localrepo: define storage backend in creation options (API)

We add an experimental config option to define the storage backend
for new repositories. By default, it uses "revlogv1," which maps to
the current and only modern supported repository format.

We add a "backend" creation option to control which backend to
use. It defaults to using the value from the config option.

newreporequirements() will now barf if it sees a "backend" value
that isn't "revlogv1." This forces extensions to monkeypatch the
function to handle requirements derivation for custom backends.

In order for this to "just work," we factored out obtaining the
default creation options into its own function and made callers
of newreporequirements() responsible for passing in valid data.
Without this, direct callers of newreporequirements() wouldn't
get the proper results.

Differential Revision: https://phab.mercurial-scm.org/D4791

mercurial/configitems.py

@@ -941,6 +941,9 coreconfigitem('progress', 'width',
 coreconfigitem('push', 'pushvars.server',
     default=False,
 )
+coreconfigitem('storage', 'new-repo-backend',
+    default='revlogv1',
+)
 coreconfigitem('storage', 'revlog.optimize-delta-parent-choice',
     default=True,
     alias=[('format', 'aggressivemergedeltas')],

mercurial/localrepo.py

@@ -2808,14 +2808,26 def instance(ui, path, create, intents=N
 def islocal(path):
     return True
 
-def newreporequirements(ui, createopts=None):
+def defaultcreateopts(ui, createopts=None):
+    """Populate the default creation options for a repository.
+
+    A dictionary of explicitly requested creation options can be passed
+    in. Missing keys will be populated.
+    """
+    createopts = dict(createopts or {})
+
+    if 'backend' not in createopts:
+        # experimental config: storage.new-repo-backend
+        createopts['backend'] = ui.config('storage', 'new-repo-backend')
+
+    return createopts
+
+def newreporequirements(ui, createopts):
     """Determine the set of requirements for a new local repository.
 
     Extensions can wrap this function to specify custom requirements for
     new repositories.
     """
-    createopts = createopts or {}
-
     # If the repo is being created from a shared repository, we copy
     # its requirements.
     if 'sharedrepo' in createopts:
@@ -2827,6 +2839,14 def newreporequirements(ui, createopts=N
 
         return requirements
 
+    if 'backend' not in createopts:
+        raise error.ProgrammingError('backend key not present in createopts; '
+                                     'was defaultcreateopts() called?')
+
+    if createopts['backend'] != 'revlogv1':
+        raise error.Abort(_('unable to determine repository requirements for '
+                            'storage backend: %s') % createopts['backend'])
+
     requirements = {'revlogv1'}
     if ui.configbool('format', 'usestore'):
         requirements.add('store')
@@ -2885,6 +2905,7 def filterknowncreateopts(ui, createopts
     they know how to handle.
     """
     known = {
+        'backend',
         'narrowfiles',
         'sharedrepo',
         'sharedrelative',
@@ -2901,6 +2922,8 def createrepository(ui, path, createopt
 
     The following keys for ``createopts`` are recognized:
 
+    backend
+       The storage backend to use.
     narrowfiles
        Set up repository to support narrow file storage.
     sharedrepo
@@ -2912,7 +2935,7 def createrepository(ui, path, createopt
     shareditems
        Set of items to share to the new repository (in addition to storage).
     """
-    createopts = createopts or {}
+    createopts = defaultcreateopts(ui, createopts=createopts)
 
     unknownopts = filterknowncreateopts(ui, createopts)
 

mercurial/upgrade.py

@@ -199,7 +199,8 class requirementformatvariant(formatvar
 
     @staticmethod
     def _newreporequirements(ui):
-        return localrepo.newreporequirements(ui)
+        return localrepo.newreporequirements(
+            ui, localrepo.defaultcreateopts(ui))
 
     @classmethod
     def fromrepo(cls, repo):
@@ -747,7 +748,8 def upgraderepo(ui, repo, run=False, opt
 
     # FUTURE there is potentially a need to control the wanted requirements via
     # command arguments or via an extension hook point.
-    newreqs = localrepo.newreporequirements(repo.ui)
+    newreqs = localrepo.newreporequirements(
+        repo.ui, localrepo.defaultcreateopts(repo.ui))
     newreqs.update(preservedrequirements(repo))
 
     noremovereqs = (repo.requirements - newreqs -