Malformed JSON in query API result, 2.52-rc.0 regression

[baryluk]

What did you do?

$ curl 'http://localhost:19092/api/v1/query?query=process_cpu_seconds_total&time=1714716585.782'; echo

{"status":"success","data":{"resultType":"vector","result":[{"metric":{"__name__":"process_cpu_seconds_total","instance":"localhost:19092","job":"prometheus"},"value":[1714716585.782,"0.29"]}{"metric":{"__name__":"process_cpu_seconds_total","job":"a","instance":"baz:20702"},"value":[1714716585.782,"2125535.148752446"]}]}}

Notice a missing comma between array elements.

What did you expect to see?

$ curl 'http://localhost:19091/api/v1/query?query=process_cpu_seconds_total&time=1714716585.782'; echo

{"status":"success","data":{"resultType":"vector","result":[{"metric":{"__name__":"process_cpu_seconds_total","job":"a","instance":"baz:20702"},"value":[1714716585.782,"23600372.135197595"]},{"metric":{"__name__":"process_cpu_seconds_total","instance":"localhost:19092","job":"prometheus"},"value":[1714716585.782,"1.2"]}]}}

What did you see instead? Under which circumstances?

See above.

It looks like there are missing comas.

Works fine in 2.51.2

System information

Linux amd64

Prometheus version

prometheus, version 2.52.0-rc.0 (branch: HEAD, revision: f170a014f31cc97d2aba47cc769e846c68f8545d)
  build user:       root@969a4b73703c
  build date:       20240430-12:21:35
  go version:       go1.22.2
  platform:         linux/amd64
  tags:             netgo,builtinassets,stringlabels

Prometheus configuration file

scrape_configs:
  - job_name: "prometheus"
    static_configs:
      - targets: ["localhost:19091"]

  - job_name: "a"
    static_configs:
      - targets:
        - "baz:20702"

[amirvejahat]

I just had a quick look at the source code. it seems that comma is missing in the marshal functions, I'd like to work on the fix.

[ArthurSens]

I just had a quick look at the source code. it seems that comma is missing in the marshal functions, I'd like to work on the fix.

Please do if you already know the fix, happy to review :)

cc @bboreham, I think it's related to #13997. It is the only PR that was not included in the benchmark

[amirvejahat]

I think it's ready to be reviewed. this is my first time contributing to an open source project, it's a big step for me. please let me know if there's something wrong with the pull request or the code.

#14047

[bboreham]

Agreed, thanks for spotting the problem.

[baryluk]

Fixed by #14047