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
#!/usr/bin/env python3
#
# mercurial - scalable distributed SCM
#
# Copyright 2005-2007 Olivia Mackall <olivia@selenic.com>
#
# This software may be used and distributed according to the terms of the
# GNU General Public License version 2 or any later version.
import os
import sys
libdir = '@LIBDIR@'
if libdir != '@' 'LIBDIR' '@':
if not os.path.isabs(libdir):
libdir = os.path.join(
os.path.dirname(os.path.realpath(__file__)), libdir
)
libdir = os.path.abspath(libdir)
sys.path.insert(0, libdir)
# Make `pip install --user ...` packages available to the official Windows
# build. Most py2 packaging installs directly into the system python
# environment, so no changes are necessary for other platforms. The Windows
# py2 package uses py2exe, which lacks a `site` module. Hardcode it according
# to the documentation.
if getattr(sys, 'frozen', None) == 'console_exe':
vi = sys.version_info
appdata = os.environ.get('APPDATA')
if appdata:
sys.path.append(
os.path.join(
appdata,
'Python',
'Python%d%d' % (vi[0], vi[1]),
'site-packages',
)
)
from hgdemandimport import tracing
with tracing.log('hg script'):
# enable importing on demand to reduce startup time
try:
import hgdemandimport
hgdemandimport.enable()
except ImportError:
sys.stderr.write(
"abort: couldn't find mercurial libraries in [%s]\n"
% ' '.join(sys.path)
)
sys.stderr.write("(check your install and PYTHONPATH)\n")
sys.exit(-1)
from mercurial import dispatch
dispatch.run()