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.

mercurial/localrepo.py

@@ -645,6 +645,8 b' def gathersupportedrequirements(ui):'
         engine = util.compengines[name]
         if engine.available() and 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 b' 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:
@@ -2943,7 +2950,9 b' 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):

mercurial/upgrade.py

@@ -325,10 +325,16 b' 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):

tests/test-repo-compengines.t

@@ -44,9 +44,9 b' A requirement specifying an unknown comp'
   $ cd zstd
   $ cat .hg/requires
   dotencode
-  exp-compression-zstd
   fncache
   generaldelta
+  revlog-compression-zstd
   revlogv1
   sparserevlog
   store