--- a/mercurial/debugcommands.py	Thu Nov 24 16:24:09 2016 -0800
+++ b/mercurial/debugcommands.py	Sun Dec 18 16:16:54 2016 -0800
@@ -34,6 +34,7 @@
     localrepo,
     lock as lockmod,
     pycompat,
+    repair,
     revlog,
     scmutil,
     setdiscovery,
@@ -873,4 +874,4 @@
     should complete almost instantaneously and the chances of a consumer being
     unable to access the repository should be low.
     """
-    raise error.Abort(_('not yet implemented'))
+    return repair.upgraderepo(ui, repo, run=run, optimize=optimize)

--- a/mercurial/repair.py	Thu Nov 24 16:24:09 2016 -0800
+++ b/mercurial/repair.py	Sun Dec 18 16:16:54 2016 -0800
@@ -358,3 +358,138 @@
         newobsstorefile.write(bytes)
     newobsstorefile.close()
     return n
+
+def upgraderequiredsourcerequirements(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 set([
+        # Introduced in Mercurial 0.9.2.
+        'revlogv1',
+        # Introduced in Mercurial 0.9.2.
+        'store',
+    ])
+
+def upgradeblocksourcerequirements(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 set([
+        # The upgrade code does not yet support these experimental features.
+        # This is an artificial limitation.
+        'manifestv2',
+        'treemanifest',
+        # This was a precursor to generaldelta and was never enabled by default.
+        # It should (hopefully) not exist in the wild.
+        'parentdelta',
+        # Upgrade should operate on the actual store, not the shared link.
+        'shared',
+    ])
+
+def upgradesupportremovedrequirements(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.
+    """
+    return set()
+
+def upgradesupporteddestrequirements(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.
+    """
+    return set([
+        'dotencode',
+        'fncache',
+        'generaldelta',
+        'revlogv1',
+        'store',
+    ])
+
+def upgradeallowednewrequirements(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.
+    """
+    return set([
+        'dotencode',
+        'fncache',
+        'generaldelta',
+    ])
+
+def upgraderepo(ui, repo, run=False, optimize=None):
+    """Upgrade a repository in place."""
+    # Avoid cycle: cmdutil -> repair -> localrepo -> cmdutil
+    from . import localrepo
+
+    repo = repo.unfiltered()
+
+    # Ensure the repository can be upgraded.
+    missingreqs = upgraderequiredsourcerequirements(repo) - repo.requirements
+    if missingreqs:
+        raise error.Abort(_('cannot upgrade repository; requirement '
+                            'missing: %s') % _(', ').join(sorted(missingreqs)))
+
+    blockedreqs = upgradeblocksourcerequirements(repo) & repo.requirements
+    if blockedreqs:
+        raise error.Abort(_('cannot upgrade repository; unsupported source '
+                            'requirement: %s') %
+                          _(', ').join(sorted(blockedreqs)))
+
+    # FUTURE there is potentially a need to control the wanted requirements via
+    # command arguments or via an extension hook point.
+    newreqs = localrepo.newreporequirements(repo)
+
+    noremovereqs = (repo.requirements - newreqs -
+                   upgradesupportremovedrequirements(repo))
+    if noremovereqs:
+        raise error.Abort(_('cannot upgrade repository; requirement would be '
+                            'removed: %s') % _(', ').join(sorted(noremovereqs)))
+
+    noaddreqs = (newreqs - repo.requirements -
+                 upgradeallowednewrequirements(repo))
+    if noaddreqs:
+        raise error.Abort(_('cannot upgrade repository; do not support adding '
+                            'requirement: %s') %
+                          _(', ').join(sorted(noaddreqs)))
+
+    unsupportedreqs = newreqs - upgradesupporteddestrequirements(repo)
+    if unsupportedreqs:
+        raise error.Abort(_('cannot upgrade repository; do not support '
+                            'destination requirement: %s') %
+                          _(', ').join(sorted(unsupportedreqs)))
+
+    def printrequirements():
+        ui.write(_('requirements\n'))
+        ui.write(_('   preserved: %s\n') %
+                 _(', ').join(sorted(newreqs & repo.requirements)))
+
+        if repo.requirements - newreqs:
+            ui.write(_('   removed: %s\n') %
+                     _(', ').join(sorted(repo.requirements - newreqs)))
+
+        if newreqs - repo.requirements:
+            ui.write(_('   added: %s\n') %
+                     _(', ').join(sorted(newreqs - repo.requirements)))
+
+        ui.write('\n')
+
+    if not run:
+        ui.write(_('performing an upgrade with "--run" will make the following '
+                   'changes:\n\n'))
+
+        printrequirements()

--- a/tests/test-upgrade-repo.t	Thu Nov 24 16:24:09 2016 -0800
+++ b/tests/test-upgrade-repo.t	Sun Dec 18 16:16:54 2016 -0800
@@ -1,5 +1,51 @@
-  $ hg init empty
-  $ cd empty
-  $ hg debugupgraderepo
-  abort: not yet implemented
+  $ cat >> $HGRCPATH << EOF
+  > [extensions]
+  > share =
+  > EOF
+
+store and revlogv1 are required in source
+
+  $ hg --config format.usestore=false init no-store
+  $ hg -R no-store debugupgraderepo
+  abort: cannot upgrade repository; requirement missing: store
+  [255]
+
+  $ hg init no-revlogv1
+  $ cat > no-revlogv1/.hg/requires << EOF
+  > dotencode
+  > fncache
+  > generaldelta
+  > store
+  > EOF
+
+  $ hg -R no-revlogv1 debugupgraderepo
+  abort: cannot upgrade repository; requirement missing: revlogv1
   [255]
+
+Cannot upgrade shared repositories
+
+  $ hg init share-parent
+  $ hg -q share share-parent share-child
+
+  $ hg -R share-child debugupgraderepo
+  abort: cannot upgrade repository; unsupported source requirement: shared
+  [255]
+
+Do not yet support upgrading manifestv2 and treemanifest repos
+
+  $ hg --config experimental.manifestv2=true init manifestv2
+  $ hg -R manifestv2 debugupgraderepo
+  abort: cannot upgrade repository; unsupported source requirement: manifestv2
+  [255]
+
+  $ hg --config experimental.treemanifest=true init treemanifest
+  $ hg -R treemanifest debugupgraderepo
+  abort: cannot upgrade repository; unsupported source requirement: treemanifest
+  [255]
+
+Cannot add manifestv2 or treemanifest requirement during upgrade
+
+  $ hg init disallowaddedreq
+  $ hg -R disallowaddedreq --config experimental.manifestv2=true --config experimental.treemanifest=true debugupgraderepo
+  abort: cannot upgrade repository; do not support adding requirement: manifestv2, treemanifest
+  [255]