[whatwg] MessagePorts in Web Workers: implementation feedback

[Drew Wilson]

Agreed that removing this requirement:

> User agents must act as if MessagePort<http://www.w3.org/TR/html5/comms.html#messageport>objects have a strong reference to their entangled
> MessagePort <http://www.w3.org/TR/html5/comms.html#messageport> object.
>
would make MessagePort implementation much easier, as it would remove the
need to track reachability across multiple threads. This requirement can get
tricky especially as both sides can be cloned, in-flight to a new owner,
etc.

My only concern is that removing this requirement introduces
non-deterministic behavior - if I have an entangled MessagePort and I
register an onmessage() handler with it, then drop my reference to it, after
which someone calls postMessage() on the entangled port, there's no way to
tell if my onmessage() handler will be invoked ; it entirely depends on
whether a GC happens first or not. That seems bad.

-atw

On Thu, May 7, 2009 at 3:28 PM, Maciej Stachowiak <mjs at apple.com> wrote:

>
> I agree with Drew's assessment that MessagePorts in combination with
> Workers are extremely complicated to implement correctly, as currently
> specified. In fact, the design seems to push towards having lockable shared
> state, even though one potential advantage of the message passing design is
> to avoid locking and shared state.
>
> Besides removing MessagePorts as a way to communicate with workers, another
> possibility is simplifying the life cycle requirements. For example, getting
> rid of the keepalive rule, whereby both MessagePorts remain live so long as
> either is otherwise live, would remove the majority of the complexity. I
> don't think the slight convenience of that rule is worth the extra
> implementation cost.
>
> On May 7, 2009, at 1:39 PM, Drew Wilson wrote:
>
> Hi all,
>
> I've been hashing through a bunch of the design issues around using
> MessagePorts within Workers with IanH and the Chrome/WebKit teams and I
> wanted to follow up with the list with my progress.
>
> The problems we've encountered are all solveable, but I've been surprised
> at the amount of work involved in implementing worker MessagePorts (and the
> resulting implications that MessagePorts have on worker
> lifecycles/reachability). My concern is that the amount of work to implement
> MessagePorts within Worker context may be so high that it will prevent
> vendors from implementing the SharedWorker API. Have other implementers
> started working on this part of the spec yet?
>
> Let me quickly run down some of the implementation issues I've run into -
> some of these may be WebKit/Chrome specific, but other browsers may run into
> some of them as well:
>
> 1) MessagePort reachability is challenging in the context of separate
> Worker heaps
>
> In WebKit, each worker has its own heap (in Chrome, they will have their
> own process as well). The spec reads:
>
> User agents must act as if MessagePort<http://www.w3.org/TR/html5/comms.html#messageport>objects have a strong reference to their entangled
> MessagePort <http://www.w3.org/TR/html5/comms.html#messageport> object.
>
> Thus, a message port can be received, given an event listener, and then
> forgotten, and so long as that event listener could receive a message, the
> channel will be maintained.
>
> Of course, if this was to occur on both sides of the channel, then both
> ports would be garbage collected, since they would not be reachable from
> live code, despite having a strong reference to each other.
>
> Furthermore, a MessagePort<http://www.w3.org/TR/html5/comms.html#messageport>object must not be garbage collected while there exists a message in a task
> queue <http://www.w3.org/TR/html5/browsers.html#task-queue> that is to be
> dispatched on that MessagePort<http://www.w3.org/TR/html5/comms.html#messageport>object, or while the
> MessagePort <http://www.w3.org/TR/html5/comms.html#messageport> object's port
> message queue <http://www.w3.org/TR/html5/comms.html#port-message-queue>is open and there exists a
> message <http://www.w3.org/TR/html5/comms.html#event-message> event in
> that queue.
>
> The end result of this is the need to track some common state across an
> entangled MessagePort pair such as: number of outstanding messages, open
> state of each end, and number of active references to each port (zero or
> non-zero). Turns out this last bit will require adding new hooks to the
> JavaScriptCore garbage collector to detect transitioning between 1 and 0
> references without actually freeing the object - not that difficult, but
> possibly something that other implementers should keep in mind.
>
> 2) MessagePorts dramatically change the worker lifecycle
>
> Having MessagePorts in worker context means that Workers can outlive their
