add file writable check in #configure

[sonots]

--config-test is very useful, but it does not check file permission. This proposition is to make a rule to check file permissions in #configure method of each plugin because I thought it is impossible to check file permissions for each configuration parameter of each plugin from Fluentd side (Fluentd can not know what each plugin wants to do)

This pull request is just the first example of the proposition.

@repeatedly What do you think? Yes, each plugin author should support file permission check in #configure method of each plugin when we decide as this is an official way.

PS. @repeatedly Could you tell me if you have a right place to put the file permission check utility function?

[kenjiskywalker]

👍

[sonots]

I will write a permission check for out_file, but the permission check will be limited because out_file would make dirs not only on configure but also on running loop dynamically.

[repeatedly]

PS. @repeatedly Could you tell me if you have a right place to put the file permission check utility function?

I'm not sure. I put some socket utilities in https://github.com/fluent/fluentd/blob/master/lib/fluent/plugin/socket_util.rb.
So similar file is good for me.

[sonots]

@repeatedly Could you review?

1. Add FileUtil#wrtitable? method
2. Add file writable check for in_debug_agent's unix_path
3. Add file writable check for out_file's path (This permission check is limited because out_file would make dirs not only on configure but also on running loop dynamically.)

[sonots]

@repeatedly ping

[repeatedly]

Should use exist? instead of exists?

[sonots]

I do not mind. I will fix.

[sonots]

@repeatedly fixed!

[repeatedly]

How about following code?

path = File.dirname(path) unless File.exist?(path) # or path = File.exist?(path) ? path : File.dirname(path)
File.writable?(path)

[sonots]

Good. Fixed

[coveralls]

Changes Unknown when pulling fd1b901 on writable_check into * on master*.