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/narrow-library.sh"
create full repo
$ hg init master
$ cd master
$ echo init > init
$ hg ci -Aqm 'initial'
$ mkdir inside
$ echo inside > inside/f1
$ mkdir outside
$ echo outside > outside/f1
$ hg ci -Aqm 'add inside and outside'
$ echo modified > inside/f1
$ hg ci -qm 'modify inside'
$ echo modified > outside/f1
$ hg ci -qm 'modify outside'
$ cd ..
$ hg clone --narrow ssh://user@dummy/master narrow --include inside
requesting all changes
adding changesets
adding manifests
adding file changes
added 4 changesets with 2 changes to 1 files
new changesets *:* (glob)
updating to branch default
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ cd narrow
$ hg debugindex -c
rev linkrev nodeid p1-nodeid p2-nodeid
0 0 9958b1af2add 000000000000 000000000000
1 1 2db4ce2a3bfe 9958b1af2add 000000000000
2 2 0980ee31a742 2db4ce2a3bfe 000000000000
3 3 4410145019b7 0980ee31a742 000000000000
$ hg update -q 0
Can update to revision with changes inside
$ hg update -q 'desc("add inside and outside")'
$ hg update -q 'desc("modify inside")'
$ find *
inside
inside/f1
$ cat inside/f1
modified
Can update to revision with changes outside
$ hg update -q 'desc("modify outside")'
$ find *
inside
inside/f1
$ cat inside/f1
modified
Can update with a deleted file inside
$ hg rm inside/f1
$ hg update -q 'desc("modify inside")'
$ hg update -q 'desc("modify outside")'
$ hg update -q 'desc("initial")'
$ hg update -q 'desc("modify inside")'
Can update with a moved file inside
$ hg mv inside/f1 inside/f2
$ hg update -q 'desc("modify outside")'
$ hg update -q 'desc("add inside and outside")'
$ hg update -q 'desc("modify inside")'