[BUG] ignore trigrams with null terminator byte when constructing full text index

[codetheweb]

Description of changes

Summarize the changes made by this PR.

The encoding/decoding method we use for packing a trigram into a u64 cannot handle null terminator bytes. E.x. decode(encode(a\0b)) == a. This resulted in index writers and the blockfile writer disagreeing on the ordering of mutations (index writers sorted with a\0b, the blockfile writer ordered with a).

To work around this, any trigrams containing null terminator bytes are now ignored. An (arguably cleaner) alternative would be to replace null terminator bytes with something like a space instead.

Test plan

How are these changes tested?

Added test currently fails on main.

Ran the full_text benchmark to confirm that no performance regression is introduced.

Documentation Changes

Are all docstrings for user-facing APIs updated if required? Do we need to make documentation changes in the docs section?

n/a

[github-actions]

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Testing, Bugs, Errors, Logs, Documentation

• Can you think of any use case in which the code does not behave as intended? Have they been tested?
• Can you think of any inputs or external events that could break the code? Is user input validated and safe? Have they been tested?
• If appropriate, are there adequate property based tests?
• If appropriate, are there adequate unit tests?
• Should any logging, debugging, tracing information be added or removed?
• Are error messages user-friendly?
• Have all documentation changes needed been made?
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System Compatibility

• Are there any potential impacts on other parts of the system or backward compatibility?
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Quality

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[Sicheng-Pan]

Although this works for now, I think we should consider about the alternative approach later. As a note we may want to use unicode codepoint in the custom use areas to encode special characters (whether it is null, document start/end, etc.)