zope.locking 1.2.2

1.2.2 (2011-01-31)

• Consolidate duplicate evolution code.

• Split generations config into its own zcml file.

1.2.1 (2010-01-20)

• Bug fix: the generation added in 1.2 did not properly clean up expired tokens, and could leave the token utility in an inconsistent state.

1.2 (2009-11-23)

• Bug fix: tokens were stored in a manner that prevented them from being cleaned up properly in the utility’s _principal_ids mapping. Make zope.locking.tokens.Token orderable to fix this, as tokens are stored as keys in BTrees.

• Add a zope.app.generations Schema Manager to clean up any lingering tokens due to this bug. Token utilities not accessible through the component registry can be cleaned up manually with zope.locking.generations.fix_token_utility.

• TokenUtility’s register method will now add the token to the utility’s database connection if the token provides IPersistent.

• Clean up the tests and docs and move some common code to testing.py.

• Fix some missing imports.

1.1

(series for Zope 3.4; eggs)

1.0

(series for Zope 3.3; no dependencies on Zope eggs)

Exclusive Locks

Exclusive locks are tokens that are owned by a single principal. No principal may be added or removed: the lock token must be ended and another started for another principal to get the benefits of the lock (whatever they have been configured to be).

Here’s an example of creating and registering an exclusive lock: the principal with an id of ‘john’ locks the demo object.

>>> lock = tokens.ExclusiveLock(demo, 'john')
>>> res = util.register(lock)
>>> res is lock
True

The lock token is now in effect. Registering the token (the lock) fired an ITokenStartedEvent, which we’ll look at now.

(Note that this example uses an events list to look at events that have fired. This is simply a list whose append method has been added as a subscriber to the zope.event.subscribers list. It’s included as a global when this file is run as a test.)

>>> ev = events[-1]
>>> verifyObject(interfaces.ITokenStartedEvent, ev)
True
>>> ev.object is lock
True

Now that the lock token is created and registered, the token utility knows about it. The utilities get method simply returns the active token for an object or None–it never returns an ended token, and in fact none of the utility methods do.

>>> util.get(demo) is lock
True
>>> util.get(Demo()) is None
True

Note that get accepts alternate defaults, like a dictionary.get:

>>> util.get(Demo(), util) is util
True

The iterForPrincipalId method returns an iterator of active locks for the given principal id.

>>> list(util.iterForPrincipalId('john')) == [lock]
True
>>> list(util.iterForPrincipalId('mary')) == []
True

The util’s __iter__ method simply iterates over all active (non-ended) tokens.

>>> list(util) == [lock]
True

The token utility disallows registration of multiple active tokens for the same object.

>>> util.register(tokens.ExclusiveLock(demo, 'mary'))
... # doctest: +ELLIPSIS
Traceback (most recent call last):
...
RegistrationError: ...
>>> util.register(tokens.SharedLock(demo, ('mary', 'jane')))
... # doctest: +ELLIPSIS
Traceback (most recent call last):
...
RegistrationError: ...
>>> util.register(tokens.Freeze(demo))
... # doctest: +ELLIPSIS
Traceback (most recent call last):
...
RegistrationError: ...

It’s also worth looking at the lock token itself. The registered lock token implements IExclusiveLock.

>>> verifyObject(interfaces.IExclusiveLock, lock)
True

It provides a number of capabilities. Arguably the most important attribute is whether the token is in effect or not: ended. This token is active, so it has not yet ended:

>>> lock.ended is None
True

When it does end, the ended attribute is a datetime in UTC of when the token ended. We’ll demonstrate that below.

Later, the creation, expiration, duration, and remaining_duration will be important; for now we merely note their existence.

>>> before_creation <= lock.started <= datetime.datetime.now(pytz.utc)
True
>>> lock.expiration is None # == forever
True
>>> lock.duration is None # == forever
True
>>> lock.remaining_duration is None # == forever
True

The end method and the related ending and expiration attributes are all part of the IEndable interface–an interface that not all tokens must implement, as we will also discuss later.

>>> interfaces.IEndable.providedBy(lock)
True

The context and __parent__ attributes point to the locked object–demo in our case. context is the intended standard API for obtaining the object, but __parent__ is important for the Zope 3 security set up, as discussed towards the end of this document.

>>> lock.context is demo
True
>>> lock.__parent__ is demo # important for security
True

Registering the lock with the token utility set the utility attribute and initialized the started attribute to the datetime that the lock began. The utility attribute should never be set by any code other than the token utility.

>>> lock.utility is util
True

Tokens always provide a principal_ids attribute that provides an iterable of the principals that are part of a token. In our case, this is an exclusive lock for ‘john’, so the value is simple.

