upgrade: extract the checking of target requirements change

This logic is fairly independant, lets move it out of the main function.

Differential Revision: https://phab.mercurial-scm.org/D9485

mercurial/upgrade.py

@@ -1,357 +1,320 b''
 # upgrade.py - functions for in place upgrade of Mercurial repository
 #
 # Copyright (c) 2016-present, Gregory Szorc
 #
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2 or any later version.
 
 from __future__ import absolute_import
 
 from .i18n import _
 from . import (
     error,
     hg,
     localrepo,
     pycompat,
 )
 
 from .upgrade_utils import (
     actions as upgrade_actions,
     engine as upgrade_engine,
 )
 
 allformatvariant = upgrade_actions.allformatvariant
 
 # search without '-' to support older form on newer client.
 #
 # We don't enforce backward compatibility for debug command so this
 # might eventually be dropped. However, having to use two different
 # forms in script when comparing result is anoying enough to add
 # backward compatibility for a while.
 legacy_opts_map = {
     b'redeltaparent': b're-delta-parent',
     b'redeltamultibase': b're-delta-multibase',
     b'redeltaall': b're-delta-all',
     b'redeltafulladd': b're-delta-fulladd',
 }
 
 
 def upgraderepo(
     ui,
     repo,
     run=False,
     optimize=None,
     backup=True,
     manifest=None,
     changelog=None,
     filelogs=None,
 ):
     """Upgrade a repository in place."""
     if optimize is None:
         optimize = []
     optimize = {legacy_opts_map.get(o, o) for o in optimize}
     repo = repo.unfiltered()
 
     revlogs = set(upgrade_engine.UPGRADE_ALL_REVLOGS)
     specentries = (
         (upgrade_engine.UPGRADE_CHANGELOG, changelog),
         (upgrade_engine.UPGRADE_MANIFEST, manifest),
         (upgrade_engine.UPGRADE_FILELOGS, filelogs),
     )
     specified = [(y, x) for (y, x) in specentries if x is not None]
     if specified:
         # we have some limitation on revlogs to be recloned
         if any(x for y, x in specified):
             revlogs = set()
             for upgrade, enabled in specified:
                 if enabled:
                     revlogs.add(upgrade)
         else:
             # none are enabled
             for upgrade, __ in specified:
                 revlogs.discard(upgrade)
 
     # Ensure the repository can be upgraded.
     upgrade_actions.check_source_requirements(repo)
 
-    newreqs = localrepo.newreporequirements(
-        repo.ui, localrepo.defaultcreateopts(repo.ui)
-    )
+    default_options = localrepo.defaultcreateopts(repo.ui)
+    newreqs = localrepo.newreporequirements(repo.ui, default_options)
     newreqs.update(upgrade_actions.preservedrequirements(repo))
 
-    noremovereqs = (
-        repo.requirements
-        - newreqs
-        - upgrade_actions.supportremovedrequirements(repo)
-    )
-    if noremovereqs:
-        raise error.Abort(
-            _(
-                b'cannot upgrade repository; requirement would be '
-                b'removed: %s'
-            )
-            % _(b', ').join(sorted(noremovereqs))
-        )
-
-    noaddreqs = (
-        newreqs
-        - repo.requirements
-        - upgrade_actions.allowednewrequirements(repo)
-    )
-    if noaddreqs:
-        raise error.Abort(
-            _(
-                b'cannot upgrade repository; do not support adding '
-                b'requirement: %s'
-            )
-            % _(b', ').join(sorted(noaddreqs))
-        )
-
-    unsupportedreqs = newreqs - upgrade_actions.supporteddestrequirements(repo)
-    if unsupportedreqs:
-        raise error.Abort(
-            _(
-                b'cannot upgrade repository; do not support '
-                b'destination requirement: %s'
-            )
-            % _(b', ').join(sorted(unsupportedreqs))
-        )
+    upgrade_actions.check_requirements_changes(repo, newreqs)
 
