Question about StagesSubject

[NickWu]

Hi, I have a question about the logic of the following code:

maya-usd/lib/mayaUsd/ufe/StagesSubject.cpp

Lines 581 to 583 in 00e63f3

	// We should have no listeners and stage map is dirty.
	TF_VERIFY(g_StageMap.isDirty());
	TF_VERIFY(fStageListeners.empty());

The comment on the above code says : We should have no listeners and stage map is dirty. So I wonder why not just returning from the function when g_StageMap is not dirty and fStageListeners is not empty? What is the reason to keep proceeding the rest of the logic when both statements of TF_VERIFY failed? Thank you.

[ppt-adsk]

Hi @NickWu, given that this is essentially an assertion, the software is in an unspecified state at this point. TF_AXIOM would cause a program exit, which is too strong. My opinion is at this point the user should be in a state where they can save their work, and that the software should be in a reasonable enough state to allow this to succeed. Your suggestion has merits; you could try it out to see if the plugin remains stable in such a case. However, my hope is that you will never hit this code path :)

Is this causing you problems, or are you simply trying to get a better understanding of the code? Cheers.

[NickWu]

Thank you for your reply!

Yes, at Animal Logic, we found that outStageData attribute (derived from MayaUsdProxyShapeBase) is being requested to be recomputed many times when moving a vehicle rig (rigged with math nodes connections) in Maya which drives the prims in usd file loaded in AL ProxyShape which is derived from MayaUsdProxyShapeBase:

maya-usd/plugin/al/lib/AL_USDMaya/AL/usdmaya/nodes/ProxyShape.cpp

Lines 1646 to 1648 in 00e63f3

	} else if (plug == outStageData()) {
	MStatus status = computeOutputTime(MPlug(plug.node(), outTime()), dataBlock, currentTime);
	return status == MS::kSuccess ? computeOutStageData(plug, dataBlock) : status;

In turn, computeOutStageData() would be called which would send MayaUsdProxyStageSetNotice:

maya-usd/plugin/al/lib/AL_USDMaya/AL/usdmaya/nodes/ProxyShape.cpp

Line 1584 in 00e63f3

MayaUsdProxyStageSetNotice(*this).Send();

which would call StagesSubject::onStageSet many times over the course of moving a vehicle rig:

maya-usd/lib/mayaUsd/ufe/StagesSubject.cpp

Line 562 in 00e63f3

void StagesSubject::onStageSet(const MayaUsdProxyStageSetNotice& notice)

As a result, we are seeing lots of error message being emitted in Maya script editor:

// Error: Failed verification: ' g_StageMap.isDirty() ' // 
// Error: Failed verification: ' fStageListeners.empty() ' // 
// Error: Failed verification: ' g_StageMap.isDirty() ' // 
// Error: Failed verification: ' fStageListeners.empty() ' // 
// Error: Failed verification: ' g_StageMap.isDirty() ' // 
// Error: Failed verification: ' fStageListeners.empty() ' // 
// Error: Failed verification: ' g_StageMap.isDirty() ' // 
// Error: Failed verification: ' fStageListeners.empty() ' // 

For some reason, this issue is only happening starting from Maya 2022.3.2-22.0.PFIX. We don't see this issue in Maya 2022.3.2-19.0.PFIX

Even though I haven't figured out what is triggering this issue and why it's only happening in Maya 2022.3.2-22.0.PFIX and newer Maya 2022 versions, but if we could return from the StagesSubject::onStageSet function when g_StageMap is not dirty and fStageListeners is not empty if it's not necessary to proceed the result of the code, we could avoid this constant error message being emitted. So I am suggesting instead of using TF_VERIFY, we could do:

if(!g_StageMap.isDirty() && !fStageListeners.empty())
    return;

One thing I am not sure about this approach is that if we do early return from the function like the question I mentioned, should we set stageSetGuardCount=false because the following code would set stageSetGuardCount=true when the if statement evaluates true:

maya-usd/lib/mayaUsd/ufe/StagesSubject.cpp

Line 580 in 00e63f3

if (stageSetGuardCount.compare_exchange_strong(expectedState, true)) {

And since we are early returning from the function, we don't set stageSetGuardCount back to false like the end of the function does:

maya-usd/lib/mayaUsd/ufe/StagesSubject.cpp

Line 606 in 00e63f3

stageSetGuardCount = false;

What do you think? Thanks!