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 no-windows
$ . "$TESTDIR/remotefilelog-library.sh"
$ hg init master
$ cd master
$ echo treemanifest >> .hg/requires
$ cat >> .hg/hgrc <<EOF
> [remotefilelog]
> server=True
> EOF
# uppercase directory name to test encoding
$ mkdir -p A/B
$ echo x > A/B/x
$ hg commit -qAm x
$ cd ..
# shallow clone from full
$ hgcloneshallow ssh://user@dummy/master shallow --noupdate
streaming all changes
4 files to transfer, 449 bytes of data
transferred 449 bytes in * seconds (*/sec) (glob)
searching for changes
no changes found
$ cd shallow
$ hg debugrequires
dotencode
dirstate-v2 (dirstate-v2 !)
exp-remotefilelog-repo-req-1
fncache
generaldelta
persistent-nodemap (rust !)
revlog-compression-zstd (zstd !)
revlogv1
share-safe
sparserevlog
store
treemanifest
$ find .hg/store/meta | sort
.hg/store/meta
.hg/store/meta/_a
.hg/store/meta/_a/00manifest.i
.hg/store/meta/_a/_b
.hg/store/meta/_a/_b/00manifest.i
$ hg update
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
1 files fetched over 1 fetches - (1 misses, 0.00% hit ratio) over *s (glob)
$ cat A/B/x
x
$ ls .hg/store/data
$ echo foo > A/B/F
$ hg add A/B/F
$ hg ci -m 'local content'
$ ls .hg/store/data
ca31988f085bfb945cb8115b78fabdee40f741aa
$ cd ..
# shallow clone from shallow
$ hgcloneshallow ssh://user@dummy/shallow shallow2 --noupdate
streaming all changes
5 files to transfer, 1008 bytes of data
transferred 1008 bytes in * seconds (*/sec) (glob)
searching for changes
no changes found
$ cd shallow2
$ hg debugrequires
dotencode
dirstate-v2 (dirstate-v2 !)
exp-remotefilelog-repo-req-1
fncache
generaldelta
persistent-nodemap (rust !)
revlog-compression-zstd (zstd !)
revlogv1
share-safe
sparserevlog
store
treemanifest
$ ls .hg/store/data
ca31988f085bfb945cb8115b78fabdee40f741aa
$ hg update
2 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ cat A/B/x
x
$ cd ..
# full clone from shallow
# - send stderr to /dev/null because the order of stdout/err causes
# flakiness here
$ hg clone --noupdate ssh://user@dummy/shallow full 2>/dev/null
streaming all changes
[100]
# getbundle full clone
$ printf '[server]\npreferuncompressed=False\n' >> master/.hg/hgrc
$ hgcloneshallow ssh://user@dummy/master shallow3
requesting all changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 0 changes to 0 files
new changesets 18d955ee7ba0
updating to branch default
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ ls shallow3/.hg/store/data
$ hg debugrequires -R shallow3/
dotencode
dirstate-v2 (dirstate-v2 !)
exp-remotefilelog-repo-req-1
fncache
generaldelta
persistent-nodemap (rust !)
revlog-compression-zstd (zstd !)
revlogv1
share-safe
sparserevlog
store
treemanifest