Promise Cancellation

CancellationToken+.md

Rationale

Cancellation is not an error and not a fulfillment. It means that the resolution value is no more needed, the underlying process to compute it can and shall be aborted (like an XMLHttpRequest). The callbacks just don't get called any more, there is no error passing through the errorback chain.

For this, callbacks which are attached to a promise can be revoked so that they will not be called regardless what happens to the promise - as if it was forever pending. To support branching of promise chains without introducing unexpected cancellations, every promise keeps track of how many callbacks are attached and not revoked. When it is attempted to be cancelled, it can ensure that no callbacks are interested in the result any more. After asserting this, it can (and should) attempt to cancel all other promises that it depends on (or alternatively, abort the non-promise primitive it is built for). Mid-chain cancellation attempts are not effective in this scenario.

The design is built on two pillors:

• callbacks themselves can be cancelled/revoked/unregistered/ignored so that they won't be called. This is done via "passive" tokens that are registered together with the callbacks
• promises can be attempted to be cancelled, triggering the abort action of the underlying task - when there are no more active callbacks This is done via a .cancel() method call on the promise

The basic idea of this draft is that handlers that were passed to then will not be executed when the cancellation token that accompanied them (usually implicitly) is cancelled:

var ajax = http.get(…);
var some = ajax.then(doSomething);
var json = ajax.then(JSON.parse);
// later:
json.cancel(); // `ajax` can't be cancelled (because `doSomething` is still in-
               // terested in it), but the `JSON.parse` won't need to be executed

Extending the idea of promises-aplus/cancellation-spec#8, going further and amend/modify the specification for then, so that handlers themselves can be prevented from execution.

Terminology

1. A CancellationToken is an object with methods for determining whether an operation can be cancelled. It doesn't need to offer a subscription mechanism for the event of becoming cancelled
2. One CancellationToken might be associated with a promise.
3. Many CancellationTokens can be registered with a promise, each optionally linked to a registered callback
4. A CancellationError is an error used to reject cancelled promises.
5. A cancelled promise is a promise that has been rejected with a CancellationError.
6. A cancelled token is a CancellationToken that is in the cancelled state, denoting that the result of an operation is no longer of interest. It might be considered a revoked token.
7. A cancelled callback is an onFulfilled or onRejected handler whose corresponding cancellationToken has been revoked. (All three might have been arguments to a .then() call). It might be considered an unregistered or ignored callback.

Requirements

The then method

Extensions are made to the following sections: 2.2.1. If onFulfilled is a function: 2.2.1.1. it must be called unless it is cancelled after promise is fulfilled, with promise’s value as its first argument. 2.2.2.1. If onRejected is a function, 2.2.2.2. it must be called unless it is cancelled after promise is rejected, with promise’s reason as its first argument.

Note: 2.2.1.3. and 2.2.2.3. ("must not be called more than once") stay in place, and still at most one of the two is called.

2.2.6.1. If/when promise is fulfilled, all respective uncancelled onFulfilled callbacks must execute in the order of their originating calls to then. 2.2.6.2. If/when promise is rejected, all respective uncancelled onRejected callbacks must execute in the order of their originating calls to then.

2.2.7.3. If onFulfilled is not a function and promise1 is fulfilled and promise2 was not cancelled, promise2 must be fulfilled with the same value as promise1. 2.2.7.4. If onRejected is not a function and promise1 is rejected and promise2 was not cancelled, promise2 must be rejected with the same reason as promise1.

(we probably need these last two in every cancellation spec anyway)

The Promise Resolution Procedure

2.3.2 If x is a promise, adopt its state 2.3.2.1 If x is pending, promise must remain pending until x is fulfilled or rejected. ***The cancellatition token associated with promise is registered on x. 2.3.2.2 If/when x is fulfilled, fulfill promise with the same value unless promise had been cancelled. 2.3.2.3 If/when x is rejected, reject promise with the same reason unless promise had been cancelled.. 2.3.2.4 When promise is cancelled, attempt to cancel x.

2.3.3.1 If then is a function, call it with x as this, first argument resolvePromise, second argument rejectPromise and fourth argument token, where 2.3.3.1.5 token is a CancellationToken reflecting the state of the token associated to promise (it can be the same object, or a proxy for it) 2.3.3.1.6 When promise is cancelled, try to invoke x.cancel() as a method (ignoring exceptions)

The cancellationToken parameter

The fourth parameter of the then method is an optional cancellationToken; a call does look like

promise = parentPromise.then(onFulfilled, onRejected, onProgress, cancellationToken)

If cancellationToken is not a CancellationToken object, create an implicit CancellationToken for the new promise. In both cases (explicit and implicit) associate it with the new promise. The state of an explicit token must not be changed by the then method.

