FEATURE: Allow Unicorn logs to be JSON formatted.

config/unicorn.conf.rb

@@ -1,5 +1,12 @@
 # See http://unicorn.bogomips.org/Unicorn/Configurator.html
 
+if ENV["UNICORN_JSON_LOG_FORMAT"]
+  unicorn_logger = Logger.new($stderr)
+  require_relative '../lib/unicorn/unicorn_json_log_formatter'
+  unicorn_logger.formatter = UnicornJSONLogFormatter.new
+  logger unicorn_logger
+end
+
 # enable out of band gc out of the box, it is low risk and improves perf a lot
 ENV['UNICORN_ENABLE_OOBGC'] ||= "1"
 

lib/unicorn/unicorn_json_log_formatter.rb

@@ -0,0 +1,21 @@
+require 'json'
+
+class UnicornJSONLogFormatter < Logger::Formatter
+  def call(severity, datetime, progname, message)
+    default = {
+      severity: severity,
+      datetime: datetime,
+      progname: progname,
+      pid: $$,
+    }
+
+    default[:message] =
+      if message.is_a?(Exception)
+        "#{message.message}: #{message.backtrace.join("\n")}"
+      else
+        message
+      end
+
+    "#{default.to_json}\n"
+  end
+end

spec/components/unicorn/unicorn_json_log_formatter_spec.rb

@@ -0,0 +1,31 @@
+require 'rails_helper'
+require 'unicorn/unicorn_json_log_formatter'
+
+RSpec.describe UnicornJSONLogFormatter do
+  context 'when message is an exception' do
+    it 'should include the backtrace' do
+      freeze_time do
+        begin
+          raise 'boom'
+        rescue => e
+          error = e
+
+          output = described_class.new.call(
+            'INFO',
+            Time.zone.now,
+            '',
+            e
+          )
+        end
+
+        output = JSON.parse(output)
+
+        expect(output["severity"]).to eq('INFO')
+        expect(output["datetime"]).to be_present
+        expect(output["progname"]).to eq('')
+        expect(output["pid"]).to be_present
+        expect(output["message"]).to match(/boom:.*/)
+      end
+    end
+  end
+end