add check_configuration option

[kangaechu]

Added check_configuration option to validate configuration.

Fixes #2178.

I prepared conf/fluent-bit_invalid.conf and test it.
5c5

$ diff conf/fluent-bit.conf conf/fluent-bit_invalid.conf
5c5
<     Flush        5
---
>     Flush

# validate config file with valid configuration file.
$ docker run fluent-bit:latest /fluent-bit/bin/fluent-bit -c /fluent-bit/etc/fluent-bit.conf -C

Fluent Bit v1.5.0
* Copyright (C) 2019-2020 The Fluent Bit Authors
* Copyright (C) 2015-2018 Treasure Data
* Fluent Bit is a CNCF sub-project under the umbrella of Fluentd
* https://fluentbit.io

configuration test is successful
$ echo $?
0

# validate config file with valid configuration file.
$ docker run fluent-bit:latest /fluent-bit/bin/fluent-bit -c /fluent-bit/etc/fluent-bit.conf --check_configuration

Fluent Bit v1.5.0
* Copyright (C) 2019-2020 The Fluent Bit Authors
* Copyright (C) 2015-2018 Treasure Data
* Fluent Bit is a CNCF sub-project under the umbrella of Fluentd
* https://fluentbit.io

configuration test is successful
$ echo $?
0

# validate config file with invalid configuration file.
$ docker run fluent-bit:latest /fluent-bit/bin/fluent-bit -c /fluent-bit/etc/fluent-bit_invalid.conf -C
Fluent Bit v1.5.0
[2020/05/17 06:00:08] [  Error] File /fluent-bit/etc/fluent-bit_invalid.conf
[2020/05/17 06:00:08] [  Error] Error in line 5: Each key must have a value
* Copyright (C) 2019-2020 The Fluent Bit Authors
* Copyright (C) 2015-2018 Treasure Data
* Fluent Bit is a CNCF sub-project under the umbrella of Fluentd
* https://fluentbit.io

$ echo $?
1

# validate config file with invalid configuration file.
$ docker run fluent-bit:latest /fluent-bit/bin/fluent-bit -c /fluent-bit/etc/fluent-bit_invalid.conf --check_configuration
Fluent Bit v1.5.0
* Copyright (C) 2019-2020 The Fluent Bit Authors
* Copyright (C) 2015-2018 Treasure Data
* Fluent Bit is a CNCF sub-project under the umbrella of Fluentd
* https://fluentbit.io

[2020/05/17 06:00:23] [  Error] File /fluent-bit/etc/fluent-bit_invalid.conf
[2020/05/17 06:00:23] [  Error] Error in line 5: Each key must have a value
$ echo $?
1

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• [N/A] Example configuration file for the change
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---

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.

[kangaechu]

I am a beginner of C programmer.
So someone have any good implementation to solve this, I'll close this.

[fujimotos]

+1 for this patch, with a couple of nitpicks about the option name:

• I'd shorten the long option to --check-config; It's probably more
  consistent with -c/--config.

• OTOH the short option -C looks good to me. Other programs (notably
  nginx and apache) seem to use -t/--test-config for that purpose,
  but we already booked -t for -t/--tag. So I think -C is fine.

[edsiper]

what about --dry-run ?

[kangaechu]

@edsiper
--dry-run is also good.
But -d option is already used for --daemon, so I will use -D instead of -d.

[edsiper]

thanks!