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
PYTHONVER=2.7.16
PYTHONNAME=python-
PREFIX=$(HOME)/bin/prefix-$(PYTHONNAME)$(PYTHONVER)
SYMLINKDIR=$(HOME)/bin
help:
@echo
@echo 'Make a custom installation of a Python version'
@echo
@echo 'Common make parameters:'
@echo ' PYTHONVER=... [$(PYTHONVER)]'
@echo ' PREFIX=... [$(PREFIX)]'
@echo ' SYMLINKDIR=... [$(SYMLINKDIR) creating $(PYTHONNAME)$(PYTHONVER)]'
@echo
@echo 'Common make targets:'
@echo ' python - install Python $$PYTHONVER in $$PREFIX'
@echo ' symlink - create a $$SYMLINKDIR/$(PYTHONNAME)$$PYTHONVER symlink'
@echo
@echo 'Example: create a temporary Python installation:'
@echo ' $$ make -f Makefile.python python PYTHONVER=${PYTHONVER} PREFIX=/tmp/p27'
@echo ' $$ /tmp/p27/bin/python -V'
@echo ' Python 2.7'
@echo
@echo 'Some external libraries are required for building Python: zlib bzip2 openssl.'
@echo 'Make sure their development packages are installed systemwide.'
# fedora: yum install zlib-devel bzip2-devel openssl-devel
# debian: apt-get install zlib1g-dev libbz2-dev libssl-dev
@echo
@echo 'To build a nice collection of interesting Python versions:'
@echo ' $$ for v in 2.{6{,.1,.2,.9},7{,.8,.10}}; do'
@echo ' make -f Makefile.python symlink PYTHONVER=$$v || break; done'
@echo 'To run a Mercurial test on all these Python versions:'
@echo ' $$ for py in `cd ~/bin && ls $(PYTHONNAME)2.*`; do'
@echo ' echo $$py; $$py run-tests.py test-http.t; echo; done'
@echo
export LANGUAGE=C
export LC_ALL=C
python: $(PREFIX)/bin/python docutils
printf 'import sys, zlib, bz2, docutils, ssl' | $(PREFIX)/bin/python
PYTHON_SRCDIR=Python-$(PYTHONVER)
PYTHON_SRCFILE=$(PYTHON_SRCDIR).tgz
$(PREFIX)/bin/python:
[ -f $(PYTHON_SRCFILE) ] || wget http://www.python.org/ftp/python/$(PYTHONVER)/$(PYTHON_SRCFILE) || curl -OL http://www.python.org/ftp/python/$(PYTHONVER)/$(PYTHON_SRCFILE) || [ -f $(PYTHON_SRCFILE) ]
rm -rf $(PYTHON_SRCDIR)
tar xf $(PYTHON_SRCFILE)
# Debian/Ubuntu disables SSLv2,3 the hard way, disable it on old Pythons too
-sed -i 's,self.*SSLv[23]_method(),0;//\0,g' $(PYTHON_SRCDIR)/Modules/_ssl.c
# Find multiarch system libraries on Ubuntu and disable fortify error when setting argv
LDFLAGS="-L/usr/lib/`dpkg-architecture -qDEB_HOST_MULTIARCH`"; \
BASECFLAGS=-U_FORTIFY_SOURCE; \
export LDFLAGS BASECFLAGS; \
cd $(PYTHON_SRCDIR) && ./configure --prefix=$(PREFIX) && make all SVNVERSION=pwd && make install
printf 'import sys, zlib, bz2, ssl' | $(PREFIX)/bin/python
rm -rf $(PYTHON_SRCDIR)
DOCUTILSVER=0.12
DOCUTILS_SRCDIR=docutils-$(DOCUTILSVER)
DOCUTILS_SRCFILE=$(DOCUTILS_SRCDIR).tar.gz
docutils: $(PREFIX)/bin/python
@$(PREFIX)/bin/python -c 'import docutils' || ( set -ex; \
[ -f $(DOCUTILS_SRCFILE) ] || wget http://downloads.sourceforge.net/project/docutils/docutils/$(DOCUTILSVER)/$(DOCUTILS_SRCFILE) || [ -f $(DOCUTILS_SRCFILE) ]; \
rm -rf $(DOCUTILS_SRCDIR); \
tar xf $(DOCUTILS_SRCFILE); \
cd $(DOCUTILS_SRCDIR) && $(PREFIX)/bin/python setup.py install --prefix=$(PREFIX); \
$(PREFIX)/bin/python -c 'import docutils'; \
rm -rf $(DOCUTILS_SRCDIR); )
symlink: python $(SYMLINKDIR)
ln -sf $(PREFIX)/bin/python $(SYMLINKDIR)/$(PYTHONNAME)$(PYTHONVER)
.PHONY: help python docutils symlink