Command Buffer Extension API Corner Cases

[bashbaug]

Related to #202.

I hit an interesting corner case today trying to update the opencl.hpp mirror I have in my SimpleOpenCLSamples repo. As background, I previously enabled very basic query and reference counting support for command buffers and semaphores, though nothing fancy like the full command buffer and semaphore support added recently. Rather than querying extension API function pointers in opencl.hpp, I relied on the extension loader to query the extension API function pointers.

Because I didn't have any fancy support for creating command buffers I used the constructor taking a cl_command_buffer_khr to construct my cl::CommandBuffer:

https://github.com/bashbaug/SimpleOpenCLSamples/blob/a434b4edcede9c6215e509f01eee7f736632993a/samples/12_commandbufferspp/main.cpp#L161

    cl::CommandBuffer cmdbuf{clCreateCommandBufferKHR(
        1,
        &commandQueue(),
        NULL,
        NULL)};

This gave me the nice reference counting support and the ability to do C++ queries - cool.

When I tried to update to the latest opencl.hpp though, I hit a problem:

1. This constructor doesn't initialize the command buffer function pointers (link).
2. Even if we wanted to initialize the command buffer function pointers in this constructor, we have no way of doing so, because we can't get a platform (or anything we could use to query a platform) to use to initialize the command buffer function pointers.

We have all of the right nullptr checks so we won't crash even when the function pointers are uninitialized, but the queries and any reference counting won't work as expected.

I don't think there's a great way to fix this, unfortunately. Some options to consider:

1. Do nothing - this is an odd usage and a corner case.
2. Remove this constructor - it's dangerous.
3. Add a parameter to this constructor (could be optional?) that gives us something we could use to query function pointers.
4. Provide an expliict function that applications can use to initialize extension API function pointers to handle cases like this one where the extension function pointers aren't initialized automatically.

Of these options I think I like (4) the best, but they all seem pretty unsatisfying, so maybe there's another better option instead?

PS: I encountered this issue for command buffers but I'm pretty sure semaphores could have the same problem.

[shajder]

@bashbaug I am not even sure if we could consider that approach as fifth option but I will reveal what I learnt only because it works at my machine. The idea comes from iffy assumption that since cl_command_buffer_khr object was properly created there is fair good chance ::clGetCommandBufferInfoKHR may be "available". Is there any sense in that ?