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
$ hgcommit() {
> hg commit -u user "$@"
> }
$ hg init clhead
$ cd clhead
$ touch foo && hg add && hgcommit -m 'foo'
adding foo
$ touch bar && hg add && hgcommit -m 'bar'
adding bar
$ touch baz && hg add && hgcommit -m 'baz'
adding baz
$ echo "flub" > foo
$ hgcommit -m "flub"
$ echo "nub" > foo
$ hgcommit -m "nub"
$ hg up -C 2
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ echo "c1" > c1
$ hg add c1
$ hgcommit -m "c1"
created new head
$ echo "c2" > c1
$ hgcommit -m "c2"
$ hg up -C 2
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ echo "d1" > d1
$ hg add d1
$ hgcommit -m "d1"
created new head
$ echo "d2" > d1
$ hgcommit -m "d2"
$ hg tag -l good
fail with three heads
$ hg up -C good
0 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg merge
abort: branch 'default' has 3 heads - please merge with an explicit rev
(run 'hg heads .' to see heads, specify rev with -r)
[255]
close one of the heads
$ hg up -C 6
1 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ hgcommit -m 'close this head' --close-branch
succeed with two open heads
$ hg up -C good
1 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ hg up -C good
0 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg merge
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
(branch merge, don't forget to commit)
$ hgcommit -m 'merged heads'
hg update -C 8
$ hg update -C 8
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
hg branch some-branch
$ hg branch some-branch
marked working directory as branch some-branch
(branches are permanent and global, did you want a bookmark?)
hg commit
$ hgcommit -m 'started some-branch'
hg commit --close-branch
$ hgcommit --close-branch -m 'closed some-branch'
hg update default
$ hg update default
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
hg merge some-branch
$ hg merge some-branch
0 files updated, 0 files merged, 0 files removed, 0 files unresolved
(branch merge, don't forget to commit)
hg commit (no reopening of some-branch)
$ hgcommit -m 'merge with closed branch'
$ cd ..