pbio/sys/program_load: Allow reading user program.

[laurensvalk]

This is an experimental pull request to facilitate the use case in https://github.com/orgs/pybricks/discussions/1567.

This extends the readable region of user data to include the user code. This seems like a reasonable case for advanced users, so we might even choose to merge this.

The write-able region is not extended.

NOTE
If your purpose is to recover an existing program, naturally you mustn't override it with a new program to use this feature. Instead, use the REPL!

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The hub.system.storage method is normally used to read and store user data. Such as user light sensor calibration values. This is 512 bytes on SPIKE Prime. The user program size (4 bytes) and program follow immediately thereafter.

So, you can theoretically extract your user code using the REPL as follows:

>>> PROGRAM_START = 512 # For SPIKE Prime. Everything before this is user data.
>>> size = hub.system.storage(PROGRAM_START, read=1)[0]
>>> print(size)
61
>>> hub.system.storage(PROGRAM_START + 4, read=61) # For large programs, read it chunk by chunk.
b'0\x00\x00\x00__main__\x00M\x06\x00\x1f\x03\x01\x10hello.py\x00\x0f\x81w\x05\rHello, world!\x00`\x08\x02\x01\x11\x02#\x004\x01YQc'

[github-actions]

Download the artifacts for this pull request:

• cityhub-firmware-build-3365-git63f494b1.zip
• essentialhub-firmware-build-3365-git63f494b1.zip
• movehub-firmware-build-3365-git63f494b1.zip
• mpy-cross.zip
• nxt-firmware-build-3365-git63f494b1.zip
• pr_number.zip
• primehub-firmware-build-3365-git63f494b1.zip
• pybricks-micropython-build-3365-git63f494b1.zip
• technichub-firmware-build-3365-git63f494b1.zip

[coveralls]

coverage: 56.572%. remained the same
when pulling a289bb1 on read-data
into 1d0f987 on master.