areaDetector with 'empty' array output

[Phil Atkin <phil.atkin@pixelanalytics.co.uk>]

Hi,

My areaDetector plugin has an option to generate an array containing a 'list' of events.  In pseudocode, it does something like the following in the processCallbacks method:

    if (this->pArrays[0]) {
        this->pArrays[0]->release();
        this->pArrays[0] = NULL;
    }
   
    this->unlock();

        // Copy the events list to the output array
        size_t count = events.size();
        dims[xDim] = count;
        dims[yDim] = 3; // x, y and type
        this->pArrays[0] = pNDArrayPool->alloc(2, dims, NDFloat32, 0, nullptr);

    this->pArrays[0]->timeStamp = pArray->timeStamp;
    this->pArrays[0]->epicsTS = pArray->epicsTS;
    this->pArrays[0]->uniqueId = pArray->uniqueId;
    pArray->pAttributeList->copy(this->pArrays[0]->pAttributeList);

    this->lock();
    // [Set some parameter values, including NDArraySizeX,Y,Z]
    this->getAttributes(this->pArrays[0]->pAttributeList);
    this->unlock();
    doCallbacksGenericPointer(this->pArrays[0], NDArrayData, 0);
    this->lock();
    callParamCallbacks();

Currently I'm experiencing a crash if there are no events in the list.  pNDArrayPool->alloc returns null if count is zero.  I guess this is neither surprising nor unreasonable, but it leaves me with the question of what I should (and should not) do if the list is empty.  I somehow need to 'output' something, otherwise plugins downstream won't be called, will they?

Thanks in advance,

Phil

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