Add decode_size_limit_bytes option.

Resolves logstash-plugins#29

Author: andrewvc

Diff for: CHANGELOG.md

@@ -1,3 +1,6 @@
+## 3.2.0
+  - Add decode_size_limit_bytes option to limit the maximum size of JSON document that can be parsed.[#35](https://github.com/logstash-plugins/logstash-codec-json_lines/pull/35)
+
 ## 3.1.0
   - Feat: event `target => namespace` support (ECS) [#41](https://github.com/logstash-plugins/logstash-codec-json_lines/pull/41)
   - Refactor: dropped support for old Logstash versions (< 6.0)

Diff for: lib/logstash/codecs/json_lines.rb

@@ -42,6 +42,11 @@ class LogStash::Codecs::JSONLines < LogStash::Codecs::Base
   # Change the delimiter that separates lines
   config :delimiter, :validate => :string, :default => "\n"
 
+  # Maximum number of bytes for a single line before a fatal exception is raised
+  # which will stop Logsash.
+  # The default is 20MB which is quite large for a JSON document
+  config :decode_size_limit_bytes, :validate => :number, :default => 20 * (1024 * 1024) # 20MB
+
   # Defines a target field for placing decoded fields.
   # If this setting is omitted, data gets stored at the root (top level) of the event.
   # The target is only relevant while decoding data into a new event.
@@ -50,7 +55,7 @@ class LogStash::Codecs::JSONLines < LogStash::Codecs::Base
   public
 
   def register
-    @buffer = FileWatch::BufferedTokenizer.new(@delimiter)
+    @buffer = FileWatch::BufferedTokenizer.new(@delimiter, @decode_size_limit_bytes)
     @converter = LogStash::Util::Charset.new(@charset)
     @converter.logger = @logger
   end

Diff for: spec/codecs/json_lines_spec.rb

@@ -118,6 +118,22 @@
       end
     end
 
+    describe "decode_size_limits_bytes" do
+      let(:maximum_payload) { "a" * subject.decode_size_limit_bytes }
+
+      it "should not raise an error if the number of bytes is not exceeded" do
+        expect {
+          subject.decode(maximum_payload)
+        }.not_to raise_error
+      end
+      
+      it "should raise an error if the max bytes are exceeded" do
+        expect {
+          subject.decode(maximum_payload << "z")
+        }.to raise_error(RuntimeError, "input buffer full")
+      end
+    end
+
   end
 
   context "#encode" do