     # Find and validate all improvements that can be made.
     alloptimizations = upgrade_actions.findoptimizations(repo)
 
     # Apply and Validate arguments.
     optimizations = []
     for o in alloptimizations:
         if o.name in optimize:
             optimizations.append(o)
             optimize.discard(o.name)
 
     if optimize:  # anything left is unknown
         raise error.Abort(
             _(b'unknown optimization action requested: %s')
             % b', '.join(sorted(optimize)),
             hint=_(b'run without arguments to see valid optimizations'),
         )
 
     deficiencies = upgrade_actions.finddeficiencies(repo)
     actions = upgrade_actions.determineactions(
         repo, deficiencies, repo.requirements, newreqs
     )
     actions.extend(
         o
         for o in sorted(optimizations)
         # determineactions could have added optimisation
         if o not in actions
     )
 
     removedreqs = repo.requirements - newreqs
     addedreqs = newreqs - repo.requirements
 
     if revlogs != upgrade_engine.UPGRADE_ALL_REVLOGS:
         incompatible = upgrade_actions.RECLONES_REQUIREMENTS & (
             removedreqs | addedreqs
         )
         if incompatible:
             msg = _(
                 b'ignoring revlogs selection flags, format requirements '
                 b'change: %s\n'
             )
             ui.warn(msg % b', '.join(sorted(incompatible)))
             revlogs = upgrade_engine.UPGRADE_ALL_REVLOGS
 
     def write_labeled(l, label):
         first = True
         for r in sorted(l):
             if not first:
                 ui.write(b', ')
             ui.write(r, label=label)
             first = False
 
     def printrequirements():
         ui.write(_(b'requirements\n'))
         ui.write(_(b'   preserved: '))
         write_labeled(
             newreqs & repo.requirements, "upgrade-repo.requirement.preserved"
         )
         ui.write((b'\n'))
         removed = repo.requirements - newreqs
         if repo.requirements - newreqs:
             ui.write(_(b'   removed: '))
             write_labeled(removed, "upgrade-repo.requirement.removed")
             ui.write((b'\n'))
         added = newreqs - repo.requirements
         if added:
             ui.write(_(b'   added: '))
             write_labeled(added, "upgrade-repo.requirement.added")
             ui.write((b'\n'))
         ui.write(b'\n')
 
     def printoptimisations():
         optimisations = [
             a for a in actions if a.type == upgrade_actions.OPTIMISATION
         ]
         optimisations.sort(key=lambda a: a.name)
         if optimisations:
             ui.write(_(b'optimisations: '))
             write_labeled(
                 [a.name for a in optimisations],
                 "upgrade-repo.optimisation.performed",
             )
             ui.write(b'\n\n')
 
     def printupgradeactions():
         for a in actions:
             ui.status(b'%s\n   %s\n\n' % (a.name, a.upgrademessage))
 
     def print_affected_revlogs():
         if not revlogs:
             ui.write((b'no revlogs to process\n'))
         else:
             ui.write((b'processed revlogs:\n'))
             for r in sorted(revlogs):
                 ui.write((b'  - %s\n' % r))
         ui.write((b'\n'))
 
     if not run:
         fromconfig = []
         onlydefault = []
 
         for d in deficiencies:
             if d.fromconfig(repo):
                 fromconfig.append(d)
             elif d.default:
                 onlydefault.append(d)
 
         if fromconfig or onlydefault:
 
             if fromconfig:
                 ui.status(
                     _(
                         b'repository lacks features recommended by '
                         b'current config options:\n\n'
                     )
                 )
                 for i in fromconfig:
                     ui.status(b'%s\n   %s\n\n' % (i.name, i.description))
 
             if onlydefault:
                 ui.status(
                     _(
                         b'repository lacks features used by the default '
                         b'config options:\n\n'
                     )
                 )
                 for i in onlydefault:
                     ui.status(b'%s\n   %s\n\n' % (i.name, i.description))
 
             ui.status(b'\n')
         else:
             ui.status(
                 _(
                     b'(no feature deficiencies found in existing '
                     b'repository)\n'
                 )
             )
 
         ui.status(
             _(
                 b'performing an upgrade with "--run" will make the following '
                 b'changes:\n\n'
             )
         )
 
         printrequirements()
         printoptimisations()
         printupgradeactions()
         print_affected_revlogs()
 
         unusedoptimize = [i for i in alloptimizations if i not in actions]
 
         if unusedoptimize:
             ui.status(
                 _(
                     b'additional optimizations are available by specifying '
                     b'"--optimize <name>":\n\n'
                 )
             )
             for i in unusedoptimize:
                 ui.status(_(b'%s\n   %s\n\n') % (i.name, i.description))
         return
 
