Support bracketed target field name

[nickcarenza]

The argument for defining the target isn't like other logstash plugins (logstash-filter-geoip). It seems to require that I supply the field without surrounding brackets. If i use brackets, it silently adds the fields to the root of the event. Strangely however, if i target a nested field like [one][two] with the brackets, it does write the fields to that location. It is not obvious to me how this line allows that since the resulting normalized_target seems like it would be [[one][two]]. Maybe that is a valid target or I am misunderstanding what that line does.

Here is the config I expect to work but causes the filter to write fields to root:

    useragent {
      source => "[device][user_agent]"
      target => "[device]"
    }

Here is the config where I specify a deep path and the fields are successfully written to it as expected:

    useragent {
      source => "[device][user_agent]"
      target => "[device][test]"
    }

And here is the config I ended up using for this filter which works as intended but unlike other plugins in how target is specified:

    useragent {
      source => "[device][user_agent]"
      target => "device"
    }

This is with version 2.0.8 but the line of the code that seems to matter looks the same on master.

[jordansissel]

Based on my interpretation of the code, the problem is that this normalized_target is not aware that field references can be nested. It looks like the regexp would accept foo and [foo] but would bug out when given nested [foo][bar] producing [[foo][bar]] as a field reference.

[nickcarenza]

The else case though sets normalized_target to "", which defaults to document root.

irb(main):001:0> target = "[test]"
=> "[test]"
irb(main):002:0> normalized_target = (target && target !~ /^\[[^\[\]]+\]$/) ? "[#{target}]" : ""
=> ""

irb(main):003:0> target = "[one][two]"
=> "[one][two]"
irb(main):004:0> normalized_target = (target && target !~ /^\[[^\[\]]+\]$/) ? "[#{target}]" : ""
=> "[[one][two]]"

irb(main):005:0> target = "test"
=> "test"
irb(main):006:0> normalized_target = (target && target !~ /^\[[^\[\]]+\]$/) ? "[#{target}]" : ""
=> "[test]"