>>> sorted(lock.principal_ids)
['john']

The only method on a basic token like the exclusive lock is end. Calling it without arguments permanently and explicitly ends the life of the token.

>>> lock.end()

Like registering a token, ending a token fires an event.

>>> ev = events[-1]
>>> verifyObject(interfaces.ITokenEndedEvent, ev)
True
>>> ev.object is lock
True

It affects attributes on the token. Again, the most important of these is ended, which is now the datetime of ending.

>>> lock.ended >= lock.started
True
>>> lock.remaining_duration == datetime.timedelta()
True

It also affects queries of the token utility.

>>> util.get(demo) is None
True
>>> list(util.iterForPrincipalId('john')) == []
True
>>> list(util) == []
True

Don’t try to end an already-ended token.

>>> lock.end()
Traceback (most recent call last):
...
EndedError

The other way of ending a token is with an expiration datetime. As we’ll see, one of the most important caveats about working with timeouts is that a token that expires because of a timeout does not fire any expiration event. It simply starts providing the expiration value for the ended attribute.

>>> one = datetime.timedelta(hours=1)
>>> two = datetime.timedelta(hours=2)
>>> three = datetime.timedelta(hours=3)
>>> four = datetime.timedelta(hours=4)
>>> lock = util.register(tokens.ExclusiveLock(demo, 'john', three))
>>> lock.duration
datetime.timedelta(0, 10800)
>>> three >= lock.remaining_duration >= two
True
>>> lock.ended is None
True
>>> util.get(demo) is lock
True
>>> list(util.iterForPrincipalId('john')) == [lock]
True
>>> list(util) == [lock]
True

The expiration time of an endable token is always the creation date plus the timeout.

>>> lock.expiration == lock.started + lock.duration
True
>>> ((before_creation + three) <=
...  (lock.expiration) <= # this value is the expiration date
...  (before_creation + four))
True

Expirations can be changed while a lock is still active, using any of the expiration, remaining_duration or duration attributes. All changes fire events. First we’ll change the expiration attribute.

>>> lock.expiration = lock.started + one
>>> lock.expiration == lock.started + one
True
>>> lock.duration == one
True
>>> ev = events[-1]
>>> verifyObject(interfaces.IExpirationChangedEvent, ev)
True
>>> ev.object is lock
True
>>> ev.old == lock.started + three
True

Next we’ll change the duration attribute.

>>> lock.duration = four
>>> lock.duration
datetime.timedelta(0, 14400)
>>> four >= lock.remaining_duration >= three
True
>>> ev = events[-1]
>>> verifyObject(interfaces.IExpirationChangedEvent, ev)
True
>>> ev.object is lock
True
>>> ev.old == lock.started + one
True

Now we’ll hack our code to make it think that it is two hours later, and then check and modify the remaining_duration attribute.

>>> def hackNow():
...     return (datetime.datetime.now(pytz.utc) +
...             datetime.timedelta(hours=2))
...
>>> import zope.locking.utils
>>> oldNow = zope.locking.utils.now
>>> zope.locking.utils.now = hackNow # make code think it's 2 hours later
>>> lock.duration
datetime.timedelta(0, 14400)
>>> two >= lock.remaining_duration >= one
True
>>> lock.remaining_duration -= one
>>> one >= lock.remaining_duration >= datetime.timedelta()
True
>>> three + datetime.timedelta(minutes=1) >= lock.duration >= three
True
>>> ev = events[-1]
>>> verifyObject(interfaces.IExpirationChangedEvent, ev)
True
>>> ev.object is lock
True
>>> ev.old == lock.started + four
True

Now, we’ll hack our code to make it think that it’s a day later. It is very important to remember that a lock ending with a timeout ends silently–that is, no event is fired.

>>> def hackNow():
...     return (
...         datetime.datetime.now(pytz.utc) + datetime.timedelta(days=1))
...
>>> zope.locking.utils.now = hackNow # make code think it is a day later
>>> lock.ended == lock.expiration
True
>>> util.get(demo) is None
True
>>> util.get(demo, util) is util # alternate default works
True
>>> lock.remaining_duration == datetime.timedelta()
True
>>> lock.end()
Traceback (most recent call last):
...
EndedError

Once a lock has ended, the timeout can no longer be changed.

>>> lock.duration = datetime.timedelta(days=2)
Traceback (most recent call last):
...
EndedError

We’ll undo the hacks, and also end the lock (that is no longer ended once the hack is finished).

