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
Create @ bookmark as main reference
$ hg init repo
$ cd repo
$ echo "[extensions]" >> $HGRCPATH
$ echo "patchbomb=" >> $HGRCPATH
$ hg book @
Create a dummy revision that must never be exported
$ echo no > no
$ hg ci -Amno -d '6 0'
adding no
Create a feature and use -B
$ hg book booktest
$ echo first > a
$ hg ci -Amfirst -d '7 0'
adding a
$ echo second > b
$ hg ci -Amsecond -d '8 0'
adding b
$ hg email --date '1981-1-1 0:1' -n -t foo -s bookmark -B booktest
From [test]: test
this patch series consists of 2 patches.
Write the introductory message for the patch series.
Cc:
displaying [PATCH 0 of 2] bookmark ...
MIME-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 7bit
Subject: [PATCH 0 of 2] bookmark
Message-Id: <patchbomb.347155260@test-hostname>
User-Agent: Mercurial-patchbomb/* (glob)
Date: Thu, 01 Jan 1981 00:01:00 +0000
From: test
To: foo
displaying [PATCH 1 of 2] first ...
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Subject: [PATCH 1 of 2] first
X-Mercurial-Node: accde9b8b6dce861c185d0825c1affc09a79cb26
X-Mercurial-Series-Index: 1
X-Mercurial-Series-Total: 2
Message-Id: <accde9b8b6dce861c185.347155261@test-hostname>
X-Mercurial-Series-Id: <accde9b8b6dce861c185.347155261@test-hostname>
In-Reply-To: <patchbomb.347155260@test-hostname>
References: <patchbomb.347155260@test-hostname>
User-Agent: Mercurial-patchbomb/* (glob)
Date: Thu, 01 Jan 1981 00:01:01 +0000
From: test
To: foo
# HG changeset patch
# User test
# Date 7 0
# Thu Jan 01 00:00:07 1970 +0000
# Node ID accde9b8b6dce861c185d0825c1affc09a79cb26
# Parent 043bd3889e5aaf7d88fe3713cf425f782ad2fb71
first
diff -r 043bd3889e5a -r accde9b8b6dc a
--- /dev/null Thu Jan 01 00:00:00 1970 +0000
+++ b/a Thu Jan 01 00:00:07 1970 +0000
@@ -0,0 +1,1 @@
+first
displaying [PATCH 2 of 2] second ...
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Subject: [PATCH 2 of 2] second
X-Mercurial-Node: 417defd1559c396ba06a44dce8dc1c2d2d653f3f
X-Mercurial-Series-Index: 2
X-Mercurial-Series-Total: 2
Message-Id: <417defd1559c396ba06a.347155262@test-hostname>
X-Mercurial-Series-Id: <accde9b8b6dce861c185.347155261@test-hostname>
In-Reply-To: <patchbomb.347155260@test-hostname>
References: <patchbomb.347155260@test-hostname>
User-Agent: Mercurial-patchbomb/* (glob)
Date: Thu, 01 Jan 1981 00:01:02 +0000
From: test
To: foo
# HG changeset patch
# User test
# Date 8 0
# Thu Jan 01 00:00:08 1970 +0000
# Node ID 417defd1559c396ba06a44dce8dc1c2d2d653f3f
# Parent accde9b8b6dce861c185d0825c1affc09a79cb26
second
diff -r accde9b8b6dc -r 417defd1559c b
--- /dev/null Thu Jan 01 00:00:00 1970 +0000
+++ b/b Thu Jan 01 00:00:08 1970 +0000
@@ -0,0 +1,1 @@
+second
Do the same and combine with -o only one must be exported
$ cd ..
$ hg clone repo repo2
updating to bookmark @
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ cd repo
$ hg up @
0 files updated, 0 files merged, 2 files removed, 0 files unresolved
(activating bookmark @)
$ hg book outgoing
$ echo 1 > x
$ hg ci -Am1 -d '8 0'
adding x
created new head
$ hg push ../repo2 -B outgoing
pushing to ../repo2
searching for changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files (+1 heads)
exporting bookmark outgoing
$ echo 2 > y
$ hg ci -Am2 -d '9 0'
adding y
$ hg email --date '1982-1-1 0:1' -n -t foo -s bookmark -B outgoing -o ../repo2
comparing with ../repo2
From [test]: test
this patch series consists of 1 patches.
Cc:
displaying [PATCH] bookmark ...
MIME-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 7bit
Subject: [PATCH] bookmark
X-Mercurial-Node: 8dab2639fd35f1e337ad866c372a5c44f1064e3c
X-Mercurial-Series-Index: 1
X-Mercurial-Series-Total: 1
Message-Id: <8dab2639fd35f1e337ad.378691260@test-hostname>
X-Mercurial-Series-Id: <8dab2639fd35f1e337ad.378691260@test-hostname>
User-Agent: Mercurial-patchbomb/* (glob)
Date: Fri, 01 Jan 1982 00:01:00 +0000
From: test
To: foo
# HG changeset patch
# User test
# Date 9 0
# Thu Jan 01 00:00:09 1970 +0000
# Node ID 8dab2639fd35f1e337ad866c372a5c44f1064e3c
# Parent 0b24b8316483bf30bfc3e4d4168e922b169dbe66
2
diff -r 0b24b8316483 -r 8dab2639fd35 y
--- /dev/null Thu Jan 01 00:00:00 1970 +0000
+++ b/y Thu Jan 01 00:00:09 1970 +0000
@@ -0,0 +1,1 @@
+2