ExpressionHelper thread-safety

[John Hendrikx]

On 24/04/2025 22:07, Christopher Schnick wrote:
>
> Hey John,
>
> Thanks for taking your time on going into the details here.
>
> About our use case: We are actually not constructing UI in a
> background thread, all nodes are initialized and added to the scene on
> the platform thread. This is done because previously instantiating
> nodes on other threads was unstable for some controls. So this issue
> has nothing to do with nodes at all and is purely focused on the
> observable value listeners. When initializing our application, there
> are some global properties like the app language property that a lot
> of other listeners and bindings depend on as a lot of stuff has to be
> changed when the language changes (Not just within nodes). So in
> practice, we have 100+ listeners added to these important setting
> properties. These listeners are added from various threads as the
> loading is parallelized. This loading is to some degree done before
> the platform is even started.
>

Okay, so you're adding listeners in parallel, but not changing the
property at the same time listeners are being added? In other words,
there is never a listener being added/removed while concurrently
listeners are being notified?  That should be relatively safe (if
properties were thread safe in that regard :))

For a short term solution, one thing you can do is to wrap these global
properties in a custom wrapper, delegating all methods to the original
FX class.  You then add `synchronized` to all the add/remove listener
methods.  The users of these properties won't know the difference, and
ExpressionHelper should no longer get in a bad state.

>
> About the listener management: Yes I'm aware that this can be tricky
> and I'm doing the best to account for all of that. The GC problem is
> always a bit tricky and has led to some issues in the past. But once
> you are familiar with it, it's manageable. We are using a somewhat
> custom implementation to create nodes, link them to properties, and
> control their lifecycle properly for GC handling for that.
>
> About the expression helper being replaced, that is unfortunate (and
> also a bit weird from my unknowing perspective, at least I don't see
> the reason for that). If you are already reworking the listeners in
> general, then it would be nice if there was a good way to synchronize
> the expressionhelper on something that is consistent.
>
ExpressionHelper replaces itself depending on the type and count of
listeners on the property; it's an optimization to use as little memory
as possible as most properties have 0 listeners, and the next most
common amount of listeners is 1.  For those two special cases,
ExpressionHelper is null (for 0 listeners) or it is one of the SingleXXX
instances (which take less memory than the variants that have an ArrayList).

The reworking is mostly to fix another problem, and potentially making
listener management synchronized can be done in either case.

>
> You mentioned that there is a case that is hard to fix where a
> callback occurs on another thread while the instances are manipulated
> on the first thread. At least for us, that shouldn't be a problem.
> This issue I reported is only about adding/removing listeners in
> different threads. So I will be following the status of your PR for
> updates.
>
As long as you're aware that such a fix (or the wrapper I mentioned
above) only gives partial thread safety and may confuse your own
listeners if they were recently added and immediately get notified... :)

The PR mentioned is not specifically created to solve this problem; if
the team agrees that making listener management thread-safe is the way
forward, it will probably be a separate PR and ticket.  The work
involved however is fairly trivial and could be done before or after the
PR mentioned is integrated.