Register this cancellation token on the parentPromise together with the onFulfilled and onRejected callbacks.

This cancellation token will also be registered with any child promises that are returned from onFulfilled or onRejected (2.2.7.1), see the Promise Resolution Procedure above for details.

Attempts to cancel

If a promise is attempted to be cancelled with an error, run the following steps:

1. If its associated token is an implicit token, test whether all the registered tokens on it are cancelled. If so, revoke the implicit token.
2. If its associated token is not cancelled, return.
3. Cancel the promise by rejecting it with error. [Note: this is necessary for handlers that have not registered a token, or that might be attached later]
4. Trigger instance-specific cancellation behaviour, e.g. for promises created via then: 4.1. If parentPromise is pending, attempt to cancel it with error. 4.2. If onFulfilled or onRejected have been called and returned a child promise, attempt to cancel that with error.
5. Signal success to the caller.

The CancellationToken

A CancellationToken is an object with a unique identity. It can get revoked, moving it into the cancelled state, which is an irreversible change.

The object has an isCancelled property, whose value must be a boolean[, or a function that returns a boolean]. It must yield true if the token is in the cancelled state, and false otherwise.

Retrieving the state of a cancellation token must not change the state, i.e. an isCancelled function must have no side effects.

The CancellationError

1. It must be an instance of Error (cancellationError instanceof Error === true).
2. It should have a name property with value "CancellationError".
3. It must have a cancelled property with value true.

The cancel method

The cancel method of a promise accepts two optional parameters:

promise.cancel(reason, token);

1. Assert: promise is still pending. Return false otherwise.
2. If reason is a CancellationError, let error be that error object, else let error be a new CancellationError with the reason as the value of its message property.
3. If token is a CancellationToken, revoke it.
4. Attempt to cancel the promise with error.

The Promise constructor

Promises not created by a call to then may handle attempts to cancel them in implementation-dependent ways.

Constructors are however encouraged to signal these to the promise creators, and optionally provide them access to the list of registered tokens. This might be done through a callback that is passed as an additional argument to the Promise constructor, or returned from the resolver call.

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Pluses:

• braching of promise chains is handled gracefully

• no explicit token passing necessary, the default is to work out of the box with existing code by creating implicit tokens

• no ambiguity when a promise is cancelled before the handlers of its resolved parent could be executed

    // Example:
    var promise = fulfilled.then(willNeverBeExecuted); promise.cancel();
    // or:
    parent.then(function() { promise.cancel() }); promise = parent.then(willNeverBeExecuted);
  

• making a promise uncancellable is trivial: .then(null, null, null, {isCancelled:false})

• forking a promise (to prevent immediate cancellation) is even more trivial: .then()

• cancelling promises "from the inside" is possible by passing an explicit token within a then chain:

    // Example (whether this is an appropriate use of promises is another question):
    function getUserchoice() {
        var token = {isCancelled: false};
        var promise = getClick("#radio").then(function(button) {
            return button.value;
        }, null, null, token);
        getClick("#close").then(function(reason) { promise.cancel(reason, token); });
        return promise;
    }
    getUserChoice() // might get rejected "by itself"
  

• the explicit token parameter and .cancel() invocation ensure interoperability between implementations

Minus:

• There's not yet a way to add a handler via calling .then() without registering a token; such would be necessary to implement finally or onCancelled. Promise implementations need an additional token-less callback-registering method, or the .then() above needs to be tweaked (e.g. to only create an implicit token when null or undefined is passed, and not to register anything when something that is not a cancellation token is passed (false, objects with isCancelled, etc).
• Adding a parameter to then is cumbersome, it should not collide with progression callbacks (that's why I have simply chosen to use the fourth parameter, better ideas welcome)

Could you please clarify:

It occurs to me that in "Attempts to Cancel" "4.2. If onFulfilled or onRejected have been called and returned a child promise, attempt to cancel that with error" will never be executed - since the receiver is always a pending promise.

Why add a parameter to .then instead of adding a different handler method?

@couchdeveloper: This is for the case var promise = parentPromise.then(onFulfilled, onRejected);. If promise (which is still pending indeed) is cancelled, then either parentPromise is going to be cancelled, or the "child" promise that is returned by onFulfilled/onRejected after the parent has been settled is going to be cancelled. (…attempted to be cancelled, I mean everywhere)

@benjamingr: That's what I'm probably going to do anyway. But this grew as a Promises/A+ proposal, and the only method that ensures interoperability is then. So if interfacing requires passing a token, I would naturally add that as an optional parameter to this existing method.
I have come to the conclusion now that instead of passing tokens, cancelling the returned promise is more appropriate for the foreign-assimilation case.