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.
Source bundle was generated with the following script:
# hg init
# echo a > a
# ln -s a l
# hg ci -Ama -d'0 0'
# mkdir b
# echo a > b/a
# chmod +x b/a
# hg ci -Amb -d'1 0'
$ hg init
$ hg -q pull "$TESTDIR/bundles/test-manifest.hg"
The next call is expected to return nothing:
$ hg manifest
$ hg co
3 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg manifest
a
b/a
l
$ hg files -vr .
2 a
2 x b/a (glob)
1 l l
$ hg files -r . -X b
a
l
$ hg manifest -v
644 a
755 * b/a
644 @ l
$ hg manifest --debug
b789fdd96dc2f3bd229c1dd8eedf0fc60e2b68e3 644 a
b789fdd96dc2f3bd229c1dd8eedf0fc60e2b68e3 755 * b/a
047b75c6d7a3ef6a2243bd0e99f94f6ea6683597 644 @ l
$ hg manifest -r 0
a
l
$ hg manifest -r 1
a
b/a
l
$ hg manifest -r tip
a
b/a
l
$ hg manifest tip
a
b/a
l
$ hg manifest --all
a
b/a
l
The next two calls are expected to abort:
$ hg manifest -r 2
abort: unknown revision '2'!
[255]
$ hg manifest -r tip tip
abort: please specify just one revision
[255]