[klodolph]

Maybe I didn't explain what's happening in OpenGL properly, because that comparison doesn't work at all.

With e.g. std::ofstream, you are wrapping a handle, such as a POSIX file descriptor. So that part of your description is accurate.

However, this is different from what happens with OpenGL. The problem is that if you wrap an OpenGL texture (which is a handle), you end up with some problems because you can only free it with the correct context active, and it might become free for other reasons besides the wrapper being destroyed. So you are no longer wrapping a handle (like std::ofstream), but you are actually creating a new handle-to-handle, and wrapping that, and explicitly managing the lifetime OpenGL resources with a separate object somewhere else.

Of course, you could just wrap an OpenGL texture, explicitly decide not to handle context invalidation, and then be careful to ensure that your objects get destroyed with the OpenGL context active. You lose some flexibility and you have to babysit RAII to be sure it "does the right thing".

So what I'm saying here is that OpenGL textures are not like file handles.

[ska]

They don't behave like file handles - but at a certain distance if you squint at it looks like the same problem. The real issue isn't that OpenGL textures don't behave like file handles, but that neither of them behave quite like objects in your language, which can lead to problems. The specifics of how the behavior differs is I think less important that the fact that it differs. Of course, file handles are not the best example because the language designers often had to put some thought into it early on; less true of something like OpenGL textures.

[klodolph]

> …but at a certain distance if you squint at it looks like the same problem…

The whole discussion here is about how these things differ in subtle ways. The problem with “squinting” at the problem is you end up e.g. using RAII and then having to redesign your system because RAII doesn’t match, and the problems weren’t obvious when you started out.

Not to make too fine a point about it here, but file handles are perfect matches for RAII / Rust lifetimes, unlike OpenGL textures. You just close the handle at end of object lifetime. You can get errors when you close but current recommendation is to ignore errors when cleaning up and have a separate path for closing/committing data on close when writing, so on most paths the destructor will nop. This works well.

An example of a mismatch with the language semantics and file handles is with GC. If you close a file handle in a C++ destructor or Rust drop(), it’s fine, those are run deterministically. If you use a JVM finalizer to close your file handle that might not happen soon enough (cue EMFILE).

Again, the reason why textures don’t work this way is because you have to release them in a certain context and they might be released outside your control.

[anyfoo]

Ok, file handles are often good matches, unless you start allowing for them being changed outside of the file object, e.g. being closed or their position pointer modified (by e.g. seeking or reading).

So my example might not have been the best, because usually, they have a closely matching lifecycle, but the greater point that I was trying to make is that managing handles of handles (of handles) is not weird if you have to do some impedance matching. Maybe file handles are often straightforward, and therefore only require one "additional layer of handle indirection", but there's probably other good examples like say, threads in thread pools, or a whole bunch of stuff that's going on inside an OS kernel, especially a multiprocessing one.

[adrianratnapala]

> Not to make too fine a point about it here, but file handles are perfect matches for RAII / Rust lifetimes, unlike OpenGL textures.

Agreed, something like an FD matches RAII very well, but you can open one file multiple times, dup fds and multiple processes can do all this. So the FDs end up mapping in some non-trivial, probably refcounted way to other objects in the kernel.

So it seems with OpenGL, this kind of mapping / bookkeeping layer gets hoisted up into the application framework -- which might be a reasonable choice.

[klodolph]

OpenGL textures point to some refcounted thing, but the handle table is not per-process but per-context, and the contexts are not thread-safe, and the contexts can become invalidated. So it is similar in many ways but there are too many differences in general.

[ska]

Yes, I am talking about this at a higher abstraction level than the specifics only of c++ & OpenGL textures. I guess that wasn't clear enough.

I was only attempting to make that point that wrapping handles with other handles, rather than being silly, is a pretty good pattern to address this problem, and it shows up all over.

The file handle thing I already pointed out; there are better examples.

[adrianratnapala]

> ... you end up with some problems because you can only free it with the correct context active, ...

I wonder if this is where Rust can actually help. I.e. could the lifetime tracking in the language allow you to construct a library where the unref was guaranteed to happen only when the right context is active?

And if such a library could be constructed, what would the loss of flexibility cost?

[steveklabnik]

Incidentally, some of the people working on safe OpenGL wrappers ran into similar issues with the blog post, thanks to similar problems.