How do you use MTLSharedTextureHandle or MTLSharedEventHandle with the C XPC interface on macOS?

Question

TL;DR: How do you encode and decode an MTLSharedTextureHandle and MTLSharedEventHandler such that it can be transported across an XPC connection inside an xpc_dictionary?

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A macOS application I'm working on makes extensive use of XPC services and was implemented using the C-based API. (i.e.: xpc_main, xpc_connection, xpc_dictionary...) This made sense at the time because certain objects, like IOSurfaces, did not support NSCoding/NSSecureCoding and had to be passed using IOSurfaceCreateXPCObject.

In macOS 10.14, Apple introduced new classes for sharing Metal textures and events between processes: MTLSharedTextureHandle and MTLSharedEventHandle. These classes support NSSecureCoding but they don't appear to have a counter-part in the C-XPC interface for encoding/decoding them.

I thought I could use something like [NSKeyedArchiver archivedDataWithRootObject:requiringSecureCoding:error] to just convert them to NSData objects, which can then be stored in an xpc_dictionary, but when I try and do that, I get the following exception:

Caught exception during archival: 
This object may only be encoded by an NSXPCCoder.

(NSXPCCoder is a private class.)

This happens for both MTLSharedTextureHandle and MTLSharedEventHandle. I could switch over to using the new NSXPCConnection API but I've already got an extensive amount of code built on the C-interface, so I'd rather not have to make the switch.

Is there any way to archive either of those two classes into a payload that can be stored in an xpc_dictionary for transfer between the service and the client?

Answer 1

MTLSharedTextureHandle only works with NSXPCConnection. If you're creating the texture from an IOSurface you can share the surface instead which is effectively the same thing. Make sure you are using the same GPU (same id<MTLDevice>) in both processes.

There is no workaround for MTLSharedEventHandle using public API.

I recommend switching to NSXPCConnection if you can. Unfortunately there isn't a good story for partially changing over using public API, you'll have to do it all at once or split your XPC service into two separate services.