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.
$ hg init test
$ cd test
$ echo foo>foo
$ hg addremove
adding foo
$ hg commit -m "1"
$ hg verify
checking changesets
checking manifests
crosschecking files in changesets and manifests
checking files
1 files, 1 changesets, 1 total revisions
$ hg clone . ../branch
updating to branch default
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ cd ../branch
$ hg co
0 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ echo bar>>foo
$ hg commit -m "2"
$ cd ../test
$ hg pull ../branch
pulling from ../branch
searching for changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files
(run 'hg update' to get a working copy)
$ hg verify
checking changesets
checking manifests
crosschecking files in changesets and manifests
checking files
1 files, 2 changesets, 2 total revisions
$ hg co
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ cat foo
foo
bar
$ hg manifest --debug
6f4310b00b9a147241b071a60c28a650827fb03d 644 foo
update to rev 0 with a date
$ hg upd -d foo 0
abort: you can't specify a revision and a date
[255]
$ cd ..