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
$ hg init
$ echo "added file1" > file1
$ echo "another line of text" >> file1
$ echo "added file2" > file2
$ hg add file1 file2
$ hg commit -m "added file1 and file2"
$ echo "changed file1" >> file1
$ hg commit -m "changed file1"
$ hg -q log
1:dfab7f3c2efb
0:c3fa057dd86f
$ hg id
dfab7f3c2efb tip
$ hg update -C 0
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg id
c3fa057dd86f
$ echo "changed file1" >> file1
$ hg id
c3fa057dd86f+
$ hg revert --no-backup --all
reverting file1
$ hg diff
$ hg status
$ hg id
c3fa057dd86f
$ hg update
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg diff
$ hg status
$ hg id
dfab7f3c2efb tip
$ hg update -C 0
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ echo "changed file1 different" >> file1
$ hg update
merging file1
warning: conflicts while merging file1! (edit, then use 'hg resolve --mark')
0 files updated, 0 files merged, 0 files removed, 1 files unresolved
use 'hg resolve' to retry unresolved file merges
[1]
$ hg diff --nodates
diff -r dfab7f3c2efb file1
--- a/file1
+++ b/file1
@@ -1,3 +1,7 @@
added file1
another line of text
+<<<<<<< working copy: c3fa057dd86f - test: added file1 and file2
+changed file1 different
+=======
changed file1
+>>>>>>> destination: dfab7f3c2efb - test: changed file1
$ hg status
M file1
? file1.orig
$ hg id
dfab7f3c2efb+ tip
$ hg revert --no-backup --all
reverting file1
$ hg diff
$ hg status
? file1.orig
$ hg id
dfab7f3c2efb tip
$ hg revert -r tip --no-backup --all
$ hg diff
$ hg status
? file1.orig
$ hg id
dfab7f3c2efb tip
$ hg update -C
0 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg diff
$ hg status
? file1.orig
$ hg id
dfab7f3c2efb tip