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 reporevlogstore
$ CONTRIBDIR="$TESTDIR/../contrib"
$ hg init repo-a
$ cd repo-a
$ echo this is file a > a
$ hg add a
$ hg commit -m first
$ echo adding to file a >> a
$ hg commit -m second
$ echo adding more to file a >> a
$ hg commit -m third
$ hg verify
checking changesets
checking manifests
crosschecking files in changesets and manifests
checking files
checked 3 changesets with 3 changes to 1 files
Dumping revlog of file a to stdout:
$ "$PYTHON" "$CONTRIBDIR/dumprevlog" .hg/store/data/a.i
file: .hg/store/data/a.i
node: 183d2312b35066fb6b3b449b84efc370d50993d0
linkrev: 0
parents: 0000000000000000000000000000000000000000 0000000000000000000000000000000000000000
length: 15
-start-
this is file a
-end-
node: b1047953b6e6b633c0d8197eaa5116fbdfd3095b
linkrev: 1
parents: 183d2312b35066fb6b3b449b84efc370d50993d0 0000000000000000000000000000000000000000
length: 32
-start-
this is file a
adding to file a
-end-
node: 8c4fd1f7129b8cdec6c7f58bf48fb5237a4030c1
linkrev: 2
parents: b1047953b6e6b633c0d8197eaa5116fbdfd3095b 0000000000000000000000000000000000000000
length: 54
-start-
this is file a
adding to file a
adding more to file a
-end-
Dump all revlogs to file repo.dump:
$ find .hg/store -name "*.i" | sort | xargs "$PYTHON" "$CONTRIBDIR/dumprevlog" > ../repo.dump
$ cd ..
Undumping into repo-b:
$ hg init repo-b
$ cd repo-b
$ "$PYTHON" "$CONTRIBDIR/undumprevlog" < ../repo.dump
.hg/store/00changelog.i
.hg/store/00manifest.i
.hg/store/data/a.i
$ cd ..
Rebuild fncache with clone --pull:
$ hg clone --pull -U repo-b repo-c
requesting all changes
adding changesets
adding manifests
adding file changes
added 3 changesets with 3 changes to 1 files
new changesets de1da620e7d8:46946d278c50
Verify:
$ hg -R repo-c verify
checking changesets
checking manifests
crosschecking files in changesets and manifests
checking files
checked 3 changesets with 3 changes to 1 files
Compare repos:
$ hg -R repo-c incoming repo-a
comparing with repo-a
searching for changes
no changes found
[1]
$ hg -R repo-a incoming repo-c
comparing with repo-c
searching for changes
no changes found
[1]