hgtagsfnodescache: inherit fnode from parent when possible
If a changeset does not update the content of `.hgtags`, it means it will use
the same file-node (for `.hgtags`) as its parents. In this case we can
directly reuse the parent's file-node.
We use this property when updating the `hgtagsfnodescache` taking a faster path
if we already have a cached value for the parents of the node we are looking
at.
Doing so provides a large performance boost when looking at a lot of fnodes,
especially on repository with very large manifest:
timing for `tagsmod.fnoderevs(ui, repo, repo.changelog.revs())`
mercurial: (41907 revisions, 1923 files)
before: 6.9 seconds
after: 2.7 seconds (-54%)
pypy: (96266 revisions, 5198 files)
before: 80 seconds
after: 20 seconds (-75%)
mozilla-central: (463411 revisions, 272080 files)
before: 7166.4 seconds
after: 47.8 seconds (-99%, x150 speedup)
On a copy of mozilla-try with about 35K heads ans 1.7M changesets, this moves
the computation from many hours to a couple of minutes, making it more
interesting to do a full warm up of this cache before computing tags (from a
cold cache).
There seems to be other performance low hanging fruits, like avoiding the use of
changectx or a more revision centric logic. However, the new code is fast enough
for my needs right now.
{
// Enforcing
"eqeqeq" : true, // true: Require triple equals (===) for comparison
"forin" : true, // true: Require filtering for..in loops with obj.hasOwnProperty()
"freeze" : true, // true: prohibits overwriting prototypes of native objects such as Array, Date etc.
"nonbsp" : true, // true: Prohibit "non-breaking whitespace" characters.
"undef" : true, // true: Require all non-global variables to be declared (prevents global leaks)
// Environments
"browser" : true // Web Browser (window, document, etc)
}