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#
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#
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# Copyright (C) 2006, 2007 Google Inc.
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#
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# This program is free software; you can redistribute it and/or modify
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# it under the terms of the GNU General Public License as published by
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# the Free Software Foundation; either version 2 of the License, or
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# (at your option) any later version.
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#
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# This program is distributed in the hope that it will be useful, but
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# WITHOUT ANY WARRANTY; without even the implied warranty of
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# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
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# General Public License for more details.
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#
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# You should have received a copy of the GNU General Public License
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# along with this program; if not, write to the Free Software
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# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
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# 02110-1301, USA.
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"""Module implementing the Ganeti locking code."""
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# pylint: disable-msg=W0613,W0201
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import threading
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# Wouldn't it be better to define LockingError in the locking module?
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# Well, for now that's how the rest of the code does it...
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from ganeti import errors
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from ganeti import utils
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class SharedLock:
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  """Implements a shared lock.
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  Multiple threads can acquire the lock in a shared way, calling
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  acquire_shared().  In order to acquire the lock in an exclusive way threads
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  can call acquire_exclusive().
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  The lock prevents starvation but does not guarantee that threads will acquire
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  the shared lock in the order they queued for it, just that they will
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  eventually do so.
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  """
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  def __init__(self):
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    """Construct a new SharedLock"""
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    # we have two conditions, c_shr and c_exc, sharing the same lock.
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    self.__lock = threading.Lock()
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    self.__turn_shr = threading.Condition(self.__lock)
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    self.__turn_exc = threading.Condition(self.__lock)
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    # current lock holders
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    self.__shr = set()
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    self.__exc = None
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    # lock waiters
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    self.__nwait_exc = 0
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    self.__nwait_shr = 0
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    self.__npass_shr = 0
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    # is this lock in the deleted state?
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    self.__deleted = False
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  def __is_sharer(self):
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    """Is the current thread sharing the lock at this time?"""
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    return threading.currentThread() in self.__shr
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  def __is_exclusive(self):
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    """Is the current thread holding the lock exclusively at this time?"""
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    return threading.currentThread() == self.__exc
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  def __is_owned(self, shared=-1):
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    """Is the current thread somehow owning the lock at this time?
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    This is a private version of the function, which presumes you're holding
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    the internal lock.
76

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    """
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    if shared < 0:
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      return self.__is_sharer() or self.__is_exclusive()
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    elif shared:
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      return self.__is_sharer()
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    else:
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      return self.__is_exclusive()
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85
  def _is_owned(self, shared=-1):
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    """Is the current thread somehow owning the lock at this time?
87

88
    Args:
89
      shared:
90
        < 0: check for any type of ownership (default)
91
        0: check for exclusive ownership
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        > 0: check for shared ownership
93

94
    """
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    self.__lock.acquire()
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    try:
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      result = self.__is_owned(shared=shared)
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    finally:
99
      self.__lock.release()
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101
    return result
102

    
103
  def __wait(self, c):
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    """Wait on the given condition, and raise an exception if the current lock
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    is declared deleted in the meantime.
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107
    Args:
108
      c: condition to wait on
109

110
    """
111
    c.wait()
112
    if self.__deleted:
113
      raise errors.LockError('deleted lock')
114

    
115
  def __exclusive_acquire(self):
116
    """Acquire the lock exclusively.
117

118
    This is a private function that presumes you are already holding the
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    internal lock. It's defined separately to avoid code duplication between
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    acquire() and delete()
121

122
    """
123
    self.__nwait_exc += 1
124
    try:
125
      # This is to save ourselves from a nasty race condition that could
126
      # theoretically make the sharers starve.
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      if self.__nwait_shr > 0 or self.__nwait_exc > 1:
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        self.__wait(self.__turn_exc)
129

    
130
      while len(self.__shr) > 0 or self.__exc is not None:
131
        self.__wait(self.__turn_exc)
132

    
133
      self.__exc = threading.currentThread()
134
    finally:
135
      self.__nwait_exc -= 1
136

    
137
    assert self.__npass_shr == 0, "SharedLock: internal fairness violation"
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139
  def acquire(self, blocking=1, shared=0):
140
    """Acquire a shared lock.
141

