Asyncio generc fails when the buffer is larger than file contents

[thelvyn]

When we perform reads with asyncio generic and provide a buffer larger than the file contents an assert and a failure occur.

Works fine with the liburing backend but I did not test it on windows.

I provided a pull request to fix the issue: #13719

[icculus]

Comment from the PR, to move discussion back here:

Okay, I looked into this, and there's definitely a bug; SDL_IOStream won't change its status from SDL_IO_STATUS_READY until it can't make progress...so if there was an error or an EOF but you got 5 bytes out of it first, the read will return 5 and the status will remain READY, until you try to read again, in which case you'll get zero and EOF/ERROR/NOT_READY/whatever.

This seems incorrect (fread() with a return > 0 can still have feof() return nonzero, SDL should probably match this), but also risky to change; it looks like all the backends do it this way at the moment.

However, testing for this case in the caller sort of sucks--you have to try to read another byte and see what happens--so we probably should correct this in SDL_IOStream and leave the generic asyncio as it is.

(It also checks if SDL_GetError() returns something other than "" to decide if there was an error, and that's probably worth cleaning up as well.)

[icculus]

I'm going to take a run at fixing the SDL_IOStream behavior and see if this causes any disasters.