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
$ hg init t
$ cd t
$ echo 1 > foo
$ hg ci -Am1 # 0
adding foo
$ hg branch branchA
marked working directory as branch branchA
(branches are permanent and global, did you want a bookmark?)
$ echo a1 > foo
$ hg ci -ma1 # 1
$ cd ..
$ hg init tt
$ cd tt
$ hg pull ../t
pulling from ../t
requesting all changes
adding changesets
adding manifests
adding file changes
added 2 changesets with 2 changes to 1 files
new changesets 495a0ec48aaf:50e089d141b7
(run 'hg update' to get a working copy)
$ hg up branchA
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ cd ../t
$ echo a2 > foo
$ hg ci -ma2 # 2
Create branch B:
$ hg up 0
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg branch branchB
marked working directory as branch branchB
$ echo b1 > foo
$ hg ci -mb1 # 3
$ cd ../tt
A new branch is there
$ hg pull -u ../t
pulling from ../t
searching for changes
adding changesets
adding manifests
adding file changes
added 2 changesets with 2 changes to 1 files (+1 heads)
new changesets 9f878dea0b96:5be59ce5067b
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
Develop both branches:
$ cd ../t
$ hg up branchA
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ echo a3 > foo
$ hg ci -ma3 # 4
$ hg up branchB
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ echo b2 > foo
$ hg ci -mb2 # 5
$ cd ../tt
Should succeed, no new heads:
$ hg pull -u ../t
pulling from ../t
searching for changes
adding changesets
adding manifests
adding file changes
added 2 changesets with 2 changes to 1 files
new changesets 7c8fe7e20c32:453e93fa00a5
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
Add a head on other branch:
$ cd ../t
$ hg up branchA
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ echo a4 > foo
$ hg ci -ma4 # 6
$ hg up branchB
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ echo b3.1 > foo
$ hg ci -m b3.1 # 7
$ hg up 5
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ echo b3.2 > foo
$ hg ci -m b3.2 # 8
created new head
$ cd ../tt
Should succeed because there is only one head on our branch:
$ hg pull -u ../t
pulling from ../t
searching for changes
adding changesets
adding manifests
adding file changes
added 3 changesets with 3 changes to 1 files (+1 heads)
new changesets da3a8a0161c6:b61cab8fe4e8
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ cd ../t
$ hg up -C branchA
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ echo a5.1 > foo
$ hg ci -ma5.1 # 9
$ hg up 6
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ echo a5.2 > foo
$ hg ci -ma5.2 # 10
created new head
$ hg up 7
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ echo b4.1 > foo
$ hg ci -m b4.1 # 11
$ hg up -C 8
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ echo b4.2 > foo
$ hg ci -m b4.2 # 12
$ cd ../tt
$ hg pull -u ../t
pulling from ../t
searching for changes
adding changesets
adding manifests
adding file changes
added 4 changesets with 4 changes to 1 files (+1 heads)
new changesets 0c4d148ae29e:ecfc3f4a6fd9
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
updated to "d740e1a584e7: a5.2"
1 other heads for branch "branchA"
Make changes on new branch on tt
$ hg up 6
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg branch branchC
marked working directory as branch branchC
$ echo b1 > bar
$ hg ci -Am "commit on branchC on tt"
adding bar
Make changes on default branch on t
$ cd ../t
$ hg up -C default
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ echo a1 > bar
$ hg ci -Am "commit on default on t"
adding bar
Pull branchC from tt
$ hg pull ../tt
pulling from ../tt
searching for changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files (+1 heads)
new changesets 7d8ffa4c0b22
13 local changesets published
(run 'hg heads' to see heads)
Make changes on default and branchC on tt
$ cd ../tt
$ hg pull ../t
pulling from ../t
searching for changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files (+1 heads)
new changesets 2b94b54b6b5f
1 local changesets published
(run 'hg heads' to see heads)
$ hg up -C default
2 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ echo a1 > bar1
$ hg ci -Am "commit on default on tt"
adding bar1
$ hg up branchC
2 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ echo a1 > bar2
$ hg ci -Am "commit on branchC on tt"
adding bar2
Make changes on default and branchC on t
$ cd ../t
$ hg up default
0 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ echo a1 > bar3
$ hg ci -Am "commit on default on t"
adding bar3
$ hg up branchC
2 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ echo a1 > bar4
$ hg ci -Am "commit on branchC on tt"
adding bar4
Pull from tt
$ hg pull ../tt
pulling from ../tt
searching for changes
adding changesets
adding manifests
adding file changes
added 2 changesets with 2 changes to 2 files (+2 heads)
new changesets eed40c14b407:e634733b0309
1 local changesets published
(run 'hg heads .' to see heads, 'hg merge' to merge)
$ cd ..