142
    Args:
143
      shared: whether to acquire in shared mode. By default an exclusive lock
144
              will be acquired.
145
      blocking: whether to block while trying to acquire or to operate in
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                try-lock mode. this locking mode is not supported yet.
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148
    """
149
    if not blocking:
150
      # We don't have non-blocking mode for now
151
      raise NotImplementedError
152

    
153
    self.__lock.acquire()
154
    try:
155
      if self.__deleted:
156
        raise errors.LockError('deleted lock')
157

    
158
      # We cannot acquire the lock if we already have it
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      assert not self.__is_owned(), "double acquire() on a non-recursive lock"
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      assert self.__npass_shr >= 0, "Internal fairness condition weirdness"
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162
      if shared:
163
        self.__nwait_shr += 1
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        try:
165
          wait = False
166
          # If there is an exclusive holder waiting we have to wait.  We'll
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          # only do this once, though, when we start waiting for the lock. Then
168
          # we'll just wait while there are no exclusive holders.
169
          if self.__nwait_exc > 0:
170
            # TODO: if !blocking...
171
            wait = True
172
            self.__wait(self.__turn_shr)
173

    
174
          while self.__exc is not None:
175
            wait = True
176
            # TODO: if !blocking...
177
            self.__wait(self.__turn_shr)
178

    
179
          self.__shr.add(threading.currentThread())
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181
          # If we were waiting note that we passed
182
          if wait:
183
            self.__npass_shr -= 1
184

    
185
        finally:
186
          self.__nwait_shr -= 1
187

    
188
        assert self.__npass_shr >= 0, "Internal fairness condition weirdness"
189
      else:
190
        # TODO: if !blocking...
191
        # (or modify __exclusive_acquire for non-blocking mode)
192
        self.__exclusive_acquire()
193

    
194
    finally:
195
      self.__lock.release()
196

    
197
    return True
198

    
199
  def release(self):
200
    """Release a Shared Lock.
201

202
    You must have acquired the lock, either in shared or in exclusive mode,
203
    before calling this function.
204

205
    """
206
    self.__lock.acquire()
207
    try:
208
      assert self.__npass_shr >= 0, "Internal fairness condition weirdness"
209
      # Autodetect release type
210
      if self.__is_exclusive():
211
        self.__exc = None
212

    
213
        # An exclusive holder has just had the lock, time to put it in shared
214
        # mode if there are shared holders waiting. Otherwise wake up the next
215
        # exclusive holder.
216
        if self.__nwait_shr > 0:
217
          # Make sure at least the ones which were blocked pass.
218
          self.__npass_shr = self.__nwait_shr
219
          self.__turn_shr.notifyAll()
220
        elif self.__nwait_exc > 0:
221
          self.__turn_exc.notify()
222

    
223
      elif self.__is_sharer():
224
        self.__shr.remove(threading.currentThread())
225

    
226
        # If there are shared holders waiting (and not just scheduled to pass)
227
        # there *must* be an exclusive holder waiting as well; otherwise what
228
        # were they waiting for?
229
        assert (self.__nwait_exc > 0 or self.__npass_shr == self.__nwait_shr), \
230
               "Lock sharers waiting while no exclusive is queueing"
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232
        # If there are no more shared holders either in or scheduled to pass,
233
        # and some exclusive holders are waiting let's wake one up.
234
        if (len(self.__shr) == 0 and
235
            self.__nwait_exc > 0 and
236
            not self.__npass_shr > 0):
237
          self.__turn_exc.notify()
238

    
239
      else:
240
        assert False, "Cannot release non-owned lock"
241

    
242
    finally:
243
      self.__lock.release()
244

    
245
  def delete(self, blocking=1):
246
    """Delete a Shared Lock.
247

248
    This operation will declare the lock for removal. First the lock will be
249
    acquired in exclusive mode if you don't already own it, then the lock
250
    will be put in a state where any future and pending acquire() fail.
251

252
    Args:
253
      blocking: whether to block while trying to acquire or to operate in
254
                try-lock mode.  this locking mode is not supported yet unless
255
                you are already holding exclusively the lock.
256

257
    """
258
    self.__lock.acquire()
259
    try:
260
      assert not self.__is_sharer(), "cannot delete() a lock while sharing it"
261

    
262
      if self.__deleted:
263
        raise errors.LockError('deleted lock')
264