--John

>
> Best
> Christopher Schnick
>
> On 24/04/2025 03:56, John Hendrikx wrote:
>>
>> Hi,
>>
>> I don't think adding synchronized in ExpressionHelper is going to
>> really solve your problem.  It will just move it elsewhere, but feel
>> free to let me know your exact scenario.  For now I will make some
>> assumptions.
>>
>> I'm assuming you are constructing UI's in a background thread, and
>> this UI requires listening to some global properties, like dark/light
>> mode, or any other configuration that must dynamically change your UI
>> that's basically global, or some global modeled state that can be
>> independently used, even without a UI.  It's certainly not an
>> unreasonable scenario in larger applications that may have a lot of
>> configuration options -- I've been there myself.  I usually call
>> these "global" models; they're not part of any specific piece of the
>> user interface.  Feel free to let me know your scenario.
>>
>> I'm fine with UI's being constructed on background threads; anything
>> that could potentially take more than a millisecond SHOULD be done on
>> a background thread, as otherwise animations will stutter.  However,
>> there are several gotcha's with connecting a UI with global models
>> that expose properties that you must be aware of:
>>
>> ## Listener Management
>>
>> Any UI component that listens to global properties must either:
>>
>> a) unregister itself when the UI component is removed or closed (this
>> can be very difficult to track as FX has no #dispose method that will
>> be called)
>> b) use a weak listener (discouraged as this can lead to phantom call
>> backs of UI's you thought no longer existed until GC runs)
>> c) only register the listener when the UI is visible, and immediately
>> unregister when it becomes invisible (this can be largely automated
>> with the "when" method of ObservableValue)
>>
>> Failing to do so means your UI component (including all its
>> children/parents as they refer to those) will never be garbage
>> collected as a global property is referring to it.
>>
>> I highly recommend using the "when" construct here.  Basically,
>> whenever you want to listen to a global property from a UI component
>> insert a "when" statement:
>>
>>        globalProperty.when(myComponentIsVisible).subscribe( ... ) or
>> addListener( ... )
>>
>> Or:
>>
>>        uiProperty.bind(globalProperty.when(myComponentIsVisible));
>>
>> This results in listeners being registered on the FX thread just
>> before your UI becomes visible to the user.  It also removes the
>> listeners on the FX thread as soon as the UI becomes invisible.  See
>> the documentation for a good condition to use with when() for this.
>>
>> ## Global properties may call listeners at unexpected times!
>>
>> When you registered on such a property in a background thread,
>> realize that as soon as you do, you may get a callback from the FX
>> thread.  At that point in time, your presumed single threaded code
>> that you are constructing on your isolated thread is being run by two
>> threads.  In other words, you can get a callback from a global
>> property halfway during construction while your components may be in
>> some half constructed state.  As FX controls are never safe to use
>> concurrently (and neither will your listener code be) this can cause
>> intermittent problems.
>>
>> All that said, let's say we do want to proceed to make listener
>> management a little bit safer and prevent ExpressionHelper from going
>> into a bad state. 
>>
>> Your proposal to just synchronize the methods in ExpressionHelper
>> will be insufficient.  ExpressionHelper replaces itself on properties
>> all the time, meaning that having a single invalidation listener on a
>> property is a different ExpressionHelper instance then when that same
>> property has 2 invalidation listeners or say just a single change
>> listener.  This is done by properties like this (from
>> ObjectPropertyBase):
>>
>> @Override
>>
>> publicvoidaddListener(InvalidationListener listener) {
>>
>> helper= ExpressionHelper.addListener(helper, this, listener);
>>
>> }
>>
>> As you can see, the actual helper is getting replaced in certain
>> cases (it "morphs" from one internal type to another depending on
>> what listener types and counts are registered).  That means that the
>> first call may be dealing with Helper#1, and the second call may also
>> be dealing with Helper#1 (blocking inside ExpressionHelper on a
>> synchronized block)... but the first call returns a new Helper,
>> including the new listener.  When then the second call runs that was
>> blocked, it will replace the Helper again but without knowledge of
>> the listener that was added by the first call.  This happens when
>> going from a single invalidation listener to two invalidation
>> listeners -- it's a different helper.
>>
>> There are two ways around that; you could synchronize at an earlier
>> level before calling ExpressionHelper, adding synchronized to the
>> above method and similar methods, in all property/bindings and read
>> only property classes (about 20 orso).  Another is to synchronize on
>> the property itself (which is passed as "this" in the above
>> snippet).  That still requires modifying 20 classes though as the
>> "removeListener" variant does not pass "this" currently, so it would
>> need to be explicitly passed for those as well to have something to
>> synchronize on.
>>
>> The PR which replaces
>> ExpressionHelper (https://github.com/openjdk/jfx/pull/1081) faces
>> similar issues, but in that PR, "this" is passed already in all