     # Else we're in the run=true case.
     ui.write(_(b'upgrade will perform the following actions:\n\n'))
     printrequirements()
     printoptimisations()
     printupgradeactions()
     print_affected_revlogs()
 
     upgradeactions = [a.name for a in actions]
 
     ui.status(_(b'beginning upgrade...\n'))
     with repo.wlock(), repo.lock():
         ui.status(_(b'repository locked and read-only\n'))
         # Our strategy for upgrading the repository is to create a new,
         # temporary repository, write data to it, then do a swap of the
         # data. There are less heavyweight ways to do this, but it is easier
         # to create a new repo object than to instantiate all the components
         # (like the store) separately.
         tmppath = pycompat.mkdtemp(prefix=b'upgrade.', dir=repo.path)
         backuppath = None
         try:
             ui.status(
                 _(
                     b'creating temporary repository to stage migrated '
                     b'data: %s\n'
                 )
                 % tmppath
             )
 
             # clone ui without using ui.copy because repo.ui is protected
             repoui = repo.ui.__class__(repo.ui)
             dstrepo = hg.repository(repoui, path=tmppath, create=True)
 
             with dstrepo.wlock(), dstrepo.lock():
                 backuppath = upgrade_engine.upgrade(
                     ui, repo, dstrepo, newreqs, upgradeactions, revlogs=revlogs
                 )
             if not (backup or backuppath is None):
                 ui.status(
                     _(b'removing old repository content%s\n') % backuppath
                 )
                 repo.vfs.rmtree(backuppath, forcibly=True)
                 backuppath = None
 
         finally:
             ui.status(_(b'removing temporary repository %s\n') % tmppath)
             repo.vfs.rmtree(tmppath, forcibly=True)
 
             if backuppath and not ui.quiet:
                 ui.warn(
                     _(b'copy of old repository backed up at %s\n') % backuppath
                 )
                 ui.warn(
                     _(
                         b'the old repository will not be deleted; remove '
                         b'it to free up disk space once the upgraded '
                         b'repository is verified\n'
                     )
                 )
 
             if upgrade_actions.sharesafe.name in addedreqs:
                 ui.warn(
                     _(
                         b'repository upgraded to share safe mode, existing'
                         b' shares will still work in old non-safe mode. '
                         b'Re-share existing shares to use them in safe mode'
                         b' New shares will be created in safe mode.\n'
                     )
                 )
             if upgrade_actions.sharesafe.name in removedreqs:
                 ui.warn(
                     _(
                         b'repository downgraded to not use share safe mode, '
                         b'existing shares will not work and needs to'
                         b' be reshared.\n'
                     )
                 )

mercurial/upgrade_utils/actions.py

@@ -1,688 +1,719 b''
 # upgrade.py - functions for in place upgrade of Mercurial repository
 #
 # Copyright (c) 2016-present, Gregory Szorc
 #
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2 or any later version.
 
 from __future__ import absolute_import
 
 from ..i18n import _
 from .. import (
     error,
     localrepo,
     requirements,
     util,
 )
 
 from ..utils import compression
 
 # list of requirements that request a clone of all revlog if added/removed
 RECLONES_REQUIREMENTS = {
     b'generaldelta',
     requirements.SPARSEREVLOG_REQUIREMENT,
 }
 
 
 def supportremovedrequirements(repo):
     """Obtain requirements that can be removed during an upgrade.
 
     If an upgrade were to create a repository that dropped a requirement,
     the dropped requirement must appear in the returned set for the upgrade
     to be allowed.
     """
     supported = {
         requirements.SPARSEREVLOG_REQUIREMENT,
         requirements.SIDEDATA_REQUIREMENT,
         requirements.COPIESSDC_REQUIREMENT,
         requirements.NODEMAP_REQUIREMENT,
         requirements.SHARESAFE_REQUIREMENT,
     }
     for name in compression.compengines:
         engine = compression.compengines[name]
         if engine.available() and engine.revlogheader():
             supported.add(b'exp-compression-%s' % name)
             if engine.name() == b'zstd':
                 supported.add(b'revlog-compression-zstd')
     return supported
 
 
 def supporteddestrequirements(repo):
     """Obtain requirements that upgrade supports in the destination.
 