    
265
      if not self.__is_exclusive():
266
        if not blocking:
267
          # We don't have non-blocking mode for now
268
          raise NotImplementedError
269
        self.__exclusive_acquire()
270

    
271
      self.__deleted = True
272
      self.__exc = None
273
      # Wake up everybody, they will fail acquiring the lock and
274
      # raise an exception instead.
275
      self.__turn_exc.notifyAll()
276
      self.__turn_shr.notifyAll()
277

    
278
    finally:
279
      self.__lock.release()
280

    
281

    
282
class LockSet:
283
  """Implements a set of locks.
284

285
  This abstraction implements a set of shared locks for the same resource type,
286
  distinguished by name. The user can lock a subset of the resources and the
287
  LockSet will take care of acquiring the locks always in the same order, thus
288
  preventing deadlock.
289

290
  All the locks needed in the same set must be acquired together, though.
291

292
  """
293
  def __init__(self, members=None):
294
    """Constructs a new LockSet.
295

296
    Args:
297
      members: initial members of the set
298

299
    """
300
    # Used internally to guarantee coherency.
301
    self.__lock = SharedLock()
302

    
303
    # The lockdict indexes the relationship name -> lock
304
    # The order-of-locking is implied by the alphabetical order of names
305
    self.__lockdict = {}
306

    
307
    if members is not None:
308
      for name in members:
309
        self.__lockdict[name] = SharedLock()
310

    
311
    # The owner dict contains the set of locks each thread owns. For
312
    # performance each thread can access its own key without a global lock on
313
    # this structure. It is paramount though that *no* other type of access is
314
    # done to this structure (eg. no looping over its keys). *_owner helper
315
    # function are defined to guarantee access is correct, but in general never
316
    # do anything different than __owners[threading.currentThread()], or there
317
    # will be trouble.
318
    self.__owners = {}
319

    
320
  def _is_owned(self):
321
    """Is the current thread a current level owner?"""
322
    return threading.currentThread() in self.__owners
323

    
324
  def _add_owned(self, name=None):
325
    """Note the current thread owns the given lock"""
326
    if name is None:
327
      if not self._is_owned():
328
        self.__owners[threading.currentThread()] = set()
329
    else:
330
      if self._is_owned():
331
        self.__owners[threading.currentThread()].add(name)
332
      else:
333
        self.__owners[threading.currentThread()] = set([name])
334

    
335

    
336
  def _del_owned(self, name=None):
337
    """Note the current thread owns the given lock"""
338

    
339
    if name is not None:
340
      self.__owners[threading.currentThread()].remove(name)
341

    
342
    # Only remove the key if we don't hold the set-lock as well
343
    if (not self.__lock._is_owned() and
344
        not self.__owners[threading.currentThread()]):
345
      del self.__owners[threading.currentThread()]
346

    
347
  def _list_owned(self):
348
    """Get the set of resource names owned by the current thread"""
349
    if self._is_owned():
350
      return self.__owners[threading.currentThread()].copy()
351
    else:
352
      return set()
353

    
354
  def __names(self):
355
    """Return the current set of names.
356

357
    Only call this function while holding __lock and don't iterate on the
358
    result after releasing the lock.
359

360
    """
361
    return self.__lockdict.keys()
362

    
363
  def _names(self):
364
    """Return a copy of the current set of elements.
365

366
    Used only for debugging purposes.
367

368
    """
369
    self.__lock.acquire(shared=1)
370
    try:
371
      result = self.__names()
372
    finally:
373
      self.__lock.release()
374
    return set(result)
375

    
376
  def acquire(self, names, blocking=1, shared=0):
377
    """Acquire a set of resource locks.
378

379
    Args:
380
      names: the names of the locks which shall be acquired.
381
             (special lock names, or instance/node names)
382
      shared: whether to acquire in shared mode. By default an exclusive lock
383
              will be acquired.
384
      blocking: whether to block while trying to acquire or to operate in
385
                try-lock mode.  this locking mode is not supported yet.
386

387
    Returns:
388
      True: when all the locks are successfully acquired
389

390
    Raises:
391
      errors.LockError: when any lock we try to acquire has been deleted
392
      before we succeed. In this case none of the locks requested will be
393
      acquired.
394

395
    """
396
    if not blocking:
397
      # We don't have non-blocking mode for now
398
      raise NotImplementedError
399