>>> zope.locking.utils.now = oldNow # undo the hack
>>> lock.end()

Make sure to register tokens. Creating a lock but not registering it puts it in a state that is not fully initialized.

>>> lock = tokens.ExclusiveLock(demo, 'john')
>>> lock.started # doctest: +ELLIPSIS
Traceback (most recent call last):
...
UnregisteredError: ...
>>> lock.ended # doctest: +ELLIPSIS
Traceback (most recent call last):
...
UnregisteredError: ...

Shared Locks

Shared locks are very similar to exclusive locks, but take an iterable of one or more principals at creation, and can have principals added or removed while they are active.

In this example, also notice a convenient characteristic of the TokenUtility register method: it also returns the token, so creation, registration, and variable assignment can be chained, if desired.

>>> lock = util.register(tokens.SharedLock(demo, ('john', 'mary')))
>>> ev = events[-1]
>>> verifyObject(interfaces.ITokenStartedEvent, ev)
True
>>> ev.object is lock
True

Here, principals with ids of ‘john’ and ‘mary’ have locked the demo object. The returned token implements ISharedLock and provides a superset of the IExclusiveLock capabilities. These next operations should all look familiar from the discussion of the ExclusiveLock tokens above.

>>> verifyObject(interfaces.ISharedLock, lock)
True
>>> lock.context is demo
True
>>> lock.__parent__ is demo # important for security
True
>>> lock.utility is util
True
>>> sorted(lock.principal_ids)
['john', 'mary']
>>> lock.ended is None
True
>>> before_creation <= lock.started <= datetime.datetime.now(pytz.utc)
True
>>> lock.expiration is None
True
>>> lock.duration is None
True
>>> lock.remaining_duration is None
True
>>> lock.end()
>>> lock.ended >= lock.started
True

As mentioned, though, the SharedLock capabilities are a superset of the ExclusiveLock ones. There are two extra methods: add and remove. These are able to add and remove principal ids as shared owners of the lock token.

>>> lock = util.register(tokens.SharedLock(demo, ('john',)))
>>> sorted(lock.principal_ids)
['john']
>>> lock.add(('mary',))
>>> sorted(lock.principal_ids)
['john', 'mary']
>>> lock.add(('alice',))
>>> sorted(lock.principal_ids)
['alice', 'john', 'mary']
>>> lock.remove(('john',))
>>> sorted(lock.principal_ids)
['alice', 'mary']
>>> lock.remove(('mary',))
>>> sorted(lock.principal_ids)
['alice']

Adding and removing principals fires appropriate events, as you might expect.

>>> lock.add(('mary',))
>>> sorted(lock.principal_ids)
['alice', 'mary']
>>> ev = events[-1]
>>> verifyObject(interfaces.IPrincipalsChangedEvent, ev)
True
>>> ev.object is lock
True
>>> sorted(ev.old)
['alice']
>>> lock.remove(('alice',))
>>> sorted(lock.principal_ids)
['mary']
>>> ev = events[-1]
>>> verifyObject(interfaces.IPrincipalsChangedEvent, ev)
True
>>> ev.object is lock
True
>>> sorted(ev.old)
['alice', 'mary']

Removing all participants in a lock ends the lock, making it ended.

>>> lock.remove(('mary',))
>>> sorted(lock.principal_ids)
[]
>>> lock.ended >= lock.started
True
>>> ev = events[-1]
>>> verifyObject(interfaces.IPrincipalsChangedEvent, ev)
True
>>> ev.object is lock
True
>>> sorted(ev.old)
['mary']
>>> ev = events[-2]
>>> verifyObject(interfaces.ITokenEndedEvent, ev)
True
>>> ev.object is lock
True

As you might expect, trying to add (or remove!) users from an ended lock is an error.

>>> lock.add(('john',))
Traceback (most recent call last):
...
EndedError
>>> lock.remove(('john',))
Traceback (most recent call last):
...
EndedError

The token utility keeps track of shared lock tokens the same as exclusive lock tokens. Here’s a quick summary in code.

>>> lock = util.register(tokens.SharedLock(demo, ('john', 'mary')))
>>> util.get(demo) is lock
True
>>> list(util.iterForPrincipalId('john')) == [lock]
True
>>> list(util.iterForPrincipalId('mary')) == [lock]
True
>>> list(util) == [lock]
True
>>> util.register(tokens.ExclusiveLock(demo, 'mary'))
... # doctest: +ELLIPSIS
Traceback (most recent call last):
...
RegistrationError: ...
>>> util.register(tokens.SharedLock(demo, ('mary', 'jane')))
... # doctest: +ELLIPSIS
Traceback (most recent call last):
...
RegistrationError: ...
>>> util.register(tokens.Freeze(demo))
... # doctest: +ELLIPSIS
Traceback (most recent call last):
...
RegistrationError: ...
>>> lock.end()

Timed expirations work the same as with exclusive locks. We won’t repeat that here, though look in the annoying.txt document in this package for the actual repeated tests.

