Float numbers in JSON stdout and http output plugin misformatted

[jennuineness]

Bug Report

Describe the bug
Hi, I noticed certain float numbers such as 0.072 in a JSON data structure is being rewritten as 0.07199999999999999 when using the Format json option for the stdout and http output plugin.

I am sending out logs to a service that only accepts numbers with Decimal positions to be less than 16 digits. I am unable to find a way to preserve the number as-is when using the Format json option through the output plugins.

Any help with a fix/workaround is much appreciated!

To Reproduce

• Rubular link if applicable:
• Example log message if applicable:

{"test": 0.072}

• Steps to reproduce the problem:

tail.conf

[SERVICE]
    Parsers_File parsers.conf
    Flush 0.1

[INPUT]
    Name            tail
    Path              /test.log
    Parser json

[OUTPUT]
    Name stdout
    Match *
    Format json
    json_date_key false

parsers.conf

[PARSER]
    Name   json
    Format json

test.log

{"test": 0.072}

1. Setup environment with files listed above.
2. Run following command in terminal where tail.conf is located: fluent-bit -c tail.conf
3. Run the following command in a new terminal where test.log is located: echo '{"test": 0.072}' >> test.log

Expected behavior
Expect output to be [{"test":0.072}] instead of [{"test":0.07199999999999999}].

Your Environment

• Version used: Fluent Bit v2.0.4
• Configuration:
• Environment name and version (e.g. Kubernetes? What version?):
• Server type and version:
• Operating System and version: MacOS Monterey v12.6
• Filters and plugins: tail Input plugin, JSON parser plugin, stdout Output plugin, and HTTP Output plugin

Additional context

Possibly, allow an option to preserve the number types without being reformatted when parsing.

[nokute78]

Note: The issue is related %.16g.
https://github.com/fluent/fluent-bit/blob/v2.0.6/src/flb_pack.c#L596

#include <stdio.h>
#include <stdlib.h>

int main() {
  char *a = "0.072";
  double val = atof(a); 

  printf("%lf, %.16g\n", val, val);
  return 0;
}

$ ./a.out 
0.072000, 0.07199999999999999

Related PR:
#1439
#2592

[github-actions]

This issue is stale because it has been open 90 days with no activity. Remove stale label or comment or this will be closed in 5 days. Maintainers can add the exempt-stale label.

[github-actions]

This issue was closed because it has been stalled for 5 days with no activity.

[vitaly-sinev]

This is still reproducible on current master — the Format json path in flb_pack.c still uses %.16g, so 0.072 serializes as 0.07199999999999999.

I've opened #12129 with a fix: a shared flb_pack_double_to_str() helper that uses the already-vendored yyjson dtoa to emit the shortest decimal string that round-trips back to the same double (so 0.072 → "0.072"), while still guaranteeing round-trip precision (unlike a plain %.15g). Non-finite values keep their previous formatting.

Could a maintainer please reopen this issue? Thanks to @nokute78 for pinpointing the %.16g root cause.