    
400
    # Check we don't already own locks at this level
401
    assert not self._is_owned(), "Cannot acquire locks in the same set twice"
402

    
403
    if names is None:
404
      # If no names are given acquire the whole set by not letting new names
405
      # being added before we release, and getting the current list of names.
406
      # Some of them may then be deleted later, but we'll cope with this.
407
      #
408
      # We'd like to acquire this lock in a shared way, as it's nice if
409
      # everybody else can use the instances at the same time. If are acquiring
410
      # them exclusively though they won't be able to do this anyway, though,
411
      # so we'll get the list lock exclusively as well in order to be able to
412
      # do add() on the set while owning it.
413
      self.__lock.acquire(shared=shared)
414
      try:
415
        # note we own the set-lock
416
        self._add_owned()
417
        names = self.__names()
418
      except:
419
        # We shouldn't have problems adding the lock to the owners list, but
420
        # if we did we'll try to release this lock and re-raise exception.
421
        # Of course something is going to be really wrong, after this.
422
        self.__lock.release()
423
        raise
424

    
425
    try:
426
      # Support passing in a single resource to acquire rather than many
427
      if isinstance(names, basestring):
428
        names = [names]
429
      else:
430
        names.sort()
431

    
432
      acquire_list = []
433
      # First we look the locks up on __lockdict. We have no way of being sure
434
      # they will still be there after, but this makes it a lot faster should
435
      # just one of them be the already wrong
436
      for lname in names:
437
        try:
438
          lock = self.__lockdict[lname] # raises KeyError if lock is not there
439
          acquire_list.append((lname, lock))
440
        except (KeyError):
441
          if self.__lock._is_owned():
442
            # We are acquiring all the set, it doesn't matter if this particular
443
            # element is not there anymore.
444
            continue
445
          else:
446
            raise errors.LockError('non-existing lock in set (%s)' % lname)
447

    
448
      # This will hold the locknames we effectively acquired.
449
      acquired = set()
450
      # Now acquire_list contains a sorted list of resources and locks we want.
451
      # In order to get them we loop on this (private) list and acquire() them.
452
      # We gave no real guarantee they will still exist till this is done but
453
      # .acquire() itself is safe and will alert us if the lock gets deleted.
454
      for (lname, lock) in acquire_list:
455
        try:
456
          lock.acquire(shared=shared) # raises LockError if the lock is deleted
457
          # now the lock cannot be deleted, we have it!
458
          self._add_owned(name=lname)
459
          acquired.add(lname)
460
        except (errors.LockError):
461
          if self.__lock._is_owned():
462
            # We are acquiring all the set, it doesn't matter if this particular
463
            # element is not there anymore.
464
            continue
465
          else:
466
            name_fail = lname
467
            for lname in self._list_owned():
468
              self.__lockdict[lname].release()
469
              self._del_owned(name=lname)
470
            raise errors.LockError('non-existing lock in set (%s)' % name_fail)
471
        except:
472
          # We shouldn't have problems adding the lock to the owners list, but
473
          # if we did we'll try to release this lock and re-raise exception.
474
          # Of course something is going to be really wrong, after this.
475
          if lock._is_owned():
476
            lock.release()
477
            raise
478

    
479
    except:
480
      # If something went wrong and we had the set-lock let's release it...
481
      if self.__lock._is_owned():
482
        self.__lock.release()
483
      raise
484

    
485
    return acquired
486

    
487
  def release(self, names=None):
488
    """Release a set of resource locks, at the same level.
489

490
    You must have acquired the locks, either in shared or in exclusive mode,
491
    before releasing them.
492

493
    Args:
494
      names: the names of the locks which shall be released.
495
             (defaults to all the locks acquired at that level).
496

497
    """
498
    assert self._is_owned(), "release() on lock set while not owner"
499

    
500
    # Support passing in a single resource to release rather than many
501
    if isinstance(names, basestring):
502
      names = [names]
503

    
504
    if names is None:
505
      names = self._list_owned()
506
    else:
507
      names = set(names)
508
      assert self._list_owned().issuperset(names), (
509
               "release() on unheld resources %s" %
510
               names.difference(self._list_owned()))
511

