compengines.t b/tests/test-repo-compengines.t
> --- a/tests/test-repo-compengines.t
> +++ b/tests/test-repo-compengines.t
> @@ -44,9 +44,9 @@ A requirement specifying an unknown comp
>    $ cd zstd
>    $ cat .hg/requires
>    dotencode
> -  exp-compression-zstd
>    fncache
>    generaldelta
> +  revlog-compression-zstd
>    revlogv1
>    sparserevlog
>    store
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