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
$ cat <<EOF >> $HGRCPATH
> [extensions]
> convert =
> [convert]
> hg.tagsbranch = 0
> EOF
$ hg init source
$ cd source
$ echo a > a
$ hg ci -qAm adda
Add a merge with one parent in the same branch
$ echo a >> a
$ hg ci -qAm changea
$ hg up -qC 0
$ hg branch branch0
marked working directory as branch branch0
(branches are permanent and global, did you want a bookmark?)
$ echo b > b
$ hg ci -qAm addb
$ hg up -qC
$ hg merge default
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
(branch merge, don't forget to commit)
$ hg ci -qm mergeab
$ hg tag -ql mergeab
$ cd ..
Miss perl... sometimes
$ cat > filter.py <<EOF
> import re
> import sys
>
> r = re.compile(r'^(?:\d+|pulling from)')
> sys.stdout.writelines([l for l in sys.stdin if r.search(l)])
> EOF
convert
$ hg convert -v --config convert.hg.clonebranches=1 source dest |
> "$PYTHON" filter.py
3 adda
2 changea
1 addb
pulling from default into branch0
1 changesets found
0 mergeab
pulling from default into branch0
1 changesets found
Add a merge with both parents and child in different branches
$ cd source
$ hg branch branch1
marked working directory as branch branch1
$ echo a > file1
$ hg ci -qAm c1
$ hg up -qC mergeab
$ hg branch branch2
marked working directory as branch branch2
$ echo a > file2
$ hg ci -qAm c2
$ hg merge branch1
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
(branch merge, don't forget to commit)
$ hg branch branch3
marked working directory as branch branch3
$ hg ci -qAm c3
$ cd ..
incremental conversion
$ hg convert -v --config convert.hg.clonebranches=1 source dest |
> "$PYTHON" filter.py
2 c1
pulling from branch0 into branch1
4 changesets found
1 c2
pulling from branch0 into branch2
4 changesets found
0 c3
pulling from branch1 into branch3
5 changesets found
pulling from branch2 into branch3
1 changesets found