     If the result of the upgrade would create requirements not in this set,
     the upgrade is disallowed.
 
     Extensions should monkeypatch this to add their custom requirements.
     """
     supported = {
         b'dotencode',
         b'fncache',
         b'generaldelta',
         b'revlogv1',
         b'store',
         requirements.SPARSEREVLOG_REQUIREMENT,
         requirements.SIDEDATA_REQUIREMENT,
         requirements.COPIESSDC_REQUIREMENT,
         requirements.NODEMAP_REQUIREMENT,
         requirements.SHARESAFE_REQUIREMENT,
     }
     for name in compression.compengines:
         engine = compression.compengines[name]
         if engine.available() and engine.revlogheader():
             supported.add(b'exp-compression-%s' % name)
             if engine.name() == b'zstd':
                 supported.add(b'revlog-compression-zstd')
     return supported
 
 
 def allowednewrequirements(repo):
     """Obtain requirements that can be added to a repository during upgrade.
 
     This is used to disallow proposed requirements from being added when
     they weren't present before.
 
     We use a list of allowed requirement additions instead of a list of known
     bad additions because the whitelist approach is safer and will prevent
     future, unknown requirements from accidentally being added.
     """
     supported = {
         b'dotencode',
         b'fncache',
         b'generaldelta',
         requirements.SPARSEREVLOG_REQUIREMENT,
         requirements.SIDEDATA_REQUIREMENT,
         requirements.COPIESSDC_REQUIREMENT,
         requirements.NODEMAP_REQUIREMENT,
         requirements.SHARESAFE_REQUIREMENT,
     }
     for name in compression.compengines:
         engine = compression.compengines[name]
         if engine.available() and engine.revlogheader():
             supported.add(b'exp-compression-%s' % name)
             if engine.name() == b'zstd':
                 supported.add(b'revlog-compression-zstd')
     return supported
 
 
 def preservedrequirements(repo):
     return set()
 
 
 DEFICIENCY = b'deficiency'
 OPTIMISATION = b'optimization'
 
 
 class improvement(object):
     """Represents an improvement that can be made as part of an upgrade.
 
     The following attributes are defined on each instance:
 
     name
        Machine-readable string uniquely identifying this improvement. It
        will be mapped to an action later in the upgrade process.
 
     type
        Either ``DEFICIENCY`` or ``OPTIMISATION``. A deficiency is an obvious
        problem. An optimization is an action (sometimes optional) that
        can be taken to further improve the state of the repository.
 
     description
        Message intended for humans explaining the improvement in more detail,
        including the implications of it. For ``DEFICIENCY`` types, should be
        worded in the present tense. For ``OPTIMISATION`` types, should be
        worded in the future tense.
 
     upgrademessage
        Message intended for humans explaining what an upgrade addressing this
        issue will do. Should be worded in the future tense.
     """
 
     def __init__(self, name, type, description, upgrademessage):
         self.name = name
         self.type = type
         self.description = description
         self.upgrademessage = upgrademessage
 
     def __eq__(self, other):
         if not isinstance(other, improvement):
             # This is what python tell use to do
             return NotImplemented
         return self.name == other.name
 
     def __ne__(self, other):
         return not (self == other)
 
     def __hash__(self):
         return hash(self.name)
 
 
 allformatvariant = []
 
 
 def registerformatvariant(cls):
     allformatvariant.append(cls)
     return cls
 
 
 class formatvariant(improvement):
     """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
 
     # 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
 
     # 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()
 
     @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.
     """
 
     # the requirement that control this format variant
     _requirement = None
 
     @staticmethod
     def _newreporequirements(ui):
         return localrepo.newreporequirements(
             ui, localrepo.defaultcreateopts(ui)
         )
 
     @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.ui)
 
 
 @registerformatvariant
 class fncache(requirementformatvariant):
     name = b'fncache'
 
     _requirement = b'fncache'
 
     default = True
 
     description = _(
         b'long and reserved filenames may not work correctly; '
         b'repository performance is sub-optimal'
     )
 
     upgrademessage = _(
         b'repository will be more resilient to storing '
         b'certain paths and performance of certain '
         b'operations should be improved'
     )
 
 
 @registerformatvariant
 class dotencode(requirementformatvariant):
     name = b'dotencode'
 
