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
==========================================================
Test file dedicated to checking side-data related behavior
==========================================================
Check data can be written/read from sidedata
============================================
$ cat << EOF >> $HGRCPATH
> [extensions]
> testsidedata=$TESTDIR/testlib/ext-sidedata.py
> EOF
$ hg init test-sidedata --config experimental.revlogv2=enable-unstable-format-and-corrupt-my-data
$ cd test-sidedata
$ echo aaa > a
$ hg add a
$ hg commit -m a --traceback
$ echo aaa > b
$ hg add b
$ hg commit -m b
$ echo xxx >> a
$ hg commit -m aa
$ hg debugsidedata -c 0
2 sidedata entries
entry-0001 size 4
entry-0002 size 32
$ hg debugsidedata -c 1 -v
2 sidedata entries
entry-0001 size 4
'\x00\x00\x006'
entry-0002 size 32
'\x98\t\xf9\xc4v\xf0\xc5P\x90\xf7wRf\xe8\xe27e\xfc\xc1\x93\xa4\x96\xd0\x1d\x97\xaaG\x1d\xd7t\xfa\xde'
$ hg debugsidedata -m 2
2 sidedata entries
entry-0001 size 4
entry-0002 size 32
$ hg debugsidedata a 1
2 sidedata entries
entry-0001 size 4
entry-0002 size 32
$ hg debug-revlog-index --verbose -c
rev rank linkrev nodeid p1-rev p1-nodeid p2-rev p2-nodeid full-size delta-base flags comp-mode data-offset chunk-size sd-comp-mode sidedata-offset sd-chunk-size
0 -1 0 7049e48789d7 -1 000000000000 -1 000000000000 54 0 0 0 0 54 plain 0 90
1 -1 1 2707720c6597 0 7049e48789d7 -1 000000000000 54 1 0 0 54 54 plain 90 90
2 -1 2 40f977031323 1 2707720c6597 -1 000000000000 55 2 0 0 108 55 plain 180 90
$ hg debug-revlog-index --verbose -m
rev rank linkrev nodeid p1-rev p1-nodeid p2-rev p2-nodeid full-size delta-base flags comp-mode data-offset chunk-size sd-comp-mode sidedata-offset sd-chunk-size
0 -1 0 b85d294330e3 -1 000000000000 -1 000000000000 43 0 0 0 0 43 plain 0 90
1 -1 1 1a0aec305c63 0 b85d294330e3 -1 000000000000 86 0 0 0 43 55 plain 90 90
2 -1 2 104258a4f75f 1 1a0aec305c63 -1 000000000000 86 1 0 0 98 55 plain 180 90
Check upgrade behavior
======================
Right now, sidedata has not upgrade support
Check that we can upgrade to sidedata
-------------------------------------
$ hg init up-no-side-data --config experimental.revlogv2=no
$ hg debugformat -v -R up-no-side-data | egrep 'changelog-v2|revlog-v2'
revlog-v2: no no no
changelog-v2: no no no
$ hg debugformat -v -R up-no-side-data --config experimental.revlogv2=enable-unstable-format-and-corrupt-my-data | egrep 'changelog-v2|revlog-v2'
revlog-v2: no yes no
changelog-v2: no no no
$ hg debugupgraderepo -R up-no-side-data --config experimental.revlogv2=enable-unstable-format-and-corrupt-my-data > /dev/null
Check that we can downgrade from sidedata
-----------------------------------------
$ hg init up-side-data --config experimental.revlogv2=enable-unstable-format-and-corrupt-my-data
$ hg debugformat -v -R up-side-data | egrep 'changelog-v2|revlog-v2'
revlog-v2: yes no no
changelog-v2: no no no
$ hg debugformat -v -R up-side-data --config experimental.revlogv2=no | egrep 'changelog-v2|revlog-v2'
revlog-v2: yes no no
changelog-v2: no no no
$ hg debugupgraderepo -R up-side-data --config experimental.revlogv2=no > /dev/null