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
$ . "$TESTDIR/histedit-helpers.sh"
$ cat >> $HGRCPATH <<EOF
> [extensions]
> histedit=
> EOF
$ hg init r
$ cd r
$ for x in a b c d e f ; do
> echo $x > $x
> hg add $x
> hg ci -m $x
> done
$ hg book -r 1 will-move-backwards
$ hg book -r 2 two
$ hg book -r 2 also-two
$ hg book -r 3 three
$ hg book -r 4 four
$ hg book -r tip five
$ hg log --graph
@ changeset: 5:652413bf663e
| bookmark: five
| tag: tip
| user: test
| date: Thu Jan 01 00:00:00 1970 +0000
| summary: f
|
o changeset: 4:e860deea161a
| bookmark: four
| user: test
| date: Thu Jan 01 00:00:00 1970 +0000
| summary: e
|
o changeset: 3:055a42cdd887
| bookmark: three
| user: test
| date: Thu Jan 01 00:00:00 1970 +0000
| summary: d
|
o changeset: 2:177f92b77385
| bookmark: also-two
| bookmark: two
| user: test
| date: Thu Jan 01 00:00:00 1970 +0000
| summary: c
|
o changeset: 1:d2ae7f538514
| bookmark: will-move-backwards
| user: test
| date: Thu Jan 01 00:00:00 1970 +0000
| summary: b
|
o changeset: 0:cb9a9f314b8b
user: test
date: Thu Jan 01 00:00:00 1970 +0000
summary: a
$ HGEDITOR=cat hg histedit 1
pick d2ae7f538514 1 b
pick 177f92b77385 2 c
pick 055a42cdd887 3 d
pick e860deea161a 4 e
pick 652413bf663e 5 f
# Edit history between d2ae7f538514 and 652413bf663e
#
# Commits are listed from least to most recent
#
# You can reorder changesets by reordering the lines
#
# Commands:
#
# e, edit = use commit, but allow edits before making new commit
# m, mess = edit commit message without changing commit content
# p, pick = use commit
# b, base = checkout changeset and apply further changesets from there
# d, drop = remove commit from history
# f, fold = use commit, but combine it with the one above
# r, roll = like fold, but discard this commit's description and date
#
$ hg histedit 1 --commands - --verbose << EOF | grep histedit
> pick 177f92b77385 2 c
> drop d2ae7f538514 1 b
> pick 055a42cdd887 3 d
> fold e860deea161a 4 e
> pick 652413bf663e 5 f
> EOF
saved backup bundle to $TESTTMP/r/.hg/strip-backup/96e494a2d553-45c027ab-histedit.hg
$ hg log --graph
@ changeset: 3:cacdfd884a93
| bookmark: five
| tag: tip
| user: test
| date: Thu Jan 01 00:00:00 1970 +0000
| summary: f
|
o changeset: 2:59d9f330561f
| bookmark: four
| bookmark: three
| user: test
| date: Thu Jan 01 00:00:00 1970 +0000
| summary: d
|
o changeset: 1:b346ab9a313d
| bookmark: also-two
| bookmark: two
| user: test
| date: Thu Jan 01 00:00:00 1970 +0000
| summary: c
|
o changeset: 0:cb9a9f314b8b
bookmark: will-move-backwards
user: test
date: Thu Jan 01 00:00:00 1970 +0000
summary: a
$ HGEDITOR=cat hg histedit 1
pick b346ab9a313d 1 c
pick 59d9f330561f 2 d
pick cacdfd884a93 3 f
# Edit history between b346ab9a313d and cacdfd884a93
#
# Commits are listed from least to most recent
#
# You can reorder changesets by reordering the lines
#
# Commands:
#
# e, edit = use commit, but allow edits before making new commit
# m, mess = edit commit message without changing commit content
# p, pick = use commit
# b, base = checkout changeset and apply further changesets from there
# d, drop = remove commit from history
# f, fold = use commit, but combine it with the one above
# r, roll = like fold, but discard this commit's description and date
#
$ hg histedit 1 --commands - --verbose << EOF | grep histedit
> pick b346ab9a313d 1 c
> pick cacdfd884a93 3 f
> pick 59d9f330561f 2 d
> EOF
saved backup bundle to $TESTTMP/r/.hg/strip-backup/59d9f330561f-073008af-histedit.hg
We expect 'five' to stay at tip, since the tipmost bookmark is most
likely the useful signal.
$ hg log --graph
@ changeset: 3:c04e50810e4b
| bookmark: five
| bookmark: four
| bookmark: three
| tag: tip
| user: test
| date: Thu Jan 01 00:00:00 1970 +0000
| summary: d
|
o changeset: 2:c13eb81022ca
| user: test
| date: Thu Jan 01 00:00:00 1970 +0000
| summary: f
|
o changeset: 1:b346ab9a313d
| bookmark: also-two
| bookmark: two
| user: test
| date: Thu Jan 01 00:00:00 1970 +0000
| summary: c
|
o changeset: 0:cb9a9f314b8b
bookmark: will-move-backwards
user: test
date: Thu Jan 01 00:00:00 1970 +0000
summary: a