in_dummy: Add flush_on_startup config.

[franciscovalentecastro]

This PR adds new flush_on_startup setting to the in_dummy input plugin. When interval_sec/nsec is set for a long time, its helpful to be able to verify the first event is generated at startup to avoid waiting.

Documentation update in fluent/fluent-bit-docs#1264.

Configuration

[INPUT]
    Dummy            {"message": "Dummy", "severity": "INFO"}
    Interval_NSec    0
    Interval_Sec     10
    Name             dummy
    Start_time_sec   0
    Flush_on_startup True

[OUTPUT]
    Name            stdout
    Match           *

Output Log

With this config a log message is generated at startup and then every 10 seconds.

./fluent-bit -c fluent-bit.conf
Fluent Bit v2.2.1
* Copyright (C) 2015-2023 The Fluent Bit Authors
* Fluent Bit is a CNCF sub-project under the umbrella of Fluentd
* https://fluentbit.io

[2023/11/27 21:22:50] [ info] [fluent bit] version=2.2.1, commit=8733b25df1, pid=1790739
[2023/11/27 21:22:50] [ info] [storage] ver=1.5.1, type=memory, sync=normal, checksum=off, max_chunks_up=128
[2023/11/27 21:22:50] [ info] [cmetrics] version=0.6.5
[2023/11/27 21:22:50] [ info] [ctraces ] version=0.3.1
[2023/11/27 21:22:50] [ info] [input:dummy:dummy.0] initializing
[2023/11/27 21:22:50] [ info] [input:dummy:dummy.0] storage_strategy='memory' (memory only)
[2023/11/27 21:22:50] [ info] [sp] stream processor started
[2023/11/27 21:22:50] [ info] [output:stdout:stdout.0] worker #0 started
[0] dummy.0: [[0.000070090, {}], {"message"=>"Dummy", "severity"=>"INFO"}]
[0] dummy.0: [[9.447917175, {}], {"message"=>"Dummy", "severity"=>"INFO"}]
[0] dummy.0: [[19.447787506, {}], {"message"=>"Dummy", "severity"=>"INFO"}]
[0] dummy.0: [[29.447908652, {}], {"message"=>"Dummy", "severity"=>"INFO"}]
^C[2023/11/27 21:23:24] [engine] caught signal (SIGINT)
[2023/11/27 21:23:24] [ warn] [engine] service will shutdown in max 5 seconds
[2023/11/27 21:23:24] [ info] [input] pausing dummy.0
[2023/11/27 21:23:24] [ info] [engine] service has stopped (0 pending tasks)
[2023/11/27 21:23:24] [ info] [input] pausing dummy.0
[2023/11/27 21:23:24] [ info] [output:stdout:stdout.0] thread worker #0 stopping...
[2023/11/27 21:23:24] [ info] [output:stdout:stdout.0] thread worker #0 stopped

Valgrind Output

valgrind --leak-check=yes ./fluent-bit   --config ./fluent-bit.conf
==1790816== Memcheck, a memory error detector
==1790816== Copyright (C) 2002-2022, and GNU GPL'd, by Julian Seward et al.
==1790816== Using Valgrind-3.19.0 and LibVEX; rerun with -h for copyright info
==1790816== Command: ./fluent-bit --config ./fluent-bit.conf
==1790816==
Fluent Bit v2.2.1
* Copyright (C) 2015-2023 The Fluent Bit Authors
* Fluent Bit is a CNCF sub-project under the umbrella of Fluentd
* https://fluentbit.io

[2023/11/27 21:23:46] [ info] [fluent bit] version=2.2.1, commit=8733b25df1, pid=1790816
[2023/11/27 21:23:46] [ info] [storage] ver=1.5.1, type=memory, sync=normal, checksum=off, max_chunks_up=128
[2023/11/27 21:23:46] [ info] [cmetrics] version=0.6.5
[2023/11/27 21:23:46] [ info] [ctraces ] version=0.3.1
[2023/11/27 21:23:46] [ info] [input:dummy:dummy.0] initializing
[2023/11/27 21:23:46] [ info] [input:dummy:dummy.0] storage_strategy='memory' (memory only)
[2023/11/27 21:23:46] [ info] [sp] stream processor started
[2023/11/27 21:23:46] [ info] [output:stdout:stdout.0] worker #0 started
==1790816== Warning: client switching stacks?  SP change: 0x6cbae58 --> 0x517e0a0
==1790816==          to suppress, use: --max-stackframe=28560824 or greater
==1790816== Warning: client switching stacks?  SP change: 0x517df88 --> 0x6cbae58
==1790816==          to suppress, use: --max-stackframe=28561104 or greater
==1790816== Warning: client switching stacks?  SP change: 0x6cbae58 --> 0x517df88
==1790816==          to suppress, use: --max-stackframe=28561104 or greater
==1790816==          further instances of this message will not be shown.
[0] dummy.0: [[0.014987779, {}], {"message"=>"Dummy", "severity"=>"INFO"}]
[0] dummy.0: [[9.137168279, {}], {"message"=>"Dummy", "severity"=>"INFO"}]
[0] dummy.0: [[19.135748536, {}], {"message"=>"Dummy", "severity"=>"INFO"}]
^C[2023/11/27 21:24:07] [engine] caught signal (SIGINT)
[2023/11/27 21:24:07] [ warn] [engine] service will shutdown in max 5 seconds
[2023/11/27 21:24:07] [ info] [input] pausing dummy.0
[2023/11/27 21:24:08] [ info] [engine] service has stopped (0 pending tasks)
[2023/11/27 21:24:08] [ info] [input] pausing dummy.0
[2023/11/27 21:24:08] [ info] [output:stdout:stdout.0] thread worker #0 stopping...
[2023/11/27 21:24:08] [ info] [output:stdout:stdout.0] thread worker #0 stopped
==1790816==
==1790816== HEAP SUMMARY:
==1790816==     in use at exit: 0 bytes in 0 blocks
==1790816==   total heap usage: 1,755 allocs, 1,755 frees, 1,875,538 bytes allocated
==1790816==
==1790816== All heap blocks were freed -- no leaks are possible
==1790816==
==1790816== For lists of detected and suppressed errors, rerun with: -s
==1790816== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

---

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Testing
Before we can approve your change; please submit the following in a comment:

• Example configuration file for the change
• Debug log output from testing the change

• Attached Valgrind output that shows no leaks or memory corruption was found

If this is a change to packaging of containers or native binaries then please confirm it works for all targets.

• Run local packaging test showing all targets (including any new ones) build.
• Set ok-package-test label to test for all targets (requires maintainer to do).

Documentation

• Documentation required for this feature

Backporting

• Backport to latest stable release.

---

Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.

[braydonk]

LGTM. @nokute78 please take a look

[nokute78]

LGTM.

Note: If user doesn't set Match, fluent-bit doesn't flush on startup.
I think it is not related this PR and I believe user should set Match.

[INPUT]
    Name dummy
    Interval_Sec 10
    Flush_On_Startup true

[OUTPUT]
    Name stdout
#   Match *