>> cases, giving it something to synchronize on.  If that PR is
>> integrated, then offering thread safe adding/removal of listeners for
>> all observable that use the new solution can be done in one central
>> location.
>>
>> Perhaps it is worth doing; as Kevin mentioned, within FX itself we've
>> run into problems with registering listeners that required quite some
>> changes in many places.  A central fix may be preferable; however it
>> can't and won't be a full fix, as you still must deal with potential
>> callbacks coming in from another thread shortly after registering --
>> a scenario that most developers will likely not be taking into
>> account while writing what they presume to be single threaded code...
>>
>> --John
>>
>>
>> On 23/04/2025 18:58, Christopher Schnick wrote:
>>> Hello,
>>>
>>> I encountered a rare exception where adding listeners to an
>>> observable value might break when they are added concurrently. This
>>> is due to ExpressionHelper not being synchronized. I thought about
>>> how to fix this on my side, but it is very difficult to do. As this
>>> is not a typical platform thread issue, in my opinion it should be
>>> possible to add listeners to one observable value from any thread
>>> without having to think about any potential synchronization issues
>>> (which I can't solve other than just running everything on one thread).
>>>
>>> Even worse, due to the size and array being two different variables
>>> and being incremented unsafely, once such a concurrent modification
>>> occurs, this invalid state will persist permanently and will cause
>>> exceptions on any further method call as well. The only solution is
>>> to restart the application.
>>>
>>> This is how a stack trace looks like when this occurs:
>>>
>>> 21:25:38:840 - error: Index 2 out of bounds for length 2
>>> java.lang.ArrayIndexOutOfBoundsException: Index 2 out of bounds for
>>> length 2
>>>     at
>>> com.sun.javafx.binding.ExpressionHelper$Generic.addListener(ExpressionHelper.java:248)
>>>     at
>>> com.sun.javafx.binding.ExpressionHelper$Generic.addListener(ExpressionHelper.java:200)
>>>     at
>>> com.sun.javafx.binding.ExpressionHelper.addListener(ExpressionHelper.java:65)
>>>     at
>>> javafx.beans.binding.ObjectBinding.addListener(ObjectBinding.java:86)
>>>     at javafx.beans.binding.StringBinding.bind(StringBinding.java:114)
>>>     at javafx.beans.binding.Bindings$7.<init>(Bindings.java:428)
>>>     at
>>> javafx.beans.binding.Bindings.createStringBinding(Bindings.java:426)
>>>     at
>>> io.xpipe.app.util.StoreStateFormat.shellEnvironment(StoreStateFormat.java:24)
>>>     at
>>> io.xpipe.ext.proc.env.ShellEnvironmentStoreProvider.informationString(ShellEnvironmentStoreProvider.java:155)
>>>     at
>>> io.xpipe.app.comp.store.StoreEntryWrapper.update(StoreEntryWrapper.java:228)
>>>     at
>>> io.xpipe.app.comp.store.StoreViewState.lambda$updateContent$1(StoreViewState.java:147)
>>>     at java.lang.Iterable.forEach(Iterable.java:75)
>>>     at
>>> io.xpipe.app.comp.store.StoreViewState.updateContent(StoreViewState.java:147)
>>>     at
>>> io.xpipe.app.comp.store.StoreViewState.init(StoreViewState.java:93)
>>>     at
>>> io.xpipe.app.core.mode.BaseMode.lambda$onSwitchTo$1(BaseMode.java:109)
>>>     at
>>> io.xpipe.app.util.ThreadHelper.lambda$load$0(ThreadHelper.java:78)
>>>     at java.lang.Thread.run(Thread.java:1447)
>>>
>>> 21:25:38:847 - error: Index 3 out of bounds for length 2
>>> java.lang.ArrayIndexOutOfBoundsException: Index 3 out of bounds for
>>> length 2
>>>     at
>>> com.sun.javafx.binding.ExpressionHelper$Generic.addListener(ExpressionHelper.java:248)
>>>     at
>>> com.sun.javafx.binding.ExpressionHelper$Generic.addListener(ExpressionHelper.java:200)
>>>     at
>>> com.sun.javafx.binding.ExpressionHelper.addListener(ExpressionHelper.java:65)
>>>     at
>>> javafx.beans.binding.ObjectBinding.addListener(ObjectBinding.java:86)
>>>     at javafx.beans.binding.StringBinding.bind(StringBinding.java:114)
>>>     at javafx.beans.binding.Bindings$7.<init>(Bindings.java:428)
>>>     at
>>> javafx.beans.binding.Bindings.createStringBinding(Bindings.java:426)
>>>     at
>>> io.xpipe.app.util.StoreStateFormat.shellEnvironment(StoreStateFormat.java:24)
>>>     at
>>> io.xpipe.ext.proc.env.ShellEnvironmentStoreProvider.informationString(ShellEnvironmentStoreProvider.java:155)
>>>     at
>>> io.xpipe.app.comp.store.StoreEntryWrapper.update(StoreEntryWrapper.java:228)
>>>     at
>>> io.xpipe.app.comp.store.StoreEntryWrapper.lambda$setupListeners$3(StoreEntryWrapper.java:143)
>>>     at
>>> io.xpipe.app.util.PlatformThread.lambda$runLaterIfNeeded$0(PlatformThread.java:318)
>>>     at
>>> com.sun.javafx.application.PlatformImpl.lambda$runLater$4(PlatformImpl.java:424)
>>>     at
>>> com.sun.glass.ui.InvokeLaterDispatcher$Future.run$$$capture(InvokeLaterDispatcher.java:95)
>>>     at
>>> com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java)
>>>
>>> This full log goes up to index 50 out of bounds due to the recurring
>>> nature of this exception.
>>>
>>> Looking at the implementation of ExpressionHelper, I don't see any
>>> harm in just synchronizing the methods, at least from my
>>> perspective. But I guess that is up to the developers to decide. The
>>> only real solution I have as an application developer is to perform
>>> all initialization on one thread or just hope that this error is
>>> rare enough, both of which aren't great options. So I hope that a
>>> potential synchronization of the ExpressionHelper methods can be
>>> considered.
>>>
>>> Best
>>> Christopher Schnick
>>>
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