fix: Python 3 bytes comparison in linearize-data.py #32978

Replace comparison of a bytes element to the string literal "\0" with an integer 0 in linearize-data.py to ensure correct behavior in Python 3. In Python 3, indexing a bytes object returns an integer, not a single-character string, so the previous comparison would always be false. This change improves compatibility and correctness of the script when detecting end-of-file or null bytes.

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For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/32978.

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Are there steps to reproduce, or a test to confirm the fix?

Are there steps to reproduce, or a test to confirm the fix?

The change is necessary because, in Python 3, indexing a bytes object returns an integer, not a single-character string. For example:

b"\0"[0] == "\0" # always False in Python 3
b"\0"[0] == 0 # True

So, the previous comparison would never detect a null byte as intended.

I was asking about an end-to-end test, like test/functional/feature_loadblock.py or by calling linearize-data.py manually