GELF output writes subsecond portion of timestamp incorrectly, losing leading zeros

[philomory]

Bug Report

Describe the bug
Given a timestamp in fluent-bit of the form e.g. 2021-07-05T12:34:56.012345678, the GELF output will render that to a timestamp as 1625488496.12 rather than 1625488496.012 or 1625488496.01 or similar. Basically, the leading zero in the fractional portion of the timestamp is lost.

To Reproduce
Log message:

{"log":"example","stream":"stdout","time":"2021-07-05T12:34:56.012345678"}

fluent-bit.conf:

[SERVICE]
    Parsers_File       parsers.conf

[INPUT]
    Name               tail
    Tag                example
    Path               /tmp/example.log
    Parser             docker
    exit_on_eof        true
    Read_from_Head     true

[OUTPUT]
    Name                      gelf
    Match                     example
    Compress                  false
    Gelf_short_message_key    log

Steps to reproduce:

1. Write the example log above to /tmp/example.log
2. Write the example config above to an appropriate location (e.g. /fluent-bit/etc/fluent-bit.conf, if using the fluent-bit container)
3. In one shell, run nc -u -l 12201 or nc -u -l -p 12201 (depending on platform) to listen for UDP packets and output them to the shell
4. In another shell, run fluent-bit -c <path-to-config-file>

Expected behavior
nc should receive the following: {"version":"1.1", "short_message":"example", "_stream":"stdout", "_time":"2021-07-05T12:34:56.012345678", "timestamp":1625488496.012}

Actual behavior
nc receives the following: {"version":"1.1", "short_message":"example", "_stream":"stdout", "_time":"2021-07-05T12:34:56.012345678", "timestamp":1625488496.12}

Your Environment

• Version used: 1.6.10, 1.7.9
• Configuration: As above
• Environment name and version (e.g. Kubernetes? What version?): EKS 1.19, also verified outside of Kubernetes
• Operating System and version: Using the official fluent-bit docker image, so, distroless
• Filters and plugins: GELF output

Additional context
The source of the bug is https://github.com/fluent/fluent-bit/blob/master/src/flb_pack_gelf.c#L765-L766, specifically in passing integer numbers of seconds and milliseconds to printf with a format string like %u.%lu. If the number of milliseconds is 13, this will be rendered as e.g. 1.13 rather than 1.013. A correct format string would be more along the lines of %u.%03lu. A pull request will follow shortly.

[nokute78]

Thank you for pointing it.
I sent a patch #3898