To make this feasible, need to switch to a proper event loop (e.g.: calloop?).

So far the best approach seems to be:

• Create a file descriptor with memfd.
• Encode the png into it, then seal it.
• Write to the file with sendfile.
• When pasting, also write to the fd with sendfile.

Need to write an adaptor that wraps sendfile into a Future.

Writing to files async is not really feasible, so writing to the file will always happen on a separate thread.

File writes are always blocking on Linux (apparently it's a POSIX thing).

I can spawn a child that does writes to a file. I need to handle the memfd to the child (e.g.: don't stream data to the child, that would be wasteful). Communicating with the child can be entirely non blocking.