Fix available_buffer_size function

[Oveln]

In the original implementation, if read < write, the available space between 0 and read would be ignored.

[jonathanpallant]

I'm staring at this and struggling to convince myself that either what we had is correct, or what you propose is correct.

Perhaps we could use some unit tests for that function, covering the various conditions?

[Oveln]

Alright, I'll write a few tests for this function. Additionally, in fact, in the official RTT implementation (https://github.com/SEGGERMicro/RTT/blob/main/RTT/SEGGER_RTT.c , line 520), the available space in the _GetAvailWriteSpace() function is calculated exactly this way.

[Oveln]

Oh... Now this seems like there's something wrong with the testing tool?

[Hoverbear]

@Oveln Seems the most recent Rust improved their error messages and broke a snapshot. It should be fixed with #1009. :) It should pass if you rebase.

[jonathanpallant]

The changes here to the algorithm look OK, as do the tests.

Please don't do the version bump though. We'll collect a bunch of changes on the main branch and then do a release PR that combines them together in one bump.

[Oveln]

Okay, but how should I modify the changelog?

[Oveln]

Fine, I consider I fix all problem.

[jonathanpallant]

Brilliant, thank you. I hope we can get this released soon.