tests/blackbox-readonly-dispatch.py

bundle2: implement consume() API on unbundlepart

We want bundle parts to not be seekable by default. That means
eliminating the generic seek() method.

A common pattern in bundle2.py is to seek to the end of the part
data. This is mainly used by the part iteration code to ensure
the underlying stream is advanced to the next bundle part.

In this commit, we establish a dedicated API for consuming a
bundle2 part data. We switch users of seek() to it.

The old implementation of seek(0, os.SEEK_END) would effectively
call self.read(). The new implementation calls self.read(32768)
in a loop. The old implementation would therefore assemble a
buffer to hold all remaining data being seeked over. For seeking
over large bundle parts, this would involve a large allocation and
a lot of overhead to collect intermediate data! This overhead can
be seen in the results for `hg perfbundleread`:

! bundle2 iterparts()
! wall 10.891305 comb 10.820000 user 7.990000 sys 2.830000 (best of 3)
! wall 8.070791 comb 8.060000 user 7.180000 sys 0.880000 (best of 3)
! bundle2 part seek()
! wall 12.991478 comb 10.390000 user 7.720000 sys 2.670000 (best of 3)
! wall 10.370142 comb 10.350000 user 7.430000 sys 2.920000 (best of 3)

Of course, skipping over large payload data isn't likely very common.
So I doubt the performance wins will be observed in the wild.

Differential Revision: https://phab.mercurial-scm.org/D1388

from __future__ import absolute_import, print_function
import os
from mercurial import (
    dispatch,
)

def testdispatch(cmd):
    """Simple wrapper around dispatch.dispatch()

    Prints command and result value, but does not handle quoting.
    """
    print("running: %s" % (cmd,))
    req = dispatch.request(cmd.split())
    result = dispatch.dispatch(req)
    print("result: %r" % (result,))

# create file 'foo', add and commit
f = open('foo', 'wb')
f.write('foo\n')
f.close()
testdispatch("add foo")
testdispatch("commit -m commit1 -d 2000-01-01 foo")

# append to file 'foo' and commit
f = open('foo', 'ab')
f.write('bar\n')
f.close()
# remove blackbox.log directory (proxy for readonly log file)
os.rmdir(".hg/blackbox.log")
# replace it with the real blackbox.log file
os.rename(".hg/blackbox.log-", ".hg/blackbox.log")
testdispatch("commit -m commit2 -d 2000-01-02 foo")

# check 88803a69b24 (fancyopts modified command table)
testdispatch("log -r 0")
testdispatch("log -r tip")