Tag caching, at last

[Greg Ward]

[quoth Matt]
> - taking your fast solution
> - writing out cache of final tag to node mapping
> - if NOTHING has changed on read by (tip revision/hash match), return
> cached tag list

[my foolish reply]
> I'll see what I can whip up!

Still working on it, but I wanted to see what you think of this
(pasting patch into gmail, so no guarantees about formatting...this is
just for design review):

# HG changeset patch
# User Greg Ward <greg-hg at gerg.ca>
# Date 1247625009 14400
# Node ID 3dfc90efa8a74a837a50c14702a35a8ed04616ec
# Parent  cc6f04bd7d2511b91a3e54d3235575a8892f7a6a
localrepo: add destroyed() method for strip/rollback to use (issue548).

This is sort of like a hook: should we make it a hook instead?

diff --git a/mercurial/localrepo.py b/mercurial/localrepo.py
--- a/mercurial/localrepo.py
+++ b/mercurial/localrepo.py
@@ -612,6 +612,7 @@
                                  % encoding.tolocal(self.dirstate.branch()))
                 self.invalidate()
                 self.dirstate.invalidate()
+                self.destroyed()
             else:
                 self.ui.warn(_("no rollback information available\n"))
         finally:
@@ -910,6 +911,16 @@
             del tr
             lock.release()

+    def destroyed(self):
+        '''Inform the repository that nodes have been destroyed.
+        Intended for use by strip and rollback, so there's a common
+        place for anything that has to be done after destroying history.'''
+        # Do nothing for now: this is a placeholder that will be used
+        # when we add tag caching.
+        # XXX it might be nice if we could take the list of destroyed
+        # nodes, but I don't see an easy way for rollback() to do that
+        pass
+
     def walk(self, match, node=None):
         '''
         walk recursively through the directory tree or a given
diff --git a/mercurial/repair.py b/mercurial/repair.py
--- a/mercurial/repair.py
+++ b/mercurial/repair.py
@@ -142,3 +142,4 @@
         if backup != "strip":
             os.unlink(chgrpfile)

+    repo.destroyed()

In my patch series, this comes right before "implement persistent tag
caching", hence the no-op implementation.  My main question is, is
this internal hook-like API appropriate, or should we bite the bullet
and add a new hook for node destruction?

Greg