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qubes/app: Allow keeping lock after load

Browse Source 
QubesOS/qubes-issues#1729
This commit is contained in:
Wojtek Porczyk 2016-10-27 17:30:06 +02:00
parent 3553b2e1d4
commit 0141e1ac73
2 changed files with 85 additions and 61 deletions
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138
qubes/app.py
View File

@ -600,7 +600,8 @@ class Qubes(qubes.PropertyHolder):
default=True,
doc='check for updates inside qubes')
def __init__(self, store=None, load=True, offline_mode=False, **kwargs):
def __init__(self, store=None, load=True, offline_mode=False, lock=False,
**kwargs):
#: logger instance for logging global messages
self.log = logging.getLogger('app')
@ -632,6 +633,7 @@ class Qubes(qubes.PropertyHolder):
super(Qubes, self).__init__(xml=None, **kwargs)
self.__load_timestamp = None
self.__locked_fh = None
#: jinja2 environment for libvirt XML templates
self.env = jinja2.Environment(
@ -642,7 +644,7 @@ class Qubes(qubes.PropertyHolder):
undefined=jinja2.StrictUndefined)
if load:
self.load()
self.load(lock=lock)
self.events_enabled = True
@ -650,7 +652,7 @@ class Qubes(qubes.PropertyHolder):
def store(self):
return self._store
def load(self):
def load(self, lock=False):
'''Open qubes.xml
:throws EnvironmentError: failure on parsing store
@ -658,26 +660,7 @@ class Qubes(qubes.PropertyHolder):
:raises lxml.etree.XMLSyntaxError: on syntax error in qubes.xml
'''
try:
fd = os.open(self._store, os.O_RDWR) # no O_CREAT
except OSError as e:
if e.errno != errno.ENOENT:
raise
raise qubes.exc.QubesException(
'Qubes XML store {!r} is missing; use qubes-create tool'.format(
self._store))
fh = os.fdopen(fd, 'rb')
if os.name == 'posix':
fcntl.lockf(fh, fcntl.LOCK_EX)
elif os.name == 'nt':
# pylint: disable=protected-access
win32file.LockFileEx(
win32file._get_osfhandle(fh.fileno()),
win32con.LOCKFILE_EXCLUSIVE_LOCK,
0, -0x10000,
pywintypes.OVERLAPPED())
fh = self._acquire_lock()
self.xml = lxml.etree.parse(fh)
# stage 1: load labels and pools
@ -741,9 +724,10 @@ class Qubes(qubes.PropertyHolder):
# get a file timestamp (before closing it - still holding the lock!),
# to detect whether anyone else have modified it in the meantime
self.__load_timestamp = os.path.getmtime(self._store)
# intentionally do not call explicit unlock
fh.close()
del fh
if not lock:
self._release_lock()
def __xml__(self):
element = lxml.etree.Element('qubes')
@ -768,7 +752,7 @@ class Qubes(qubes.PropertyHolder):
return element
def save(self):
def save(self, lock=True):
'''Save all data to qubes.xml
There are several problems with saving :file:`qubes.xml` which must be
@ -779,36 +763,15 @@ class Qubes(qubes.PropertyHolder):
- Attempts to write two or more files concurrently. This is done by
sophisticated locking.
:param bool lock: keep file locked after saving
:throws EnvironmentError: failure on saving
'''
while True:
fd_old = os.open(self._store, os.O_RDWR | os.O_CREAT)
if os.name == 'posix':
fcntl.lockf(fd_old, fcntl.LOCK_EX)
elif os.name == 'nt':
# pylint: disable=protected-access
overlapped = pywintypes.OVERLAPPED()
win32file.LockFileEx(
win32file._get_osfhandle(fd_old),
win32con.LOCKFILE_EXCLUSIVE_LOCK, 0, -0x10000, overlapped)
if not self.__locked_fh:
self._acquire_lock(for_save=True)
# While we were waiting for lock, someone could have unlink()ed (or
# rename()d) our file out of the filesystem. We have to ensure we
# got lock on something linked to filesystem. If not, try again.
if os.fstat(fd_old) == os.stat(self._store):
