upgrade: move descriptions and selection logic in individual classes

Our goal here is to get top level definition for all the format variants. Having
them defined outside of the function enabled other users of that logic.

They are two keys components of a format variant:

1) the name and various descriptions of its effect,

2) the code that checks if the repo is using this variant and if the config
   enables it.

That second items make us pick a class-based approach, since different variants
requires different code (even if in practice, many can reuse the same logic).
Each variants define its own class that is then used like a singleton.  The
class-based approach also clarify the definitions part a bit since each are
simple assignment in an indented block.

The 'fromdefault' and 'fromconfig' are respectively replaced by a class
attribute and a method to be called at the one place where "fromconfig"
matters.

Overall, they are many viable approach for this, but this is the one I picked.

mercurial/upgrade.py

@@ -140,93 +140,153 b' class improvement(object):'
         return hash(self.name)
 
 class formatvariant(improvement):
-    """an improvement subclass dedicated to repository format
+    """an improvement subclass dedicated to repository format"""
+    type = deficiency
+    ### The following attributes should be defined for each class:
+
+    # machine-readable string uniquely identifying this improvement. it will be
+    # mapped to an action later in the upgrade process.
+    name = None
 
-    extra attributes:
+    # message intended for humans explaining the improvement in more detail,
+    # including the implications of it ``deficiency`` types, should be worded
+    # in the present tense.
+    description = None
+
+    # message intended for humans explaining what an upgrade addressing this
+    # issue will do. should be worded in the future tense.
+    upgrademessage = None
 
-    fromdefault (``deficiency`` types only)
-       Boolean indicating whether the current (deficient) state deviates
-       from Mercurial's default configuration.
+    # value of current Mercurial default for new repository
+    default = None
+
+    def __init__(self):
+        raise NotImplementedError()
+
+    @staticmethod
+    def fromrepo(repo):
+        """current value of the variant in the repository"""
+        raise NotImplementedError()
 
-    fromconfig (``deficiency`` types only)
-       Boolean indicating whether the current (deficient) state deviates
-       from the current Mercurial configuration.
+    @staticmethod
+    def fromconfig(repo):
+        """current value of the variant in the configuration"""
+        raise NotImplementedError()
+
+class requirementformatvariant(formatvariant):
+    """formatvariant based on a 'requirement' name.
+
+    Many format variant are controlled by a 'requirement'. We define a small
+    subclass to factor the code.
     """
 
-    def __init__(self, name, description, upgrademessage, fromdefault,
-                 fromconfig):
-        super(formatvariant, self).__init__(name, deficiency, description,
-                                            upgrademessage)
-        self.fromdefault = fromdefault
-        self.fromconfig = fromconfig
+    # the requirement that control this format variant
+    _requirement = None
+
+    @staticmethod
+    def _newreporequirements(repo):
+        return localrepo.newreporequirements(repo)
+
+    @classmethod
+    def fromrepo(cls, repo):
+        assert cls._requirement is not None
+        return cls._requirement in repo.requirements
+
+    @classmethod
+    def fromconfig(cls, repo):
+        assert cls._requirement is not None
+        return cls._requirement in cls._newreporequirements(repo)
+
+class fncache(requirementformatvariant):
+    name = 'fncache'
+
+    _requirement = 'fncache'
+
+    default = True
+
+    description = _('long and reserved filenames may not work correctly; '
+                    'repository performance is sub-optimal')
+
+    upgrademessage = _('repository will be more resilient to storing '
+                       'certain paths and performance of certain '
+                       'operations should be improved')
+
+class dotencode(requirementformatvariant):
+    name = 'dotencode'
+
+    _requirement = 'dotencode'
+
+    default = True
+
+    description = _('storage of filenames beginning with a period or '
+                    'space may not work correctly')
+
+    upgrademessage = _('repository will be better able to store files '
+                       'beginning with a space or period')
+
+class generaldelta(requirementformatvariant):
+    name = 'generaldelta'
+
+    _requirement = 'generaldelta'
+
+    default = True
+
+    description = _('deltas within internal storage are unable to '
+                    'choose optimal revisions; repository is larger and '
+                    'slower than it could be; interaction with other '
+                    'repositories may require extra network and CPU '
+                    'resources, making "hg push" and "hg pull" slower')
+
+    upgrademessage = _('repository storage will be able to create '
+                       'optimal deltas; new repository data will be '
+                       'smaller and read times should decrease; '
+                       'interacting with other repositories using this '
+                       'storage model should require less network and '
+                       'CPU resources, making "hg push" and "hg pull" '
+                       'faster')
+
+class removecldeltachain(formatvariant):
+    name = 'removecldeltachain'
+
+    default = True
+
+    description = _('changelog storage is using deltas instead of '
+                    'raw entries; changelog reading and any '
+                    'operation relying on changelog data are slower '
+                    'than they could be')
+
+    upgrademessage = _('changelog storage will be reformated to '
+                       'store raw entries; changelog reading will be '
+                       'faster; changelog size may be reduced')
+
+    @staticmethod
+    def fromrepo(repo):
+        # Mercurial 4.0 changed changelogs to not use delta chains. Search for
+        # changelogs with deltas.
+        cl = repo.changelog
+        chainbase = cl.chainbase
+        return all(rev == chainbase(rev) for rev in cl)
+
+    @staticmethod
+    def fromconfig(repo):
+        return True
 