    
512
    # First of all let's release the "all elements" lock, if set.
513
    # After this 'add' can work again
514
    if self.__lock._is_owned():
515
      self.__lock.release()
516
      self._del_owned()
517

    
518
    for lockname in names:
519
      # If we are sure the lock doesn't leave __lockdict without being
520
      # exclusively held we can do this...
521
      self.__lockdict[lockname].release()
522
      self._del_owned(name=lockname)
523

    
524
  def add(self, names, acquired=0, shared=0):
525
    """Add a new set of elements to the set
526

527
    Args:
528
      names: names of the new elements to add
529
      acquired: pre-acquire the new resource?
530
      shared: is the pre-acquisition shared?
531

532
    """
533

    
534
    assert not self.__lock._is_owned(shared=1), (
535
           "Cannot add new elements while sharing the set-lock")
536

    
537
    # Support passing in a single resource to add rather than many
538
    if isinstance(names, basestring):
539
      names = [names]
540

    
541
    # If we don't already own the set-level lock acquire it in an exclusive way
542
    # we'll get it and note we need to release it later.
543
    release_lock = False
544
    if not self.__lock._is_owned():
545
      release_lock = True
546
      self.__lock.acquire()
547

    
548
    try:
549
      invalid_names = set(self.__names()).intersection(names)
550
      if invalid_names:
551
        # This must be an explicit raise, not an assert, because assert is
552
        # turned off when using optimization, and this can happen because of
553
        # concurrency even if the user doesn't want it.
554
        raise errors.LockError("duplicate add() (%s)" % invalid_names)
555

    
556
      for lockname in names:
557
        lock = SharedLock()
558

    
559
        if acquired:
560
          lock.acquire(shared=shared)
561
          # now the lock cannot be deleted, we have it!
562
          try:
563
            self._add_owned(name=lockname)
564
          except:
565
            # We shouldn't have problems adding the lock to the owners list,
566
            # but if we did we'll try to release this lock and re-raise
567
            # exception.  Of course something is going to be really wrong,
568
            # after this.  On the other hand the lock hasn't been added to the
569
            # __lockdict yet so no other threads should be pending on it. This
570
            # release is just a safety measure.
571
            lock.release()
572
            raise
573

    
574
        self.__lockdict[lockname] = lock
575

    
576
    finally:
577
      # Only release __lock if we were not holding it previously.
578
      if release_lock:
579
        self.__lock.release()
580

    
581
    return True
582

    
583
  def remove(self, names, blocking=1):
584
    """Remove elements from the lock set.
585

586
    You can either not hold anything in the lockset or already hold a superset
587
    of the elements you want to delete, exclusively.
588

589
    Args:
590
      names: names of the resource to remove.
591
      blocking: whether to block while trying to acquire or to operate in
592
                try-lock mode.  this locking mode is not supported yet unless
593
                you are already holding exclusively the locks.
594

595
    Returns:
596
      A list of lock which we removed. The list is always equal to the names
597
      list if we were holding all the locks exclusively.
598

599
    """
600
    if not blocking and not self._is_owned():
601
      # We don't have non-blocking mode for now
602
      raise NotImplementedError
603

    
604
    # Support passing in a single resource to remove rather than many
605
    if isinstance(names, basestring):
606
      names = [names]
607

    
608
    # If we own any subset of this lock it must be a superset of what we want
609
    # to delete. The ownership must also be exclusive, but that will be checked
610
    # by the lock itself.
611
    assert not self._is_owned() or self._list_owned().issuperset(names), (
612
      "remove() on acquired lockset while not owning all elements")
613

    
614
    removed = []
615

    
616
    for lname in names:
617
      # Calling delete() acquires the lock exclusively if we don't already own
618
      # it, and causes all pending and subsequent lock acquires to fail. It's
619
      # fine to call it out of order because delete() also implies release(),
620
      # and the assertion above guarantees that if we either already hold
621
      # everything we want to delete, or we hold none.
622
      try:
623
        self.__lockdict[lname].delete()
624
        removed.append(lname)
625
      except (KeyError, errors.LockError):
626
        # This cannot happen if we were already holding it, verify:
627
        assert not self._is_owned(), "remove failed while holding lockset"
628
      else:
629
        # If no LockError was raised we are the ones who deleted the lock.
630
        # This means we can safely remove it from lockdict, as any further or
631
        # pending delete() or acquire() will fail (and nobody can have the lock
632
        # since before our call to delete()).
633
        #
634
        # This is done in an else clause because if the exception was thrown
635
        # it's the job of the one who actually deleted it.
636
        del self.__lockdict[lname]
637
        # And let's remove it from our private list if we owned it.
638
        if self._is_owned():
639
          self._del_owned(name=lname)
640

