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
$ cat > echo.py <<EOF
> #!$PYTHON
> import os
> import sys
> try:
> import msvcrt
> msvcrt.setmode(sys.stdout.fileno(), os.O_BINARY)
> msvcrt.setmode(sys.stderr.fileno(), os.O_BINARY)
> except ImportError:
> pass
>
> for k in ('HG_FILE', 'HG_MY_ISLINK', 'HG_OTHER_ISLINK', 'HG_BASE_ISLINK'):
> print(k, os.environ[k])
> EOF
Create 2 heads containing the same file, once as
a file, once as a link. Bundle was generated with:
# hg init t
# cd t
# echo a > a
# hg ci -qAm t0 -d '0 0'
# echo l > l
# hg ci -qAm t1 -d '1 0'
# hg up -C 0
# ln -s a l
# hg ci -qAm t2 -d '2 0'
# echo l2 > l2
# hg ci -qAm t3 -d '3 0'
$ hg init t
$ cd t
$ hg -q unbundle "$TESTDIR/bundles/test-merge-symlinks.hg"
$ hg up -C 3
3 files updated, 0 files merged, 0 files removed, 0 files unresolved
Merge them and display *_ISLINK vars
merge heads
$ hg merge --tool="\"$PYTHON\" ../echo.py"
merging l
HG_FILE l
HG_MY_ISLINK 1
HG_OTHER_ISLINK 0
HG_BASE_ISLINK 0
0 files updated, 1 files merged, 0 files removed, 0 files unresolved
(branch merge, don't forget to commit)
Test working directory symlink bit calculation wrt copies,
especially on non-supporting systems.
merge working directory
$ hg up -C 2
1 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ hg copy l l2
$ HGMERGE="\"$PYTHON\" ../echo.py" hg up 3
merging l2
HG_FILE l2
HG_MY_ISLINK 1
HG_OTHER_ISLINK 0
HG_BASE_ISLINK 0
0 files updated, 1 files merged, 0 files removed, 0 files unresolved
$ cd ..