update: add a Rust fast-path when updating from null (and clean)
This case is easy to detect and we have all we need to generate a valid
working copy and dirstate entirely in Rust, which speeds things up
considerably:
On my machine updating a repo of ~300k files goes from 10.00s down to 4.2s,
all while consuming 50% less system time, with all caches hot.
Something to note is that further improvements will probably happen
with the upcoming `InnerRevlog` series that does smarter
mmap hanlding, especially for filelogs.
Here are benchmark numbers on a machine with only 4 cores (and no SMT enabled)
```
### data-env-vars.name = heptapod-public-2024-03-25-ds2-pnm
# benchmark.name = hg.command.update
# bin-env-vars.hg.py-re2-module = default
# bin-env-vars.hg.changeset.node = <this change>
# benchmark.variants.atomic-update = no
# benchmark.variants.scenario = null-to-tip
# benchmark.variants.worker = default
default: 5.328762 ~~~~~
rust: 1.308654 (-75.44%, -4.02)
### data-env-vars.name = mercurial-devel-2024-03-22-ds2-pnm
# benchmark.name = hg.command.update
# bin-env-vars.hg.py-re2-module = default
# bin-env-vars.hg.changeset.node = <this change>
# benchmark.variants.atomic-update = no
# benchmark.variants.scenario = null-to-tip
# benchmark.variants.worker = default
default: 1.693271 ~~~~~
rust: 1.151053 (-32.02%, -0.54)
### data-env-vars.name = mozilla-unified-2024-03-22-ds2-pnm
# benchmark.name = hg.command.update
# bin-env-vars.hg.py-re2-module = default
# bin-env-vars.hg.changeset.node = <this change>
# benchmark.variants.atomic-update = no
# benchmark.variants.scenario = null-to-tip
# benchmark.variants.worker = default
default: 38.901613 ~~~~~
rust: 11.637880 (-70.08%, -27.26)
### data-env-vars.name = netbsd-xsrc-public-2024-09-19-ds2-pnm
# benchmark.name = hg.command.update
# bin-env-vars.hg.py-re2-module = default
# bin-env-vars.hg.changeset.node = <this change>
# benchmark.variants.atomic-update = no
# benchmark.variants.scenario = null-to-tip
# benchmark.variants.worker = default
default: 4.793727 ~~~~~
rust: 1.505905 (-68.59%, -3.29)
```
https://bz.mercurial-scm.org/1175
$ hg init repo
$ cd repo
$ touch a
$ hg ci -Am0
adding a
$ hg mv a a1
$ hg ci -m1
$ hg co 0
1 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ hg mv a a2
$ hg up
note: possible conflict - a was renamed multiple times to:
a1
a2
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg ci -m2
$ touch a
$ hg ci -Am3
adding a
$ hg mv a b
$ hg ci -Am4 a
$ hg ci --debug --traceback -Am5 b
committing files:
b
warning: can't find ancestor for 'b' copied from 'a'!
committing manifest
committing changelog
updating the branch cache
committed changeset 5:83a687e8a97c80992ba385bbfd766be181bfb1d1
$ hg verify -q
$ hg export --git tip
# HG changeset patch
# User test
# Date 0 0
# Thu Jan 01 00:00:00 1970 +0000
# Node ID 83a687e8a97c80992ba385bbfd766be181bfb1d1
# Parent 1d1625283f71954f21d14c3d44d0ad3c019c597f
5
diff --git a/b b/b
new file mode 100644
https://bz.mercurial-scm.org/show_bug.cgi?id=4476
$ hg init foo
$ cd foo
$ touch a && hg ci -Aqm a
$ hg mv a b
$ echo b1 >> b
$ hg ci -Aqm b1
$ hg up 0
1 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ hg mv a b
$ echo b2 >> b
$ hg ci -Aqm b2
$ hg graft 1
grafting 1:5974126fad84 "b1"
merging b
warning: conflicts while merging b! (edit, then use 'hg resolve --mark')
abort: unresolved conflicts, can't continue
(use 'hg resolve' and 'hg graft --continue')
[1]
$ echo a > b
$ echo b3 >> b
$ hg resolve --mark b
(no more unresolved files)
continue: hg graft --continue
$ hg graft --continue
grafting 1:5974126fad84 "b1"
$ hg log -f b -T 'changeset: {rev}:{node|short}\nsummary: {desc}\n\n'
changeset: 3:376d30ccffc0
summary: b1
changeset: 2:416baaa2e5e4
summary: b2
changeset: 0:3903775176ed
summary: a