Catch all format errors and return a meaningful message

lib/logstash-logger/formatter/base.rb

@@ -7,6 +7,7 @@ module Formatter
     HOST = ::Socket.gethostname
 
     class Base < ::Logger::Formatter
+      FAILED_TO_FORMAT_MSG = 'Failed to format log event'
       attr_accessor :error_logger
       include ::LogStashLogger::TaggedLogging::Formatter
 
@@ -19,6 +20,9 @@ def initialize(customize_event: nil, error_logger: LogStashLogger.configuration.
       def call(severity, time, _progname, message)
         event = build_event(message, severity, time)
         format_event(event) unless event.cancelled?
+      rescue StandardError => e
+        log_error(e)
+        FAILED_TO_FORMAT_MSG
       end
 
       private

spec/formatter/base_spec.rb

@@ -23,6 +23,22 @@
         expect(subject.call(severity, time, progname, message)).to be_nil
       end
     end
+
+    context "when exception is raised" do
+      before do
+        allow(subject).to receive(:error_logger).and_return(Logger.new('/dev/null'))
+        allow(subject).to receive(:format_event).and_throw
+      end
+
+      it "logs an exception" do
+        expect(subject).to receive(:log_error)
+        subject.call(severity, time, progname, message)
+      end
+
+      it "retruns a failed to format message" do
+        expect(subject.call(severity, time, progname, message)).to eq(LogStashLogger::Formatter::Base::FAILED_TO_FORMAT_MSG)
+      end
+    end
   end
 
   describe '#force_utf8_encoding' do