Ref-counting for pristine texts

I have committed the ref counting for pristine texts (r1058523) and have
had a bit more insight into when to perform the deletion.

Deleting unreferenced texts on closing a "wcroot" is too late - too
large a granularity - for the way the WC code is currently structured
because this doesn't happen until a (long-lived) pool is cleaned up, as
Bert pointed out.

Deleting unreferenced texts after running the Work Queue is too soon -
too fine a granularity - for the way "commit" is currently structured.
A simple test of this approach showed that by the time the post-commit
processing tries to install a reference to a pristine text whose
checksum was noted earlier, in some cases that pristine row has already
been purged.

At the moment I think the practical solution is to insert calls to the
pristine cleanup at the end of each main libsvn_wc public API that may
have freed up one or more pristines.

Wherever we do it, if it's at all frequent, the search for unreferenced
pristines needs to be efficient. At present I use "SELECT ... FROM
PRISTINE WHERE refcount = 0". I believe that can most easily be made
efficient by adding an index on the 'refcount' column, so that's what I
intend to do.

- Julian
Received on 2011-01-13 20:02:34 CET