bundlerepo: dynamically create repository type from base repository

bundlerepo: dynamically create repository type from base repository

Previously, bundlerepository inherited from localrepo.localrepository.
You simply instantiated a bundlerepository and its __init__ called
localrepo.localrepository.__init__. Things were simple.

Unfortunately, this strategy is limiting because it assumes that
the base repository is a localrepository instance. And it assumes
various properties of localrepository, such as the arguments its
__init__ takes. And it prevents us from changing behavior of
localrepository.__init__ without also having to change derived classes.

Previous and ongoing work to abstract storage revealed these
limitations.

This commit changes the initialization strategy of bundle repositories
to dynamically create a type to represent the repository. Instead of
a static type, we instantiate a new local repo instance via
localrepo.instance(). We then combine its __class__ with
bundlerepository to produce a new type. This ensures that no matter
how localrepo.instance() decides to create a repository object, we
can derive a bundle repo object from it. i.e. localrepo.instance()
could return a type that isn't a localrepository and it would "just
work."

Well, it would "just work" if bundlerepository's custom implementations
only accessed attributes in the documented repository interface. I'm
pretty sure it violates the interface contract in a handful of
places. But we can worry about that another day. This change gets us
closer to doing more clever things around instantiating repository
instances without having to worry about teaching bundlerepository about
them.

.. api::

   ``bundlerepo.bundlerepository`` is no longer usable on its own.

   The class is combined with the class of the base repository it is
   associated with at run-time.

   New bundlerepository instances can be obtained by calling
   ``bundlerepo.instance()`` or ``bundlerepo.makebundlerepository()``.

Differential Revision: https://phab.mercurial-scm.org/D4555

--- a/mercurial/bundlerepo.py	Tue Sep 11 19:16:32 2018 -0700
+++ b/mercurial/bundlerepo.py	Tue Sep 11 19:50:07 2018 -0700
@@ -255,7 +255,7 @@
             pass
     return filespos
 
-class bundlerepository(localrepo.localrepository):
+class bundlerepository(object):
     """A repository instance that is a union of a local repo and a bundle.
 
     Instances represent a read-only repository composed of a local repository
@@ -263,25 +263,19 @@
     conceptually similar to the state of a repository after an
     ``hg unbundle`` operation. However, the contents of the bundle are never
     applied to the actual base repository.
+
+    Instances constructed directly are not usable as repository objects.
+    Use instance() or makebundlerepository() to create instances.
     """
-    def __init__(self, ui, repopath, bundlepath):
-        self._tempparent = None
-        try:
-            localrepo.localrepository.__init__(self, ui, repopath)
-        except error.RepoError:
-            self._tempparent = pycompat.mkdtemp()
-            localrepo.instance(ui, self._tempparent, create=True)
-            localrepo.localrepository.__init__(self, ui, self._tempparent)
+    def __init__(self, bundlepath, url, tempparent):
+        self._tempparent = tempparent
+        self._url = url
+
         self.ui.setconfig('phases', 'publish', False, 'bundlerepo')
 
-        if repopath:
-            self._url = 'bundle:' + util.expandpath(repopath) + '+' + bundlepath
-        else:
-            self._url = 'bundle:' + bundlepath
-
         self.tempfile = None
         f = util.posixfile(bundlepath, "rb")
-        bundle = exchange.readbundle(ui, f, bundlepath)
+        bundle = exchange.readbundle(self.ui, f, bundlepath)
 
         if isinstance(bundle, bundle2.unbundle20):
             self._bundlefile = bundle
@@ -311,7 +305,7 @@
             if bundle.compressed():
                 f = self._writetempbundle(bundle.read, '.hg10un',
                                           header='HG10UN')
-                bundle = exchange.readbundle(ui, f, bundlepath, self.vfs)
+                bundle = exchange.readbundle(self.ui, f, bundlepath, self.vfs)
 
             self._bundlefile = bundle
             self._cgunpacker = bundle
@@ -484,7 +478,41 @@
 
 def makebundlerepository(ui, repopath, bundlepath):
     """Make a bundle repository object based on repo and bundle paths."""
-    return bundlerepository(ui, repopath, bundlepath)
+    if repopath:
+        url = 'bundle:%s+%s' % (util.expandpath(repopath), bundlepath)
+    else:
+        url = 'bundle:%s' % bundlepath
+
+    # Because we can't make any guarantees about the type of the base
+    # repository, we can't have a static class representing the bundle
+    # repository. We also can't make any guarantees about how to even
+    # call the base repository's constructor!
+    #
+    # So, our strategy is to go through ``localrepo.instance()`` to construct
+    # a repo instance. Then, we dynamically create a new type derived from
+    # both it and our ``bundlerepository`` class which overrides some
+    # functionality. We then change the type of the constructed repository
+    # to this new type and initialize the bundle-specific bits of it.
+
+    try:
+        parentrepo = localrepo.instance(ui, repopath, create=False)
+        tempparent = None
+    except error.RepoError:
+        tempparent = pycompat.mkdtemp()
+        try:
+            parentrepo = localrepo.instance(ui, tempparent, create=True)
+        except Exception:
+            shutil.rmtree(tempparent)
+            raise
+
+    class derivedbundlerepository(bundlerepository, parentrepo.__class__):
+        pass
+
+    repo = parentrepo
+    repo.__class__ = derivedbundlerepository
+    bundlerepository.__init__(repo, bundlepath, url, tempparent)
+
+    return repo
 
 class bundletransactionmanager(object):
     def transaction(self):