bdiff: balance recursion to avoid quadratic behavior (issue4704)
For highly structured files like JSON or XML dumps with large numbers
of duplicate lines (eg braces) and isolated matching lines, bdiff
could find large numbers of equally good spans. Because it prefers
earlier matches, this would result in pathologically unbalance
recursion that resulted in quadratic performance.
This patch makes it prefer matches closer to the middle that tend to
balance recursion. This change improves the speed of a pathological
test case from 1100s to 9s.
Included is a smaller test that has a roughly 50x safety margin on the
performance it accepts. It's likely to fail on pure builds because
difflib also has a recursion-balancing problem.
# Dummy extension that adds a delay after acquiring a lock.
#
# This extension can be used to test race conditions between lock acquisition.
from __future__ import absolute_import
import os
import time
from mercurial import (
lock as lockmod,
)
class delaylock(lockmod.lock):
def lock(self):
delay = float(os.environ.get('HGPRELOCKDELAY', '0.0'))
if delay:
time.sleep(delay)
res = super(delaylock, self).lock()
delay = float(os.environ.get('HGPOSTLOCKDELAY', '0.0'))
if delay:
time.sleep(delay)
return res
def extsetup(ui):
lockmod.lock = delaylock