When allocations were freed in a different order from the reverse of how they were allocated (leaving holes), the heap would get into an inconsistent state, eventually resulting in crashes.
free_memory() relies on having allocations in order, but allocate_memory() did not guarantee that: It reused the first allocation with a NULL ptr without ensuring that it was between low_address and high_address. When it belongs to a hole in the allocated memory, such an allocation is not really free for reuse, because free_memory() still needs its length.
Instead, explicitly mark allocations available for reuse with a special (invalid) value in the length field. Only allocations that lie between low_address and high_address are marked that way.
It was incorrect to NULL out the pointer to our heap allocated buffer in
`reset`, because subsequent to framebuffer_reset, but while
the heap was still active, we could call `get_bufinfo` again,
leading to a fresh allocation on the heap that is about to be destroyed.
Typical stack trace:
```
#1 0x0006c368 in sharpdisplay_framebuffer_get_bufinfo
#2 0x0006ad6e in _refresh_display
#3 0x0006b168 in framebufferio_framebufferdisplay_background
#4 0x00069d22 in displayio_background
#5 0x00045496 in supervisor_background_tasks
#6 0x000446e8 in background_callback_run_all
#7 0x00045546 in supervisor_run_background_tasks_if_tick
#8 0x0005b042 in common_hal_neopixel_write
#9 0x00044c4c in clear_temp_status
#10 0x000497de in spi_flash_flush_keep_cache
#11 0x00049a66 in supervisor_external_flash_flush
#12 0x00044b22 in supervisor_flash_flush
#13 0x0004490e in filesystem_flush
#14 0x00043e18 in cleanup_after_vm
#15 0x0004414c in run_repl
#16 0x000441ce in main
```
When this happened -- which was inconsistent -- the display would keep
some heap allocation across reset which is exactly what we need to avoid.
NULLing the pointer in reconstruct follows what RGBMatrix does, and that
code is a bit more battle-tested anyway.
If I had a motivation for structuring the SharpMemory code differently,
I can no longer recall it.
Testing performed: Ran my complicated calculator program over multiple
iterations without observing signs of heap corruption.
Closes: #3473
As originally written, the TRANSLATE_SOURCES_EXC was incorrect. If
more than one build directory existed, "make translate"
(and make check-translate) would error.
I corrected the problem, commented it, and added a number of additional
exclude directives.
I reviewed the PR and should have caught this, but did not.
Having zero RGB pins may not have been caught, nor having a non-multiple-of-6
value. Generally, users will only have 6 RGB pins unless they are driving
multiple matrices in parallel. No existing breakouts exist to do this, and
there are probably not any efficient pinouts to be had anyway.
In #3482, @cwalther noted that, hypothetically, a zero byte allocation
could be made in the RGBMatrix constructor. Ensure that width is positive.
Height was already checked against the number of RGB pins if it was specified,
so zero is ruled out there as well.
