How to handle 'tag pushing revisions' and stacking

[John Arbash Meinel]

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I don't think we've accurately sorted out how pushing all tags and
stacking should work. I'm tempted to back out the pushing-copies-tags in
the short term, but I wanted to open it up for discussion.

For those that haven't followed it, the background is generally
available here:
  https://bugs.launchpad.net/bugs/745664

The summary is that if you have a branch that you are pushing up to
launchpad:

1) The first push works as normal, because there is a special code path
for 'create and populate a new branch'.

2) The second push sees "hey you have a branch there, do you have all
the revisions for all of your tags"? And then proceeds to upload a
revision for every tag. It then follows that up by pushing the
inventories for the parent of every tag you have.

3) Third and subsequent pushes don't push up new content, but *do* issue
a 'get_parent_map' check on the set of tags that you have, because they
are checking if any of them need to be uploaded.

My immediate response is that we should revert the "push all referenced
tags" change, until we have an appropriate response for this.

Specifically, even if we change the logic to say "tagged revisions
should be available either in the stacked branch, or in its fallbacks",
then it would improve (2). However, (3+) all now have to issue a giant
"get_parent_map" call 2 times. First they would hit the stacked branch,
to find that nothing is present there, and then they hit the stacked-on
branch, to find that "ok, nothing has changed since the last time I
pushed, yes all tags really are still there".


There are some real performance and amount-of-data transferred issues.
bzr isn't a very large tree, and has about 151 tags. My 'second push'
case cost 1.2MB of transfer to copy up the revisions (and their
associated parent revision inventories) for the tags referenced in bzr.
(Note that it is always the *complete* inventory for all parents, modulo
shared chk pages. However, tags age, our oldest tag in bzr is about
2006, and there aren't many shared chk pages from 2006 to 2011.)

Emacs has 1129 tags, and the oldest is from 1995 (and there are a lot
from 1995). Right now, every stacked branch will end up transferring a
whole lot of extra content. And even if we fix that, they still use a
lot of bandwidth to check if the revisions are present.

Thoughts?

John
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