    
641
    return removed
642

    
643

    
644
# Locking levels, must be acquired in increasing order.
645
# Current rules are:
646
#   - at level LEVEL_CLUSTER resides the Big Ganeti Lock (BGL) which must be
647
#   acquired before performing any operation, either in shared or in exclusive
648
#   mode. acquiring the BGL in exclusive mode is discouraged and should be
649
#   avoided.
650
#   - at levels LEVEL_NODE and LEVEL_INSTANCE reside node and instance locks.
651
#   If you need more than one node, or more than one instance, acquire them at
652
#   the same time.
653
#  - level LEVEL_CONFIG contains the configuration lock, which you must acquire
654
#  before reading or changing the config file.
655
LEVEL_CLUSTER = 0
656
LEVEL_NODE = 1
657
LEVEL_INSTANCE = 2
658
LEVEL_CONFIG = 3
659

    
660
LEVELS = [LEVEL_CLUSTER,
661
          LEVEL_NODE,
662
          LEVEL_INSTANCE,
663
          LEVEL_CONFIG]
664

    
665
# Lock levels which are modifiable
666
LEVELS_MOD = [LEVEL_NODE, LEVEL_INSTANCE]
667

    
668
# Constant for the big ganeti lock and config lock
669
BGL = 'BGL'
670
CONFIG = 'config'
671

    
672

    
673
class GanetiLockManager:
674
  """The Ganeti Locking Library
675

676
  The purpouse of this small library is to manage locking for ganeti clusters
677
  in a central place, while at the same time doing dynamic checks against
678
  possible deadlocks. It will also make it easier to transition to a different
679
  lock type should we migrate away from python threads.
680

681
  """
682
  _instance = None
683

    
684
  def __init__(self, nodes=None, instances=None):
685
    """Constructs a new GanetiLockManager object.
686

687
    There should be only a GanetiLockManager object at any time, so this
688
    function raises an error if this is not the case.
689

690
    Args:
691
      nodes: list of node names
692
      instances: list of instance names
693

694
    """
695
    assert self.__class__._instance is None, "double GanetiLockManager instance"
696
    self.__class__._instance = self
697

    
698
    # The keyring contains all the locks, at their level and in the correct
699
    # locking order.
700
    self.__keyring = {
701
      LEVEL_CLUSTER: LockSet([BGL]),
702
      LEVEL_NODE: LockSet(nodes),
703
      LEVEL_INSTANCE: LockSet(instances),
704
      LEVEL_CONFIG: LockSet([CONFIG]),
705
    }
706

    
707
  def _names(self, level):
708
    """List the lock names at the given level.
709
    Used for debugging/testing purposes.
710

711
    Args:
712
      level: the level whose list of locks to get
713

714
    """
715
    assert level in LEVELS, "Invalid locking level %s" % level
716
    return self.__keyring[level]._names()
717

    
718
  def _is_owned(self, level):
719
    """Check whether we are owning locks at the given level
720

721
    """
722
    return self.__keyring[level]._is_owned()
723

    
724
  def _list_owned(self, level):
725
    """Get the set of owned locks at the given level
726

727
    """
728
    return self.__keyring[level]._list_owned()
729

    
730
  def _upper_owned(self, level):
731
    """Check that we don't own any lock at a level greater than the given one.
732

733
    """
734
    # This way of checking only works if LEVELS[i] = i, which we check for in
735
    # the test cases.
736
    return utils.any((self._is_owned(l) for l in LEVELS[level + 1:]))
737

    
738
  def _BGL_owned(self):
739
    """Check if the current thread owns the BGL.
740

741
    Both an exclusive or a shared acquisition work.
742

743
    """
744
    return BGL in self.__keyring[LEVEL_CLUSTER]._list_owned()
745

    
746
  def _contains_BGL(self, level, names):
747
    """Check if acting on the given level and set of names will change the
748
    status of the Big Ganeti Lock.
749

