store: append to fncache if there are only new files to write Before this patch, if we have to add a new entry to fncache, we write the whole fncache again which slows things down on large fncache which have millions of entries. Addition of a new entry is common operation while pulling new files or commiting a new file. This patch adds a new fncache.addls set which keeps track of the additions happening and store them. When we write the fncache, we will just read the addls set and append those entries at the end of fncache. We make sure that the entries are new entries by loading the fncache and making sure entry does not exists there. In future if we can check if an entry is new without loading the fncache, that will speed up things more. Performance numbers for commiting a new file: mercurial repo before: 0.08784651756286621 after: 0.08474504947662354 mozilla-central before: 1.83314049243927 after: 1.7054164409637451 netbeans before: 0.7953150272369385 after: 0.7202838659286499 pypy before: 0.17805707454681396 after: 0.13431048393249512 In our internal repo, the performance improvement is in seconds. I have used octobus's ASV perf benchmark thing to get the above numbers. I also see some minute perf improvements related to creating a new commit without a new file, but I believe that's just some noise. Differential Revision: https://phab.mercurial-scm.org/D5301