     _requirement = b'dotencode'
 
     default = True
 
     description = _(
         b'storage of filenames beginning with a period or '
         b'space may not work correctly'
     )
 
     upgrademessage = _(
         b'repository will be better able to store files '
         b'beginning with a space or period'
     )
 
 
 @registerformatvariant
 class generaldelta(requirementformatvariant):
     name = b'generaldelta'
 
     _requirement = b'generaldelta'
 
     default = True
 
     description = _(
         b'deltas within internal storage are unable to '
         b'choose optimal revisions; repository is larger and '
         b'slower than it could be; interaction with other '
         b'repositories may require extra network and CPU '
         b'resources, making "hg push" and "hg pull" slower'
     )
 
     upgrademessage = _(
         b'repository storage will be able to create '
         b'optimal deltas; new repository data will be '
         b'smaller and read times should decrease; '
         b'interacting with other repositories using this '
         b'storage model should require less network and '
         b'CPU resources, making "hg push" and "hg pull" '
         b'faster'
     )
 
 
 @registerformatvariant
 class sharesafe(requirementformatvariant):
     name = b'exp-sharesafe'
     _requirement = requirements.SHARESAFE_REQUIREMENT
 
     default = False
 
     description = _(
         b'old shared repositories do not share source repository '
         b'requirements and config. This leads to various problems '
         b'when the source repository format is upgraded or some new '
         b'extensions are enabled.'
     )
 
     upgrademessage = _(
         b'Upgrades a repository to share-safe format so that future '
         b'shares of this repository share its requirements and configs.'
     )
 
 
 @registerformatvariant
 class sparserevlog(requirementformatvariant):
     name = b'sparserevlog'
 
     _requirement = requirements.SPARSEREVLOG_REQUIREMENT
 
     default = True
 
     description = _(
         b'in order to limit disk reading and memory usage on older '
         b'version, the span of a delta chain from its root to its '
         b'end is limited, whatever the relevant data in this span. '
         b'This can severly limit Mercurial ability to build good '
         b'chain of delta resulting is much more storage space being '
         b'taken and limit reusability of on disk delta during '
         b'exchange.'
     )
 
     upgrademessage = _(
         b'Revlog supports delta chain with more unused data '
         b'between payload. These gaps will be skipped at read '
         b'time. This allows for better delta chains, making a '
         b'better compression and faster exchange with server.'
     )
 
 
 @registerformatvariant
 class sidedata(requirementformatvariant):
     name = b'sidedata'
 
     _requirement = requirements.SIDEDATA_REQUIREMENT
 
     default = False
 
     description = _(
         b'Allows storage of extra data alongside a revision, '
         b'unlocking various caching options.'
     )
 
     upgrademessage = _(b'Allows storage of extra data alongside a revision.')
 
 
 @registerformatvariant
 class persistentnodemap(requirementformatvariant):
     name = b'persistent-nodemap'
 
     _requirement = requirements.NODEMAP_REQUIREMENT
 
     default = False
 
     description = _(
         b'persist the node -> rev mapping on disk to speedup lookup'
     )
 
     upgrademessage = _(b'Speedup revision lookup by node id.')
 
 
 @registerformatvariant
 class copiessdc(requirementformatvariant):
     name = b'copies-sdc'
 
     _requirement = requirements.COPIESSDC_REQUIREMENT
 
     default = False
 
     description = _(b'Stores copies information alongside changesets.')
 
     upgrademessage = _(
         b'Allows to use more efficient algorithm to deal with ' b'copy tracing.'
     )
 
 
 @registerformatvariant
 class removecldeltachain(formatvariant):
     name = b'plain-cl-delta'
 
     default = True
 
     description = _(
         b'changelog storage is using deltas instead of '
         b'raw entries; changelog reading and any '
         b'operation relying on changelog data are slower '
         b'than they could be'
     )
 
     upgrademessage = _(
         b'changelog storage will be reformated to '
         b'store raw entries; changelog reading will be '
         b'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
 
 
 @registerformatvariant
 class compressionengine(formatvariant):
     name = b'compression'
     default = b'zlib'
 
     description = _(
         b'Compresion algorithm used to compress data. '
         b'Some engine are faster than other'
     )
 
     upgrademessage = _(
         b'revlog content will be recompressed with the new algorithm.'
     )
 
