Lack of scratch page may result in lost data at power loss

[Wassasin]

During the exchange of pages a page may be lost at brownout because it only exists in RAM. This can happen during or shortly after this statement: https://github.com/jhbruhn/moonboot/blob/efadc79b9edaeec0366ae6ab2276290bc09a46cd/src/boot/mod.rs#L252-L254

Fixing this would require an intermediate scratch page to write each page to, as well as an appropriate representation of this intermediate state in the State page.

[Wassasin]

This is referred to in https://github.com/jhbruhn/moonboot/blob/efadc79b9edaeec0366ae6ab2276290bc09a46cd/TODO.md?plain=1#L2

[jhbruhn]

Yes, I can confirm this is a problem, which is why I didn't advertise the stateful exchange as a feature (yet).

The scratchpad would need to have a size of PAGE_SIZE. This would obviously reduce the usable size for the firmware, but IMO that is a very good tradeoff to make it an always included feature.

Could this be realised as part of the bootloader via some macro/linker magic, without modifying the linker script?

We could get into problems if this ever supports different memory devices (e.g. external flash/EEPROM). Where will the scratchpad be stored then?

[Wassasin]

See #2 for a proposal on how to get the bootloader power-interrupt safe. As for external flash, perhaps we should change internal_memory to be an abstraction over all types of memory, internal, external and RAM, and use a reference to Bank to make it select the right kind of memory.