bisect: avoid copying ancestor list for non-merge commits
During a bisection, hg needs to compute a list of all ancestors for every
candidate commit. This is accomplished via a bottom-up traversal of the set of
candidates, during which each revision's ancestor list is populated using the
ancestor list of its parent(s). Previously, this involved copying the entire
list, which could be very long in if the bisection range was large.
To help improve this, we can observe that each candidate commit is visited
exactly once, at which point its ancestor list is copied into its children's
lists and then dropped. In the case of non-merge commits, a commit's ancestor
list consists exactly of its parent's list plus itself. This means that we can
trivially reuse the parent's existing list for one of its non-merge children,
which avoids copying entirely if that commit is the parent's only child. This
makes bisections over linear ranges of commits much faster.
During some informal testing in the large publicly-available `mozilla-central`
repository, this noticeably sped up bisections over large ranges of history:
Setup:
$ cd mozilla-central
$ hg bisect --reset
$ hg bisect --good 0
$ hg log -r tip -T '{rev}\n'
628417
Test:
$ time hg bisect --bad tip --noupdate
Before:
real 3m35.927s
user 3m35.553s
sys 0m0.319s
After:
real 1m41.142s
user 1m40.810s
sys 0m0.285s
Testing that convert.hg.preserve-hash=true can be used to make hg
convert from hg repo to hg repo preserve hashes, even if the
computation of the files list in commits change slightly between hg
versions.
$ cat <<'EOF' >> "$HGRCPATH"
> [extensions]
> convert =
> EOF
$ cat <<'EOF' > changefileslist.py
> from mercurial import (changelog, extensions, metadata)
> def wrap(orig, clog, manifest, files, *args, **kwargs):
> files = metadata.ChangingFiles(touched=[b"a"])
> return orig(clog, manifest, files, *args, **kwargs)
> def extsetup(ui):
> extensions.wrapfunction(changelog.changelog, 'add', wrap)
> EOF
$ hg init repo
$ cd repo
$ echo a > a; hg commit -qAm a
$ echo b > a; hg commit -qAm b
$ hg up -qr 0; echo c > c; hg commit -qAm c
$ hg merge -qr 1
$ hg commit -m_ --config extensions.x=../changefileslist.py
$ hg log -r . -T '{node|short} {files|json}\n'
c085bbe93d59 ["a"]
Now that we have a commit with a files list that's not what the
current hg version would create, check that convert either fixes it or
keeps it depending on config:
$ hg convert -q . ../convert
$ hg --cwd ../convert log -r tip -T '{node|short} {files|json}\n'
b7c4d4bbacd3 []
$ rm -rf ../convert
$ hg convert -q . ../convert --config convert.hg.preserve-hash=true
$ hg --cwd ../convert log -r tip -T '{node|short} {files|json}\n'
c085bbe93d59 ["a"]
$ rm -rf ../convert