> parent window(s) - I can create a worker, pass off an entangled MessagePort
> to another window (say, to a different domain), then close the original
> window, and the worker should stay alive. In the case of WebKit, this causes
> some problems for things like worker-initiated network requests - if workers
> can continue to run even though there are no open windows for that origin,
> then it becomes problematic to perform network requests (part of this is due
> to the architecture of WebKit which requires proxying network requests to
> window context, but part of this is just a general problem of "how do you
> handle things like HTTP Auth when there are no open windows for that
> origin?")
>
> Finally, the spec defines a fairly broad definition of what makes a worker
> reachable - here's an excerpt from my WebKit Shared Worker design doc, where
> I summarize the spec (possibly incorrectly - feel free to correct any
> misconceptions):
>
>> PermissibleThe spec specifies that a worker is *permissible* based on
>> whether it has a reachable MessagePort that has been entangled *at some
>> point in the past* with an active window (or with a worker who is itself
>> permissible). Basically, if a worker has *ever* been entangled with an
>> active window, or if it's ever been entangled with a worker who is itself
>> permissible (i.e. it's associated with an active window via a chain of
>> workers that have been entangled at some point in the past) then it's
>> permissible.
>>
>> The reason why the "at some point in the past" language is present is to
>> allow a page to create a fire-and-forget worker (for example, a worker that
>> does a set of long network operations) without having to keep a reference to
>> that worker around.
>>
>> Once the referent windows close, the worker should also close, as being
>> permissible is a necessary (but not sufficient) criteria for being
>> runnable.
>> Active neededA permissible worker is *active needed* if:
>>
>>    1. it has pending timers/network requests/DB activity, or
>>    2. it is currently entangled with an active window, or another active
>>    needed worker.
>>
>>
>> The intent behind #1 is to enable fire-and-forget workers that don't exit
>> until they are idle. The intent behind #2 is that an idle worker shouldn't
>> exit as long as it's reachable by an active window (possibly chained through
>> other workers).
>>
> The end result is that for each worker we need to keep track of a big list
> of every window it's ever been entangled with. As workers become entangled
> with other workers, they each inherit the list of entangled windows from the
> other worker. As windows become inactive, we then walk the lists of every
> worker to remove references to the window and properly shutdown the worker
> as appropriate. All of this with the appropriate cross-thread
> synchronization, of course :)
>
> Likewise, determining when a worker is active needed requires tracking a
> graph of entangled message ports, and walking that graph to determine
> whether a given worker is reachable by any active window. Typically this is
> only needed when either a window closes, or when a worker goes idle.
>
> Again, none of these issues individually are insurmountable, but in total
> they add up to a significant amount of work for what should be a fairly
> incremental improvement (going from dedicated workers to shared workers).
> Have other vendors started investigating what it takes to implement
> SharedWorkers (and therefore MessagePorts in workers)?
>
> Another approach for SharedWorkers would be to give them an implicit
> MessagePort-esque API like dedicated Workers and not allow passing in
> MessagePorts to postMessage(). This would mean that references to workers
> can't really be passed around to other windows/workers, but rather are kept
> per-origin. Dedicated workers could work as they do now in Firefox/WebKit
> (with no MessagePorts). The SharedWorker lifecycle could be significantly
> simplified such that a SharedWorker is permissible as long as there's an
> active window under the same origin (no more walking some distributed
> cross-thread dependency graph).
>
> The thing we'd give up is the capabilities-based API that MessagePorts
> provide, but I'd argue that the workaround is simple: the creating window
> can just act as a proxy for the worker. IMO, the implementation burden far
> outstrips the benefit of allowing direct foreign access to workers.
> Literally 90% of the work on my plate for SharedWorkers seems to derive from
> MessagePorts in one form or another, which seems completely wrong.
>
> I'd like to hear your thoughts on this - are people open to removing
> MessagePort support from Workers?
>
> -atw
>
>
>
>
>
>
>
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