transaction: delete callbacks after use Before this change, localrepository instances that performed multiple transactions would leak transaction objects. This could occur when running `hg convert`. When running `hg convert`, the leak would be ~90 MB per 10,000 changesets as measured with the Mercurial repo itself. The leak I tracked down involved the "validate" closure from localrepository.transaction(). It appeared to be keeping a reference to the original transaction via __closure__. __del__ semantics and a circular reference involving the repo object may have also come into play. Attempting to refactor the "validate" closure proved to be difficult because the "tr" reference in that closure may reference an object that isn't created until transaction.__init__ is called. And the "validate" closure is passed as an argument to transaction.__init__. Plus there is a giant warning comment in "validate" about how hacky it is. I did not want to venture into the dragon den. Anyway, we've had problems with transactions causing leaks before. The solution then (14e683d6b273) is the same as the solution in this patch: drop references to callbacks after they are called. This not only breaks cycles in core Mercurial but can help break cycles in extensions that accidentally introduce them. While I only tracked down a leak due to self.validator, since this is the 2nd time I've tracked down leaks due to transaction callbacks I figure enough is enough and we should prevent the class of leak from occurring regardless of the variable. That's why all callback variables are now nuked.