BDB: implementing 'upgrade' for fs-successor-ids

[Daniel Shahaf]

To implement 'svnadmin upgrade' for fs-successor-ids, it seems we want
code to do the following:

  def construct_successors_in_existing_fs(fs):
    def handle_range(range):
      for revision in range:
        for node-rev-id in fs->revisions[revision]:
          Add to 'successors' table: key=fs->nodes[node-rev-id].pred-id 
value=node-rev-id

    For range in 0..999, 1000..1999, ..., N*1000..youngest:
      in a txn, handle_range(range)

    in a txn, handle_range( (old youngest)+1..[txn's value of "youngest"] )

Now, handle_range() needs to work with two tables at once --- reading
from NODES and writing to SUCCESSORS --- and I haven't found any
precedent in subversion/libsvn_fs_base/bdb/*.c for code that reads two
tables at once.  (locks-table.c reads from bfd->lock_tokens, but it
doesn't read from bfd->locks in that function; and everything else only
reads from the table the file is named after.)

So, I'm asking what's the proper layout for this code: do I make
a function in bdb/successors-table.c that reads from NODES and writes to
SUCCESSORS?  Or do I have a higher-level function (in, say, node-rev.c)
that runs within a txn, reading NODES and calling SUCCESSORS?

Thanks!

Daniel