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
#require no-windows
$ . "$TESTDIR/remotefilelog-library.sh"
$ hg init master
$ cd master
$ cat >> .hg/hgrc <<EOF
> [remotefilelog]
> server=True
> EOF
$ echo x > foo
$ echo y > bar
$ hg commit -qAm one
$ cd ..
# partial shallow clone
$ hg clone --shallow ssh://user@dummy/master shallow --noupdate --config remotefilelog.includepattern=foo
streaming all changes
3 files to transfer, 336 bytes of data (no-zstd !)
transferred 336 bytes in * seconds (* */sec) (glob) (no-zstd !)
3 files to transfer, 338 bytes of data (zstd !)
transferred 338 bytes in * seconds (* */sec) (glob) (zstd !)
searching for changes
no changes found
$ cat >> shallow/.hg/hgrc <<EOF
> [remotefilelog]
> cachepath=$PWD/hgcache
> debug=True
> includepattern=foo
> reponame = master
> [extensions]
> remotefilelog=
> EOF
$ ls shallow/.hg/store/data
bar.i
# update partial clone
$ cd shallow
$ hg update
2 files updated, 0 files merged, 0 files removed, 0 files unresolved
1 files fetched over 1 fetches - (1 misses, 0.00% hit ratio) over *s (glob)
$ cat foo
x
$ cat bar
y
$ cd ..
# pull partial clone
$ cd master
$ echo a >> foo
$ echo b >> bar
$ hg commit -qm two
$ cd ../shallow
$ hg pull
pulling from ssh://user@dummy/master
searching for changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 0 changes to 0 files
new changesets a9688f18cb91
(run 'hg update' to get a working copy)
$ hg update
2 files updated, 0 files merged, 0 files removed, 0 files unresolved
1 files fetched over 1 fetches - (1 misses, 0.00% hit ratio) over *s (glob)
$ cat foo
x
a
$ cat bar
y
b
$ cd ..