 def finddeficiencies(repo):
     """returns a list of deficiencies that the repo suffer from"""
-    newreporeqs = localrepo.newreporequirements(repo)
-
     deficiencies = []
 
     # We could detect lack of revlogv1 and store here, but they were added
     # in 0.9.2 and we don't support upgrading repos without these
     # requirements, so let's not bother.
 
-    if 'fncache' not in repo.requirements:
-        deficiencies.append(formatvariant(
-            name='fncache',
-            description=_('long and reserved filenames may not work correctly; '
-                          'repository performance is sub-optimal'),
-            upgrademessage=_('repository will be more resilient to storing '
-                             'certain paths and performance of certain '
-                             'operations should be improved'),
-            fromdefault=True,
-            fromconfig='fncache' in newreporeqs))
-
-    if 'dotencode' not in repo.requirements:
-        deficiencies.append(formatvariant(
-            name='dotencode',
-            description=_('storage of filenames beginning with a period or '
-                          'space may not work correctly'),
-            upgrademessage=_('repository will be better able to store files '
-                             'beginning with a space or period'),
-            fromdefault=True,
-            fromconfig='dotencode' in newreporeqs))
-
-    if 'generaldelta' not in repo.requirements:
-        deficiencies.append(formatvariant(
-            name='generaldelta',
-            description=_('deltas within internal storage are unable to '
-                          'choose optimal revisions; repository is larger and '
-                          'slower than it could be; interaction with other '
-                          'repositories may require extra network and CPU '
-                          'resources, making "hg push" and "hg pull" slower'),
-            upgrademessage=_('repository storage will be able to create '
-                             'optimal deltas; new repository data will be '
-                             'smaller and read times should decrease; '
-                             'interacting with other repositories using this '
-                             'storage model should require less network and '
-                             'CPU resources, making "hg push" and "hg pull" '
-                             'faster'),
-            fromdefault=True,
-            fromconfig='generaldelta' in newreporeqs))
-
-    # Mercurial 4.0 changed changelogs to not use delta chains. Search for
-    # changelogs with deltas.
-    cl = repo.changelog
-    for rev in cl:
-        chainbase = cl.chainbase(rev)
-        if chainbase != rev:
-            deficiencies.append(formatvariant(
-                name='removecldeltachain',
-                description=_('changelog storage is using deltas instead of '
-                              'raw entries; changelog reading and any '
-                              'operation relying on changelog data are slower '
-                              'than they could be'),
-                upgrademessage=_('changelog storage will be reformated to '
-                                 'store raw entries; changelog reading will be '
-                                 'faster; changelog size may be reduced'),
-                fromdefault=True,
-                fromconfig=True))
-            break
+    if not fncache.fromrepo(repo):
+        deficiencies.append(fncache)
+    if not dotencode.fromrepo(repo):
+        deficiencies.append(dotencode)
+    if not generaldelta.fromrepo(repo):
+        deficiencies.append(generaldelta)
+    if not removecldeltachain.fromrepo(repo):
+        deficiencies.append(removecldeltachain)
 
     return deficiencies
 
@@ -680,9 +740,9 b' def upgraderepo(ui, repo, run=False, opt'
         onlydefault = []
 
         for d in deficiencies:
-            if d.fromconfig:
+            if d.fromconfig(repo):
                 fromconfig.append(d)
-            elif d.fromdefault:
+            elif d.default:
                 onlydefault.append(d)
 
         if fromconfig or onlydefault: