cborutil: implement sans I/O decoder
The vendored CBOR package decodes by calling read(n) on an object.
There are a number of disadvantages to this:
* Uses blocking I/O. If sufficient data is not available, the decoder
will hang until it is.
* No support for partial reads. If the read(n) returns less data than
requested, the decoder raises an error.
* Requires the use of a file like object. If the original data is in
say a buffer, we need to "cast" it to e.g. a BytesIO to appease the
decoder.
In addition, the vendored CBOR decoder doesn't provide flexibility
that we desire. Specifically:
* It buffers indefinite length bytestrings instead of streaming them.
* It doesn't allow limiting the set of types that can be decoded. This
property is useful when implementing a "hardened" decoder that is
less susceptible to abusive input.
* It doesn't provide sufficient "hook points" and introspection to
institute checks around behavior. These are useful for implementing
a "hardened" decoder.
This all adds up to a reasonable set of justifications for writing our
own decoder.
So, this commit implements our own CBOR decoder.
At the heart of the decoder is a function that decodes a single "item"
from a buffer. This item can be a complete simple value or a special
value, such as "start of array." Using this function, we can build a
decoder that effectively iterates over the stream of decoded items and
builds up higher-level values, such as arrays, maps, sets, and indefinite
length bytestrings. And we can do this without performing I/O in the
decoder itself.
The core of the sans I/O decoder will probably not be used directly.
Instead, it is expected that we'll build utility functions for invoking
the decoder given specific input types. This will allow extreme
flexibility in how data is delivered to the decoder.
I'm pretty happy with the state of the decoder modulo the TODO items
to track wanted features to help with a "hardened" decoder. The one
thing I could be convinced to change is the handling of semantic tags.
Since we only support a single semantic tag (sets), I thought it would
be easier to handle them inline in decodeitem(). This is simpler now.
But if we add support for other semantic tags, it will likely be easier
to move semantic tag handling outside of decodeitem(). But, properly
supporting semantic tags opens up a whole can of worms, as many
semantic tags imply new types. I'm optimistic we won't need these in
Mercurial. But who knows.
I'm also pretty happy with the test coverage. Writing comprehensive
tests for partial decoding did flush out a handful of bugs. One
general improvement to testing would be fuzz testing for partial
decoding. I may implement that later. I also anticipate switching the
wire protocol code to this new decoder will flush out any lingering
bugs.
Differential Revision: https://phab.mercurial-scm.org/D4414
from __future__ import absolute_import
import copy
import errno
import os
import silenttestrunner
import tempfile
import types
import unittest
from mercurial import (
error,
lock,
vfs as vfsmod,
)
testlockname = 'testlock'
# work around http://bugs.python.org/issue1515
if types.MethodType not in copy._deepcopy_dispatch:
def _deepcopy_method(x, memo):
return type(x)(x.__func__, copy.deepcopy(x.__self__, memo), x.im_class)
copy._deepcopy_dispatch[types.MethodType] = _deepcopy_method
class lockwrapper(lock.lock):
def __init__(self, pidoffset, *args, **kwargs):
# lock.lock.__init__() calls lock(), so the pidoffset assignment needs
# to be earlier
self._pidoffset = pidoffset
super(lockwrapper, self).__init__(*args, **kwargs)
def _getpid(self):
return super(lockwrapper, self)._getpid() + self._pidoffset
class teststate(object):
def __init__(self, testcase, dir, pidoffset=0):
self._testcase = testcase
self._acquirecalled = False
self._releasecalled = False
self._postreleasecalled = False
self.vfs = vfsmod.vfs(dir, audit=False)
self._pidoffset = pidoffset
def makelock(self, *args, **kwargs):
l = lockwrapper(self._pidoffset, self.vfs, testlockname,
releasefn=self.releasefn, acquirefn=self.acquirefn,
*args, **kwargs)
l.postrelease.append(self.postreleasefn)
return l
def acquirefn(self):
self._acquirecalled = True
def releasefn(self):
self._releasecalled = True
def postreleasefn(self):
self._postreleasecalled = True
def assertacquirecalled(self, called):
self._testcase.assertEqual(
self._acquirecalled, called,
'expected acquire to be %s but was actually %s' % (
self._tocalled(called),
self._tocalled(self._acquirecalled),
))
def resetacquirefn(self):
self._acquirecalled = False
def assertreleasecalled(self, called):
self._testcase.assertEqual(
self._releasecalled, called,
'expected release to be %s but was actually %s' % (
self._tocalled(called),
self._tocalled(self._releasecalled),
))
def assertpostreleasecalled(self, called):
self._testcase.assertEqual(
self._postreleasecalled, called,
'expected postrelease to be %s but was actually %s' % (
self._tocalled(called),
self._tocalled(self._postreleasecalled),
))
def assertlockexists(self, exists):
actual = self.vfs.lexists(testlockname)
self._testcase.assertEqual(
actual, exists,
'expected lock to %s but actually did %s' % (
self._toexists(exists),
self._toexists(actual),
))
def _tocalled(self, called):
