localrepo: move new repo requirements into standalone function (API)

localrepo: move new repo requirements into standalone function (API)

This patch extracts the code for determining requirements for a new
repo into a standalone function. By doing so, future code that will
perform an in-place repository upgrade (e.g. to generaldelta) can
examine the set of proposed new requirements and possibly take
additional actions (such as adding dotencode or fncache) when
performing the upgrade.

This patch is marked as API because _baserequirements (which was added
in b090601a80d1 so extensions could override it) has been removed and
will presumably impact whatever extension it was added for. Consumers
should be able to monkeypatch the new function to achieve the same
functionality.

The "create" argument has been dropped because the function is only
called in one location and "create" is always true in that case.

While it makes logical sense for this code to be a method so extensions
can implement a custom repo class / method to override it, this won't
actually work. This is because requirements determination occurs during
localrepository.__init__ and this is before the "reposetup"
"callback" is fired. So, the only way for extensions to customize
requirements would be to overwrite localrepo.localrepository or to
monkeypatch a function on a module during extsetup(). Since we try to
keep localrepository small, we use a standalone function. There is
probably room to offer extensions a "hook" point to alter repository
creation. But that is scope bloat.

--- a/mercurial/localrepo.py	Mon Feb 15 13:19:07 2016 -0800
+++ b/mercurial/localrepo.py	Mon Feb 15 13:20:20 2016 -0800
@@ -242,9 +242,6 @@
     # only functions defined in module of enabled extensions are invoked
     featuresetupfuncs = set()
 
-    def _baserequirements(self, create):
-        return ['revlogv1']
-
     def __init__(self, baseui, path=None, create=False):
         self.requirements = set()
         self.wvfs = scmutil.vfs(path, expandpath=True, realpath=True)
@@ -282,28 +279,13 @@
 
         if not self.vfs.isdir():
             if create:
-                requirements = set(self._baserequirements(create))
-                if self.ui.configbool('format', 'usestore', True):
-                    requirements.add("store")
-                    if self.ui.configbool('format', 'usefncache', True):
-                        requirements.add("fncache")
-                        if self.ui.configbool('format', 'dotencode', True):
-                            requirements.add('dotencode')
-
-                if scmutil.gdinitconfig(self.ui):
-                    requirements.add("generaldelta")
-                if self.ui.configbool('experimental', 'treemanifest', False):
-                    requirements.add("treemanifest")
-                if self.ui.configbool('experimental', 'manifestv2', False):
-                    requirements.add("manifestv2")
-
-                self.requirements = requirements
+                self.requirements = newreporequirements(self)
 
                 if not self.wvfs.exists():
                     self.wvfs.makedirs()
                 self.vfs.makedir(notindexed=True)
 
-                if 'store' in requirements:
+                if 'store' in self.requirements:
                     self.vfs.mkdir("store")
 
                     # create an invalid changelog
@@ -1969,3 +1951,27 @@
 
 def islocal(path):
     return True
+
+def newreporequirements(repo):
+    """Determine the set of requirements for a new local repository.
+
+    Extensions can wrap this function to specify custom requirements for
+    new repositories.
+    """
+    ui = repo.ui
+    requirements = set(['revlogv1'])
+    if ui.configbool('format', 'usestore', True):
+        requirements.add('store')
+        if ui.configbool('format', 'usefncache', True):
+            requirements.add('fncache')
+            if ui.configbool('format', 'dotencode', True):
+                requirements.add('dotencode')
+
+    if scmutil.gdinitconfig(ui):
+        requirements.add('generaldelta')
+    if ui.configbool('experimental', 'treemanifest', False):
+        requirements.add('treemanifest')
+    if ui.configbool('experimental', 'manifestv2', False):
+        requirements.add('manifestv2')
+
+    return requirements