py/objarray: Raise error on out-of-bound memoryview slice start.

[andrewleech]

32 bit platforms only support a slice offset start of 24 bit max due to the limited size of https://github.com/micropython/micropython/blob/9dfabcd6d3d080aced888e8474e921f11dc979bb/py/objarray.h#L47

As it stands, if a memoryview object is sliced with a start index of 16MB or more then it's aliased back to the start of the array providing incorrect results.

I initially tried to increase the size of size_t free in the link above when used on a 32bit platform, however this breaks a lot more. Not sure why, presuably the total size of that structure is hardcoded / assumed elsewhere perhaps?

For now, this PR just raises an exception if the start index is too high to prevent accidental use of the wrong data.

[codecov-commenter]

Codecov Report

Merging #9028 (5c4153e) into master (d6bc34a) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #9028   +/-   ##
=======================================
  Coverage   98.50%   98.50%           
=======================================
  Files         155      155           
  Lines       20537    20539    +2     
=======================================
+ Hits        20229    20231    +2     
  Misses        308      308           

Impacted Files	Coverage Δ
py/objarray.h	100.00% <ø> (ø)
py/objarray.c	100.00% <100.00%> (ø)

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[andrewleech]

It's rather rare that 32bit platforms will need to essentially memory map a >16MB file. For my own use case I've got a workaround using uctypes that's also quite performant. As such I think a section about this on the https://docs.micropython.org/en/latest/genrst/modules.html#array page is probably enough, possibly with my workaround snippet as example.

Instead of:

 memoryview(buff)[start:end]

you can use

uctypes.bytearray_at(uctypes.address_of(buff)+start, end-start)

[github-actions[bot]]

Code size report:

   bare-arm:    +0 +0.000% 
minimal x86:    +0 +0.000% 

[dpgeorge]

I've simplified the error message and updated the test so that it works on 32-bit and 64-bit archs.

[dpgeorge]

Thanks for the fix, this was definitely a bad gotcha.