Writing XML directly to area detector's NDAttributesFile PV

["Wolfman, Mark via Tech-talk" <tech-talk at aps.anl.gov>]

Hi, all.

I'm configuring our area detectors to save ROI stats, etc into the HDF files alongside the image data. I'm looking for a way to dynamically set the NDAttributes that get included by the HDF file plugin.

As a test, I'm using the XML snippet pasted below on our ADSimDet detector (ADCore version 3.12.1). If I put the XML into a file, and put the file path into the PV 25idSimDet:HDF1:NDAttributesFile, then I see the correct attribute in the HDF file. 

The documentation [1] makes it sound like I can put the XML directly into the PV and it should also work as long as "<Attributes>" is in the string, but I do not see the attribute in the HDF5 file, and I get an IOC console error "asynNDArrayDriver::readNDAttributesFile error opening file " followed by the XML I put in.

I've also tried putting the XML into the camera's NDAttributesFile PV with similar results, and both with and without the "<?xml ?>" declaration. Any ideas on what I'm doing wrong?

The use case here is that I want Bluesky to include some signals in the AD's HDF file, and also include the right resources in its saved scan entry to find the values later through our data API. Ophyd-async includes support to do this [2] but since my IOC does not let me put the XML into the NDAttributesFile PV, this doesn't work.

Here is the XML:

> <?xml version="1.0" encoding="UTF-8"?>
> <Attributes><Attribute name="sim_det_async-sum" type="EPICS_PV" source="25idSimDet:Stats1:Total_RBV" datatype="DOUBLE" description="Sum of the array" /></Attributes>

Thanks,
Mark

[1] https://urldefense.us/v3/__https://areadetector.github.io/areaDetector/ADCore/NDArray.html*asynndarraydriver__;Iw!!G_uCfscf7eWS!dh0BK5QPgcb-IqTxPFl9IiRRsnBYXRh9MQuybGRWbJh-lrGyjMmjBFSPWaY-MwisKfNGRzLMGIbdenqWWNX2xw$ 
[2] https://urldefense.us/v3/__https://github.com/bluesky/ophyd-async/blob/main/src/ophyd_async/plan_stubs/_nd_attributes.py__;!!G_uCfscf7eWS!dh0BK5QPgcb-IqTxPFl9IiRRsnBYXRh9MQuybGRWbJh-lrGyjMmjBFSPWaY-MwisKfNGRzLMGIbdenqlXczNSA$ 

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