Automatically choose most correct codec for bytestream inputs.

Now, if you try to use `plain` or `json` codecs with either `stdin` or
`tcp` inputs, it will automatically select `line` or `json_lines` codec
respectively.

Right now, many users try to use json codec with tcp input. This
*should* work, but due to technical limitations[1], it does not.
To compensate, we have always provided a json_lines codec to help
parse "streams of line-delimited json" which is what most folks
seem to be handling these days.

[1] I haven't found a JSON library capable of processing a byte stream.
Jackson has one, but it requires an input stream model which logstash
does not currently expose. Future work will make this possible.

Related: #1391

lib/logstash/inputs/base.rb

@@ -118,4 +118,20 @@ class LogStash::Inputs::Base < LogStash::Plugin
       event[field] = value
     end
   end
+
+  protected
+  def fix_streaming_codecs
+    require "logstash/codecs/plain"
+    require "logstash/codecs/line"
+    require "logstash/codecs/json"
+    require "logstash/codecs/json_lines"
+    case @codec
+      when LogStash::Codecs::Plain
+        @logger.info("Automatically switching from #{@codec.class.config_name} to line codec", :plugin => self.class.config_name)
+        @codec = LogStash::Codecs::Line.new
+      when LogStash::Codecs::JSON
+        @logger.info("Automatically switching from #{@codec.class.config_name} to json_lines codec", :plugin => self.class.config_name)
+        @codec = LogStash::Codecs::JSONLines.new
+    end
+  end
 end # class LogStash::Inputs::Base

lib/logstash/inputs/stdin.rb

@@ -16,6 +16,7 @@ class LogStash::Inputs::Stdin < LogStash::Inputs::Base
   public
   def register
     @host = Socket.gethostname
+    fix_streaming_codecs
   end # def register
 
   def run(queue) 

lib/logstash/inputs/tcp.rb

@@ -59,6 +59,7 @@ class LogStash::Inputs::Tcp < LogStash::Inputs::Base
 
   public
   def register
+    fix_streaming_codecs
     require "socket"
     require "timeout"
     require "openssl"