     @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 = b'zlib'
         for req in repo.requirements:
             prefix = req.startswith
             if prefix(b'revlog-compression-') or prefix(b'exp-compression-'):
                 compression = req.split(b'-', 2)[2]
         return compression
 
     @classmethod
     def fromconfig(cls, repo):
         compengines = repo.ui.configlist(b'format', b'revlog-compression')
         # return the first valid value as the selection code would do
         for comp in compengines:
             if comp in util.compengines:
                 return comp
 
         # no valide compression found lets display it all for clarity
         return b','.join(compengines)
 
 
 @registerformatvariant
 class compressionlevel(formatvariant):
     name = b'compression-level'
     default = b'default'
 
     description = _(b'compression level')
 
     upgrademessage = _(b'revlog content will be recompressed')
 
     @classmethod
     def fromrepo(cls, repo):
         comp = compressionengine.fromrepo(repo)
         level = None
         if comp == b'zlib':
             level = repo.ui.configint(b'storage', b'revlog.zlib.level')
         elif comp == b'zstd':
             level = repo.ui.configint(b'storage', b'revlog.zstd.level')
         if level is None:
             return b'default'
         return bytes(level)
 
     @classmethod
     def fromconfig(cls, repo):
         comp = compressionengine.fromconfig(repo)
         level = None
         if comp == b'zlib':
             level = repo.ui.configint(b'storage', b'revlog.zlib.level')
         elif comp == b'zstd':
             level = repo.ui.configint(b'storage', b'revlog.zstd.level')
         if level is None:
             return b'default'
         return bytes(level)
 
 
 def finddeficiencies(repo):
     """returns a list of deficiencies that the repo suffer from"""
     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.
 
     for fv in allformatvariant:
         if not fv.fromrepo(repo):
             deficiencies.append(fv)
 
     return deficiencies
 
 
 ALL_OPTIMISATIONS = []
 
 
 def register_optimization(obj):
     ALL_OPTIMISATIONS.append(obj)
     return obj
 
 
 register_optimization(
     improvement(
         name=b're-delta-parent',
         type=OPTIMISATION,
         description=_(
             b'deltas within internal storage will be recalculated to '
             b'choose an optimal base revision where this was not '
             b'already done; the size of the repository may shrink and '
             b'various operations may become faster; the first time '
             b'this optimization is performed could slow down upgrade '
             b'execution considerably; subsequent invocations should '
             b'not run noticeably slower'
         ),
         upgrademessage=_(
             b'deltas within internal storage will choose a new '
             b'base revision if needed'
         ),
     )
 )
 
 register_optimization(
     improvement(
         name=b're-delta-multibase',
         type=OPTIMISATION,
         description=_(
             b'deltas within internal storage will be recalculated '
             b'against multiple base revision and the smallest '
             b'difference will be used; the size of the repository may '
             b'shrink significantly when there are many merges; this '
             b'optimization will slow down execution in proportion to '
             b'the number of merges in the repository and the amount '
             b'of files in the repository; this slow down should not '
             b'be significant unless there are tens of thousands of '
             b'files and thousands of merges'
         ),
         upgrademessage=_(
             b'deltas within internal storage will choose an '
             b'optimal delta by computing deltas against multiple '
             b'parents; may slow down execution time '
             b'significantly'
         ),
     )
 )
 
 register_optimization(
     improvement(
         name=b're-delta-all',
         type=OPTIMISATION,
         description=_(
             b'deltas within internal storage will always be '
             b'recalculated without reusing prior deltas; this will '
             b'likely make execution run several times slower; this '
             b'optimization is typically not needed'
         ),
         upgrademessage=_(
             b'deltas within internal storage will be fully '
             b'recomputed; this will likely drastically slow down '
             b'execution time'
         ),
     )
 )
 
 register_optimization(
     improvement(
         name=b're-delta-fulladd',
         type=OPTIMISATION,
         description=_(
             b'every revision will be re-added as if it was new '
             b'content. It will go through the full storage '
             b'mechanism giving extensions a chance to process it '
             b'(eg. lfs). This is similar to "re-delta-all" but even '
             b'slower since more logic is involved.'
         ),
         upgrademessage=_(
             b'each revision will be added as new content to the '
             b'internal storage; this will likely drastically slow '
             b'down execution time, but some extensions might need '
             b'it'
         ),
     )
 )
 
 
 def findoptimizations(repo):
     """Determine optimisation that could be used during upgrade"""
     # These are unconditionally added. There is logic later that figures out
     # which ones to apply.
     return list(ALL_OPTIMISATIONS)
 
 
 def determineactions(repo, deficiencies, sourcereqs, destreqs):
     """Determine upgrade actions that will be performed.
 
