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
test branch selection options
$ hg init branch
$ cd branch
$ hg branch a
marked working directory as branch a
(branches are permanent and global, did you want a bookmark?)
$ echo a > foo
$ hg ci -d '0 0' -Ama
adding foo
$ echo a2 > foo
$ hg ci -d '0 0' -ma2
$ hg up 0
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg branch c
marked working directory as branch c
$ echo c > foo
$ hg ci -d '0 0' -mc
$ hg tag -l z
$ cd ..
$ hg clone -r 0 branch branch2
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files
new changesets 5b65ba7c951d
updating to branch a
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ cd branch2
$ hg up 0
0 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg branch b
marked working directory as branch b
$ echo b > foo
$ hg ci -d '0 0' -mb
$ hg up 0
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg --encoding utf-8 branch æ
marked working directory as branch \xc3\xa6 (esc)
$ echo ae1 > foo
$ hg ci -d '0 0' -mae1
$ hg up 0
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg --encoding utf-8 branch -f æ
marked working directory as branch \xc3\xa6 (esc)
$ echo ae2 > foo
$ hg ci -d '0 0' -mae2
created new head
$ hg up 0
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg branch -f b
marked working directory as branch b
$ echo b2 > foo
$ hg ci -d '0 0' -mb2
created new head
unknown branch and fallback
$ hg in -qbz
abort: unknown branch 'z'
[10]
$ hg in -q ../branch#z
2:f25d57ab0566
$ hg out -qbz
abort: unknown branch 'z'
[10]
in rev c branch a
$ hg in -qr c ../branch#a
1:dd6e60a716c6
2:f25d57ab0566
$ hg in -qr c -b a
1:dd6e60a716c6
2:f25d57ab0566
out branch .
$ hg out -q ../branch#.
1:b84708d77ab7
4:65511d0e2b55
$ hg out -q -b .
1:b84708d77ab7
4:65511d0e2b55
out branch . non-ascii
$ hg --encoding utf-8 up æ
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg --encoding latin1 out -q ../branch#.
2:df5a44224d4e
3:4f4a5125ca10
$ hg --encoding latin1 out -q -b .
2:df5a44224d4e
3:4f4a5125ca10
clone branch b
$ cd ..
$ hg clone branch2#b branch3
adding changesets
adding manifests
adding file changes
added 3 changesets with 3 changes to 1 files (+1 heads)
new changesets 5b65ba7c951d:65511d0e2b55
updating to branch b
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg -q -R branch3 heads b
2:65511d0e2b55
1:b84708d77ab7
$ hg -q -R branch3 parents
2:65511d0e2b55
$ rm -rf branch3
clone rev a branch b
$ hg clone -r a branch2#b branch3
adding changesets
adding manifests
adding file changes
added 3 changesets with 3 changes to 1 files (+1 heads)
new changesets 5b65ba7c951d:65511d0e2b55
updating to branch a
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg -q -R branch3 heads b
2:65511d0e2b55
1:b84708d77ab7
$ hg -q -R branch3 parents
0:5b65ba7c951d
$ rm -rf branch3