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fix(loki sink): Loki fails to parse protobuf payload with '"' in label value. #24155

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fix(loki sink): Loki fails to parse protobuf payload with '"' in label value. #24155

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@alexgavrushenko alexgavrushenko commented Nov 5, 2025 •
edited by thomasqueirozb
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Summary

When using Loki sink with JSON enconding, label values with '"' and '' chars are pushed to Loki normally, while Protobuf encoding causes errors like: 'couldn't parse labels: ... parse error: unexpected character inside braces' or 'couldn't parse labels: ... parse error: unexpected string'

Vector configuration

Default Vector configuration with Loki sink

How did you test this PR?

There is a unit-test

Change Type

  • Bug fix
  • New feature
  • Non-functional (chore, refactoring, docs)
  • Performance

Is this a breaking change?

  • Yes
  • No

Does this PR include user facing changes?

  • Yes. Please add a changelog fragment based on our guidelines.
  • No. A maintainer will apply the no-changelog label to this PR.

References

N/A

Notes

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    • We recommend adding a pre-push hook, please see this template.
    • Alternatively, we recommend running the following locally before pushing to the remote branch:
      • make fmt
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      • make test
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@github-actions github-actions bot added the domain: sinks Anything related to the Vector's sinks label Nov 5, 2025
@alexgavrushenko alexgavrushenko force-pushed the loki-escape-label-values branch 2 times, most recently from 1733221 to b868b5d Compare November 5, 2025 15:50
@alexgavrushenko alexgavrushenko force-pushed the loki-escape-label-values branch from b868b5d to 2c9fbd5 Compare November 5, 2025 15:50
@github-actions github-actions bot removed the domain: sinks Anything related to the Vector's sinks label Nov 5, 2025
@alexgavrushenko alexgavrushenko marked this pull request as ready for review November 5, 2025 19:49
@alexgavrushenko alexgavrushenko requested a review from a team as a code owner November 5, 2025 19:49
@thomasqueirozb
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thomasqueirozb commented Dec 11, 2025

Hey @alexgavrushenko, thanks for your contribution. While this fix should work, a version of this code is already present in src/sinks/prometheus/collector.rs.

vector/src/sinks/prometheus/collector.rs

Lines 298 to 313 in a4b2b8e

fn format_tag(key: &str, mut value: &str) -> String {
// For most tags, this is just `{KEY}="{VALUE}"` so allocate optimistically
let mut result = String::with_capacity(key.len() + value.len() + 3);
result.push_str(key);
result.push_str("=\"");
while let Some(i) = value.find(['\\', '"']) {
result.push_str(&value[..i]);
result.push('\\');
// Ugly but works because we know the character at `i` is ASCII
result.push(value.as_bytes()[i] as char);
value = &value[i + 1..];
}
result.push_str(value);
result.push('"');
result
}

It would be best if both parts of the code shared this implementation.

This is also considered a user facing change and therefore needs a changelog.

Thanks!

@thomasqueirozb thomasqueirozb self-assigned this Dec 11, 2025
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@alexgavrushenko @thomasqueirozb