if called:
return 'called'
else:
return 'not called'
def _toexists(self, exists):
if exists:
return 'exist'
else:
return 'not exist'
class testlock(unittest.TestCase):
def testlock(self):
state = teststate(self, tempfile.mkdtemp(dir=os.getcwd()))
lock = state.makelock()
state.assertacquirecalled(True)
lock.release()
state.assertreleasecalled(True)
state.assertpostreleasecalled(True)
state.assertlockexists(False)
def testrecursivelock(self):
state = teststate(self, tempfile.mkdtemp(dir=os.getcwd()))
lock = state.makelock()
state.assertacquirecalled(True)
state.resetacquirefn()
lock.lock()
# recursive lock should not call acquirefn again
state.assertacquirecalled(False)
lock.release() # brings lock refcount down from 2 to 1
state.assertreleasecalled(False)
state.assertpostreleasecalled(False)
state.assertlockexists(True)
lock.release() # releases the lock
state.assertreleasecalled(True)
state.assertpostreleasecalled(True)
state.assertlockexists(False)
def testlockfork(self):
state = teststate(self, tempfile.mkdtemp(dir=os.getcwd()))
lock = state.makelock()
state.assertacquirecalled(True)
# fake a fork
forklock = copy.deepcopy(lock)
forklock._pidoffset = 1
forklock.release()
state.assertreleasecalled(False)
state.assertpostreleasecalled(False)
state.assertlockexists(True)
# release the actual lock
lock.release()
state.assertreleasecalled(True)
state.assertpostreleasecalled(True)
state.assertlockexists(False)
def testinheritlock(self):
d = tempfile.mkdtemp(dir=os.getcwd())
parentstate = teststate(self, d)
parentlock = parentstate.makelock()
parentstate.assertacquirecalled(True)
# set up lock inheritance
with parentlock.inherit() as lockname:
parentstate.assertreleasecalled(True)
parentstate.assertpostreleasecalled(False)
parentstate.assertlockexists(True)
childstate = teststate(self, d, pidoffset=1)
childlock = childstate.makelock(parentlock=lockname)
childstate.assertacquirecalled(True)
childlock.release()
childstate.assertreleasecalled(True)
childstate.assertpostreleasecalled(False)
childstate.assertlockexists(True)
parentstate.resetacquirefn()
parentstate.assertacquirecalled(True)
parentlock.release()
parentstate.assertreleasecalled(True)
parentstate.assertpostreleasecalled(True)
parentstate.assertlockexists(False)
def testmultilock(self):
d = tempfile.mkdtemp(dir=os.getcwd())
state0 = teststate(self, d)
lock0 = state0.makelock()
state0.assertacquirecalled(True)
with lock0.inherit() as lock0name:
state0.assertreleasecalled(True)
state0.assertpostreleasecalled(False)
state0.assertlockexists(True)
state1 = teststate(self, d, pidoffset=1)
lock1 = state1.makelock(parentlock=lock0name)
state1.assertacquirecalled(True)
# from within lock1, acquire another lock
with lock1.inherit() as lock1name:
# since the file on disk is lock0's this should have the same
# name
self.assertEqual(lock0name, lock1name)
state2 = teststate(self, d, pidoffset=2)
lock2 = state2.makelock(parentlock=lock1name)
state2.assertacquirecalled(True)
lock2.release()
state2.assertreleasecalled(True)
state2.assertpostreleasecalled(False)
state2.assertlockexists(True)
state1.resetacquirefn()
state1.assertacquirecalled(True)
lock1.release()
state1.assertreleasecalled(True)
state1.assertpostreleasecalled(False)
state1.assertlockexists(True)
lock0.release()
def testinheritlockfork(self):
d = tempfile.mkdtemp(dir=os.getcwd())
parentstate = teststate(self, d)
parentlock = parentstate.makelock()
parentstate.assertacquirecalled(True)
# set up lock inheritance
with parentlock.inherit() as lockname:
childstate = teststate(self, d, pidoffset=1)
childlock = childstate.makelock(parentlock=lockname)
childstate.assertacquirecalled(True)
# fork the child lock
forkchildlock = copy.deepcopy(childlock)
forkchildlock._pidoffset += 1
forkchildlock.release()
childstate.assertreleasecalled(False)
childstate.assertpostreleasecalled(False)
childstate.assertlockexists(True)
# release the child lock
childlock.release()
childstate.assertreleasecalled(True)
childstate.assertpostreleasecalled(False)
childstate.assertlockexists(True)
parentlock.release()
def testinheritcheck(self):
d = tempfile.mkdtemp(dir=os.getcwd())
state = teststate(self, d)
def check():
raise error.LockInheritanceContractViolation('check failed')
lock = state.makelock(inheritchecker=check)
state.assertacquirecalled(True)
with self.assertRaises(error.LockInheritanceContractViolation):
with lock.inherit():
pass
lock.release()
def testfrequentlockunlock(self):
"""This tests whether lock acquisition fails as expected, even if
(1) lock can't be acquired (makelock fails by EEXIST), and
(2) locker info can't be read in (readlock fails by ENOENT) while
retrying 5 times.
"""
d = tempfile.mkdtemp(dir=os.getcwd())
state = teststate(self, d)
def emulatefrequentlock(*args):
raise OSError(errno.EEXIST, "File exists")
def emulatefrequentunlock(*args):
raise OSError(errno.ENOENT, "No such file or directory")
state.vfs.makelock = emulatefrequentlock
state.vfs.readlock = emulatefrequentunlock
try:
state.makelock(timeout=0)
self.fail("unexpected lock acquisition")
except error.LockHeld as why:
self.assertTrue(why.errno == errno.ETIMEDOUT)
self.assertTrue(why.locker == "")
state.assertlockexists(False)
if __name__ == '__main__':
silenttestrunner.main(__name__)