util: improve iterfile so it chooses code path wisely
We have performance concerns on "iterfile" as it is 4X slower on normal
files. While modern systems have the nice property that reading a "fast"
(on-disk) file cannot be interrupted and should be made use of.
This patch dumps the related knowledge in comments. And "iterfile" chooses
code paths wisely:
1. If it's CPython 3, or PyPY, use the fast path.
2. If fp is a normal file, use the fast path.
3. If fp is not a normal file and CPython version >= 2.7.4, use the same
workaround (4x slower) as before.
4. If fp is not a normal file and CPython version < 2.7.4, use another
workaround (2x slower but may block longer then necessary) which
basically re-invents the buffer + readline logic in Python.
This will give us good confidence on both correctness and performance
dealing with EINTR in iterfile(fp) for all known supported Python versions.
https://bz.mercurial-scm.org/2137
Setup:
create a little extension that has 3 side-effects:
1) ensure changelog data is not inlined
2) make revlog to use lazyparser
3) test that repo.lookup() works
1 and 2 are preconditions for the bug; 3 is the bug.
$ cat > commitwrapper.py <<EOF
> from mercurial import extensions, node, revlog
>
> def reposetup(ui, repo):
> class wraprepo(repo.__class__):
> def commit(self, *args, **kwargs):
> result = super(wraprepo, self).commit(*args, **kwargs)
> tip1 = node.short(repo.changelog.tip())
> tip2 = node.short(repo.lookup(tip1))
> assert tip1 == tip2
> ui.write('new tip: %s\n' % tip1)
> return result
> repo.__class__ = wraprepo
>
> def extsetup(ui):
> revlog._maxinline = 8 # split out 00changelog.d early
> revlog._prereadsize = 8 # use revlog.lazyparser
> EOF
$ cat >> $HGRCPATH <<EOF
> [extensions]
> commitwrapper = `pwd`/commitwrapper.py
> EOF
$ hg init repo1
$ cd repo1
$ echo a > a
$ hg commit -A -m'add a with a long commit message to make the changelog a bit bigger'
adding a
new tip: 553596fad57b
Test that new changesets are visible to repo.lookup():
$ echo a >> a
$ hg commit -m'one more commit to demonstrate the bug'
new tip: 799ae3599e0e
$ hg tip
changeset: 1:799ae3599e0e
tag: tip
user: test
date: Thu Jan 01 00:00:00 1970 +0000
summary: one more commit to demonstrate the bug
$ cd ..