fix(file source): Handle legacy fingerprint checksums from < v0.14.0

[jszwedko]

Adds logic to convert from checkpoints written by Vector before version 0.14.0 to 0.14.0 checkpoints by seeing if any checkpoints, that don't match an existing file, match if the old checkpoint strategy is used.

Also adds alias for first_line_checksum strategy which would have broken checkpoints if released.

Ideally I'd like to include this in 0.15.0 to avoid needing to release a 0.15.1 with it.

This will be released as a 0.13.1.

Fixes: #8182

TODO:

• Figure out why test_checkpointer_fingerprint_upgrades_legacy_checksum is failing. It seems to calculate a different checksum than I get when running 0.13.1 directly despite the upgrade working when I run this branch using 0.13.1 checkpoints.
• Add tests to assert the literal value we are writing to the checkpoint file to avoid unexpected regressions

Signed-off-by: Jesse Szwedko jesse@szwedko.me

[bruceg]

Looks reasonable, and I appreciate moving more logic into the maybe upgrade function, though I do have a question about the buffer parameter.

[bruceg]

This buffer does not appear to be used by any of the callers after calling the function. Could it be created within this function to avoid the additional parameter?

In fact, it doesn't look like the contents are used within this function either. I don't think this function is on the hot path (startup only) so reusing the buffer shouldn't be critical to performance. Am I missing something?

[jszwedko]

Agreed, I was following the pattern of the other functions, but think we can just create the buffer in here since it is only called once at start-up.

[jszwedko]

Actually, I realized the default buffer size 1 MB so I may replace this. I can see why we'd want to reuse them in-case there are a lot of files to avoid allocating 1 MB each time.