break
else:
os.close(fd_old)
if self.__load_timestamp:
current_file_timestamp = os.path.getmtime(self._store)
if current_file_timestamp != self.__load_timestamp:
os.close(fd_old)
raise qubes.exc.QubesException(
"Someone else modified qubes.xml in the meantime")
fh_new = tempfile.NamedTemporaryFile(prefix=self._store, delete=False)
fh_new = tempfile.NamedTemporaryFile(
prefix=self._store, delete=False)
lxml.etree.ElementTree(self.__xml__()).write(
fh_new, encoding='utf-8', pretty_print=True)
fh_new.flush()
@ -816,13 +779,70 @@ class Qubes(qubes.PropertyHolder):
os.chown(fh_new.name, -1, grp.getgrnam('qubes').gr_gid)
os.rename(fh_new.name, self._store)
# intentionally do not call explicit unlock to not unlock the file
# before all buffers are flushed
fh_new.close()
# update stored mtime, in case of multiple save() calls without
# loading qubes.xml again
self.__load_timestamp = os.path.getmtime(self._store)
os.close(fd_old)
# this releases lock for all other processes,
# but they should instantly block on the new descriptor
self.__locked_fh.close()
self.__locked_fh = fh_new
if not lock:
self._release_lock()
def _acquire_lock(self, for_save=False):
assert self.__locked_fh is None, 'double lock'
while True:
try:
fd = os.open(self._store,
os.O_RDWR | (os.O_CREAT * int(for_save)))
except OSError as e:
if not for_save and e.errno == errno.ENOENT:
raise qubes.exc.QubesException(
'Qubes XML store {!r} is missing; '
'use qubes-create tool'.format(self._store))
raise
# While we were waiting for lock, someone could have unlink()ed
# (or rename()d) our file out of the filesystem. We have to
# ensure we got lock on something linked to filesystem.
# If not, try again.
if os.fstat(fd) != os.stat(self._store):
os.close(fd)
continue
if self.__load_timestamp and \
os.path.getmtime(self._store) != self.__load_timestamp:
os.close(fd)
raise qubes.exc.QubesException(
'Someone else modified qubes.xml in the meantime')
break
if os.name == 'posix':
fcntl.lockf(fd, fcntl.LOCK_EX)
elif os.name == 'nt':
# pylint: disable=protected-access
overlapped = pywintypes.OVERLAPPED()
win32file.LockFileEx(
win32file._get_osfhandle(fd),
win32con.LOCKFILE_EXCLUSIVE_LOCK, 0, -0x10000, overlapped)
self.__locked_fh = os.fdopen(fd, 'r+b')
return self.__locked_fh
def _release_lock(self):
assert self.__locked_fh is not None, 'double release'
# intentionally do not call explicit unlock to not unlock the file
# before all buffers are flushed
self.__locked_fh.close()
self.__locked_fh = None
def load_initial_values(self):
self.labels = {
@ -848,10 +868,10 @@ class Qubes(qubes.PropertyHolder):
qubes.vm.adminvm.AdminVM(self, None, qid=0, name='dom0'))
@classmethod
def create_empty_store(cls, *args, **kwargs):
def create_empty_store(cls, lock=False, *args, **kwargs):
self = cls(*args, load=False, **kwargs)
self.load_initial_values()
self.save()
self.save(lock=lock)
return self

8
qubes/core2migration.py
View File

@ -220,8 +220,9 @@ class Core2Qubes(qubes.Qubes):
if 'vm' in locals():
del self.domains[vm]
def load(self):
def load(self, lock=False):
qubes_store_file = open(self._store, 'r')
self._acquire_lock(qubes_store_file)
try:
qubes_store_file.seek(0)
@ -267,5 +268,8 @@ class Core2Qubes(qubes.Qubes):
# and load other defaults (default netvm, updatevm etc)
self.load_globals(tree.getroot())
def save(self):
if not lock:
self._release_lock()
def save(self, lock=False):
raise NotImplementedError("Saving old qubes.xml not supported")