Tracking "branch heads"

Hi all --

for converting from CVS to Mercurial (and probably other DVCSes), it
would be nice for the OutputOption to know when it is processing the
last revision on a particular branch.  Eg. for Mercurial output, one
could choose to mark that branch 'closed' or dummy-merge it into
another branch.  I believe that would be useful for Git and Bazaar
output as well.

I've peeked at the code, and it looks like the dumb way to do this is
to add code to my custom OutputOption that duplicates the loop over
all svn revisions in OutputPass.run().  That's fine if you're
converting 1000 revisions in 5 minutes, but I'm converting 104,000
revisions and it takes ~14 hours.  (Yeah, there is a performance
problem there.  Not sure if it's my code or Mercurial or just the
plain fact that there are a lot more files in our rev 104000 than in
rev 1000 or rev 20000.)

So it looks like the clever thing to do is modify CreateRevsPass to
build a dictionary mapping branch name to highest rev number on that
branch, then ensure that dictionary is persisted somehow and available
to OutputPass/OutputOption.

Questions:

1) any interest in such a patch being accepted upstream?  I'm pretty
sure it will be orthogonal to my "add cvs2hg" patch series, so I could
send it in as soon as it works, if you're interested.

2) terminology: in Mercurial, we would call this "a branch head" (or
"the branch head" when dealing with CVS/svn data).  AFAIK, CVS has no
standard terminology.  What does Subversion call "the highest-numbered
rev on branch X"?  What should cvs2svn call it internally?

3) how should I persist the dict?  it'll be small enough to just write
a pickle in the tmp dir: is that acceptable?

4) how should I make it available to OutputPass/OutputOption: just
make the filename a constant somewhere and let the OutputOption author
figure it out?

Thanks --

Greg

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