narrow: keep bookmarks temporarily stripped for as long as commits are

The narrow extension also has support for shallowness and for
inserting older commits on pull. It works by temporarily stripping
newer commits, adding the older commits, then re-applying the stripped
bundle. The regular Mercurial server uses that when you widen,
although it shouldn't be necessary there. Our Google-internal server
does it when the user requests an older commit.

Our Google-internal tests fail since 7caf632e30c3 (filecache:
unimplement __set__() and __delete__() (API), 2018-10-20). I haven't
quite understood the problem, but it's related to the way we
temporarily hide bookmarks while the commits they point to are
stripped. When a transaction is started, Mercurial tries to read
various things from the repo for the transaction summary. That leads
to computation of hidden commits, which leads to an attempt to find
commits pinned by bookmarks. This is the reason we temporarily hide
the bookmarks. I think the aforementioned commit makes the restored
bookmarks visible earlier than before (which seems like an
improvement), so we can no longer incorrectly rely on the
repo._bookmarks field being cached too long (IIUC).

This patch makes it so we restore the temporarily hidden bookmarks
only after the temporary bundle has been re-applied. It also adapts
the code to update the repo.__bookmarks field using the pattern
described in the aforementioned commit instead of writing directly to
the fiels.

I have spent many hours trying to understand what was going on here,
but I still don't know if this can also happen without our custom
server. So this patch unfortunately does not add any tests; I have
only been able to test the fix using our Google-internal tests.

Differential Revision: https://phab.mercurial-scm.org/D5398

hgext/narrow/narrowbundle2.py

@@ -20,6 +20,7 b' from mercurial import ('
     changegroup,
     error,
     exchange,
+    localrepo,
     narrowspec,
     repair,
     repository,
@@ -179,14 +180,13 b' def _handlechangespec(op, inpart):'
 
     if clkills:
         # preserve bookmarks that repair.strip() would otherwise strip
-        bmstore = repo._bookmarks
+        op._bookmarksbackup = repo._bookmarks
         class dummybmstore(dict):
             def applychanges(self, repo, tr, changes):
                 pass
-        repo._bookmarks = dummybmstore()
+        localrepo.localrepository._bookmarks.set(repo, dummybmstore())
         chgrpfile = repair.strip(op.ui, repo, list(clkills), backup=True,
                                  topic='widen')
-        repo._bookmarks = bmstore
         if chgrpfile:
             op._widen_uninterr = repo.ui.uninterruptable()
             op._widen_uninterr.__enter__()
@@ -270,5 +270,10 b' def setup():'
         origcghandler(op, inpart)
         if util.safehasattr(op, '_widen_bundle'):
             handlechangegroup_widen(op, inpart)
+        if util.safehasattr(op, '_bookmarksbackup'):
+            localrepo.localrepository._bookmarks.set(op.repo,
+                                                     op._bookmarksbackup)
+            del op._bookmarksbackup
+
     wrappedcghandler.params = origcghandler.params
     bundle2.parthandlermapping['changegroup'] = wrappedcghandler