     Given a list of improvements as returned by ``finddeficiencies`` and
     ``findoptimizations``, determine the list of upgrade actions that
     will be performed.
 
     The role of this function is to filter improvements if needed, apply
     recommended optimizations from the improvements list that make sense,
     etc.
 
     Returns a list of action names.
     """
     newactions = []
 
     for d in deficiencies:
         name = d._requirement
 
         # If the action is a requirement that doesn't show up in the
         # destination requirements, prune the action.
         if name is not None and name not in destreqs:
             continue
 
         newactions.append(d)
 
     # FUTURE consider adding some optimizations here for certain transitions.
     # e.g. adding generaldelta could schedule parent redeltas.
 
     return newactions
 
 
 ###  Code checking if a repository can got through the upgrade process at all. #
 
 
 def requiredsourcerequirements(repo):
     """Obtain requirements required to be present to upgrade a repo.
 
     An upgrade will not be allowed if the repository doesn't have the
     requirements returned by this function.
     """
     return {
         # Introduced in Mercurial 0.9.2.
         b'revlogv1',
         # Introduced in Mercurial 0.9.2.
         b'store',
     }
 
 
 def blocksourcerequirements(repo):
     """Obtain requirements that will prevent an upgrade from occurring.
 
     An upgrade cannot be performed if the source repository contains a
     requirements in the returned set.
     """
     return {
         # The upgrade code does not yet support these experimental features.
         # This is an artificial limitation.
         requirements.TREEMANIFEST_REQUIREMENT,
         # This was a precursor to generaldelta and was never enabled by default.
         # It should (hopefully) not exist in the wild.
         b'parentdelta',
         # Upgrade should operate on the actual store, not the shared link.
         requirements.SHARED_REQUIREMENT,
     }
 
 
 def check_source_requirements(repo):
     """Ensure that no existing requirements prevent the repository upgrade"""
 
     required = requiredsourcerequirements(repo)
     missingreqs = required - repo.requirements
     if missingreqs:
         msg = _(b'cannot upgrade repository; requirement missing: %s')
         missingreqs = b', '.join(sorted(missingreqs))
         raise error.Abort(msg % missingreqs)
 
     blocking = blocksourcerequirements(repo)
     blockingreqs = blocking & repo.requirements
     if blockingreqs:
         m = _(b'cannot upgrade repository; unsupported source requirement: %s')
         blockingreqs = b', '.join(sorted(blockingreqs))
         raise error.Abort(m % blockingreqs)
+
+
+### Verify the validity of the planned requirement changes ####################
+
+
+def check_requirements_changes(repo, new_reqs):
+    old_reqs = repo.requirements
+
+    support_removal = supportremovedrequirements(repo)
+    no_remove_reqs = old_reqs - new_reqs - support_removal
+    if no_remove_reqs:
+        msg = _(b'cannot upgrade repository; requirement would be removed: %s')
+        no_remove_reqs = b', '.join(sorted(no_remove_reqs))
+        raise error.Abort(msg % no_remove_reqs)
+
+    support_addition = allowednewrequirements(repo)
+    no_add_reqs = new_reqs - old_reqs - support_addition
+    if no_add_reqs:
+        m = _(b'cannot upgrade repository; do not support adding requirement: ')
+        no_add_reqs = b', '.join(sorted(no_add_reqs))
+        raise error.Abort(m + no_add_reqs)
+
+    supported = supporteddestrequirements(repo)
+    unsupported_reqs = new_reqs - supported
+    if unsupported_reqs:
+        msg = _(
+            b'cannot upgrade repository; do not support destination '
+            b'requirement: %s'
+        )
+        unsupported_reqs = b', '.join(sorted(unsupported_reqs))
+        raise error.Abort(msg % unsupported_reqs)