750
    """
751
    return level == LEVEL_CLUSTER and (names is None or BGL in names)
752

    
753
  def acquire(self, level, names, blocking=1, shared=0):
754
    """Acquire a set of resource locks, at the same level.
755

756
    Args:
757
      level: the level at which the locks shall be acquired.
758
             It must be a memmber of LEVELS.
759
      names: the names of the locks which shall be acquired.
760
             (special lock names, or instance/node names)
761
      shared: whether to acquire in shared mode. By default an exclusive lock
762
              will be acquired.
763
      blocking: whether to block while trying to acquire or to operate in
764
                try-lock mode.  this locking mode is not supported yet.
765

766
    """
767
    assert level in LEVELS, "Invalid locking level %s" % level
768

    
769
    # Check that we are either acquiring the Big Ganeti Lock or we already own
770
    # it. Some "legacy" opcodes need to be sure they are run non-concurrently
771
    # so even if we've migrated we need to at least share the BGL to be
772
    # compatible with them. Of course if we own the BGL exclusively there's no
773
    # point in acquiring any other lock, unless perhaps we are half way through
774
    # the migration of the current opcode.
775
    assert (self._contains_BGL(level, names) or self._BGL_owned()), (
776
            "You must own the Big Ganeti Lock before acquiring any other")
777

    
778
    # Check we don't own locks at the same or upper levels.
779
    assert not self._upper_owned(level), ("Cannot acquire locks at a level"
780
           " while owning some at a greater one")
781

    
782
    # Acquire the locks in the set.
783
    return self.__keyring[level].acquire(names, shared=shared,
784
                                         blocking=blocking)
785

    
786
  def release(self, level, names=None):
787
    """Release a set of resource locks, at the same level.
788

789
    You must have acquired the locks, either in shared or in exclusive mode,
790
    before releasing them.
791

792
    Args:
793
      level: the level at which the locks shall be released.
794
             It must be a memmber of LEVELS.
795
      names: the names of the locks which shall be released.
796
             (defaults to all the locks acquired at that level).
797

798
    """
799
    assert level in LEVELS, "Invalid locking level %s" % level
800
    assert (not self._contains_BGL(level, names) or
801
            not self._upper_owned(LEVEL_CLUSTER)), (
802
            "Cannot release the Big Ganeti Lock while holding something"
803
            " at upper levels")
804

    
805
    # Release will complain if we don't own the locks already
806
    return self.__keyring[level].release(names)
807

    
808
  def add(self, level, names, acquired=0, shared=0):
809
    """Add locks at the specified level.
810

811
    Args:
812
      level: the level at which the locks shall be added.
813
             It must be a memmber of LEVELS_MOD.
814
      names: names of the locks to acquire
815
      acquired: whether to acquire the newly added locks
816
      shared: whether the acquisition will be shared
817
    """
818
    assert level in LEVELS_MOD, "Invalid or immutable level %s" % level
819
    assert self._BGL_owned(), ("You must own the BGL before performing other"
820
           " operations")
821
    assert not self._upper_owned(level), ("Cannot add locks at a level"
822
           " while owning some at a greater one")
823
    return self.__keyring[level].add(names, acquired=acquired, shared=shared)
824

    
825
  def remove(self, level, names, blocking=1):
826
    """Remove locks from the specified level.
827

828
    You must either already own the locks you are trying to remove exclusively
829
    or not own any lock at an upper level.
830

831
    Args:
832
      level: the level at which the locks shall be removed.
833
             It must be a memmber of LEVELS_MOD.
834
      names: the names of the locks which shall be removed.
835
             (special lock names, or instance/node names)
836
      blocking: whether to block while trying to operate in try-lock mode.
837
                this locking mode is not supported yet.
838

839
    """
840
    assert level in LEVELS_MOD, "Invalid or immutable level %s" % level
841
    assert self._BGL_owned(), ("You must own the BGL before performing other"
842
           " operations")
843
    # Check we either own the level or don't own anything from here up.
844
    # LockSet.remove() will check the case in which we don't own all the needed
845
    # resources, or we have a shared ownership.
846
    assert self._is_owned(level) or not self._upper_owned(level), (
847
           "Cannot remove locks at a level while not owning it or"
848
           " owning some at a greater one")
849
    return self.__keyring[level].remove(names, blocking=blocking)