EndableFreezes

An endable freeze token is similar to a lock token except that it grants the ‘lock’ to no one.

>>> token = util.register(tokens.EndableFreeze(demo))
>>> verifyObject(interfaces.IEndableFreeze, token)
True
>>> ev = events[-1]
>>> verifyObject(interfaces.ITokenStartedEvent, ev)
True
>>> ev.object is token
True
>>> sorted(token.principal_ids)
[]
>>> token.end()

Endable freezes are otherwise identical to exclusive locks. See annoying.txt for the comprehensive copy-and-paste tests duplicating the exclusive lock tests. Notice that an EndableFreeze will never be a part of an iterable of tokens by principal: by definition, a freeze is associated with no principals.

Freezes

Freezes are similar to EndableFreezes, except they are not endable. They are intended to be used by system level operations that should permanently disable certain changes, such as changes to the content of an archived object version.

Creating them is the same…

>>> token = util.register(tokens.Freeze(demo))
>>> verifyObject(interfaces.IFreeze, token)
True
>>> ev = events[-1]
>>> verifyObject(interfaces.ITokenStartedEvent, ev)
True
>>> ev.object is token
True
>>> sorted(token.principal_ids)
[]

But they can’t go away…

>>> token.end()
Traceback (most recent call last):
...
AttributeError: 'Freeze' object has no attribute 'end'

They also do not have expirations, duration, remaining durations, or ended dates. They are permanent, unless you go into the database to muck with implementation-specific data structures.

There is no API way to end a Freeze. We’ll need to make a new object for the rest of our demonstrations, and this token will exist through the remaining examples.

>>> old_demo = demo
>>> demo = Demo()

TokenBrokers

Token brokers adapt an object, which is the object whose tokens are brokered, and uses this object as a security context. They provide a few useful methods: lock, lockShared, freeze, and get. The TokenBroker expects to be a trusted adapter.

TokenHandlers

TokenHandlers are useful for endable tokens with one or more principals–that is, locks, but not freezes. They are intended to be protected with a lower external security permission then the usual token methods and attributes, and then impose their own checks on the basis of the current interaction. They are very much policy, and other approaches may be useful. They are intended to be registered as trusted adapters.

For exclusive locks and shared locks, then, we have token handlers. Generally, token handlers give access to all of the same capabilities as their corresponding tokens, with the following additional constraints and capabilities:

• expiration, duration, and remaining_duration all may be set only if all the principals in the current interaction are owners of the wrapped token; and

• release removes some or all of the principals in the interaction if all the principals in the current interaction are owners of the wrapped token.

Note that end is unaffected: this is effectively “break lock”, while release is effectively “unlock”. Permissions should be set accordingly.

Shared lock handlers have two additional methods that are discussed in their section.

Warnings

• The token utility will register a token for an object if it can. It does not check to see if it is actually the local token utility for the given object. This should be arranged by clients of the token utility, and verified externally if desired.

• Tokens are stored as keys in BTrees, and therefore must be orderable (i.e., they must implement __cmp__).

Intended Security Configuration

Utilities are typically unprotected in Zope 3–or more accurately, have no security assertions and are used with no security proxy–and the token utility expects to be so. As such, the broker and handler objects are expected to be the objects used by view code, and so associated with security proxies. All should have appropriate __parent__ attribute values. The ability to mutate the tokens–end, add and remove methods, for instance–should be protected with an administrator-type permission such as ‘zope.Security’. Setting the timeout properties on the token should be protected in the same way. Setting the handlers attributes can have a less restrictive setting, since they calculate security themselves on the basis of lock membership.

On the adapter, the end method should be protected with the same or similar permission. Calling methods such as lock and lockShared should be protected with something like ‘zope.ManageContent’. Getting attributes should be ‘zope.View’ or ‘zope.Public’, and unlocking and setting the timeouts, since they are already protected to make sure the principal is a member of the lock, can probably be ‘zope.Public’.

These settings can be abused relatively easily to create an insecure system–for instance, if a user can get an adapter to IPrincipalLockable for another principal–but are a reasonable start.

>>> broker.__parent__ is demo
True
>>> handler.__parent__ is lock
True

Random Thoughts

As a side effect of the design, it is conceivable that multiple lock utilities could be in use at once, governing different aspects of an object; however, this may never itself be of use.