Fix a concurrency issue with the Webserver when overrideing Puma STDERR and STDOUT.

This PR changes how we modify the STDOUT/STDERR in puma, instande of
using `const_set`, we override the constants using a module.
This operation should be thread safe and will make sure the STDERR are
correctly visible.

After when we receive the logger we can swap the null logger with the
real thing in a thread safe way.

Fixes: #5912

Fixes #5918

logstash-core/lib/logstash/patches/puma.rb

@@ -0,0 +1,44 @@
+# encoding: utf-8
+#
+# Patch to replace the usage of STDERR and STDOUT
+# see: https://github.com/elastic/logstash/issues/5912
+module LogStash
+  class NullLogger
+    def self.debug(message)
+    end
+  end
+
+  # Puma uses by default the STDERR an the STDOUT for all his error
+  # handling, the server class accept custom a events object that can accept custom io object,
+  # so I just wrap the logger into an IO like object.
+  class IOWrappedLogger
+    def initialize(new_logger)
+      @logger_lock = Mutex.new
+      @logger = new_logger
+    end
+
+    def sync=(v)
+      # noop
+    end
+
+    def logger=(logger)
+      @logger_lock.synchronize { @logger = logger }
+    end
+
+    def puts(str)
+      # The logger only accept a str as the first argument
+      @logger_lock.synchronize { @logger.debug(str.to_s) }
+    end
+    alias_method :write, :puts
+    alias_method :<<, :puts
+  end
+
+end
+
+# Reopen the puma class to create a scoped STDERR and STDOUT
+# This operation is thread safe since its done at the class level
+# and force JRUBY to flush his classes cache.
+module Puma
+  STDERR = LogStash::IOWrappedLogger.new(LogStash::NullLogger)
+  STDOUT = LogStash::IOWrappedLogger.new(LogStash::NullLogger)
+end

logstash-core/lib/logstash/webserver.rb

@@ -2,7 +2,9 @@
 require "logstash/api/rack_app"
 require "puma"
 require "puma/server"
+require "logstash/patches/puma"
 require "concurrent"
+require "thread"
 
 module LogStash
   class WebServer
@@ -70,32 +72,13 @@ module LogStash
       @server.stop(true) if @server
     end
 
-    # Puma uses by default the STDERR an the STDOUT for all his error
-    # handling, the server class accept custom a events object that can accept custom io object,
-    # so I just wrap the logger into an IO like object.
-    class IOWrappedLogger
-      def initialize(logger)
-        @logger = logger
-      end
-
-      def sync=(v)
-      end
-
-      def puts(str)
-        # The logger only accept a str as the first argument
-        @logger.debug(str.to_s)
-      end
-      alias_method :write, :puts
-      alias_method :<<, :puts
-    end
-
     def start_webserver(port)
       # wrap any output that puma could generate into a wrapped logger
       # use the puma namespace to override STDERR, STDOUT in that scope.
-      io_wrapped_logger = IOWrappedLogger.new(@logger)
+      Puma::STDERR.logger = logger
+      Puma::STDOUT.logger = logger
 
-      ::Puma.const_set("STDERR", io_wrapped_logger) unless ::Puma.const_defined?("STDERR")
-      ::Puma.const_set("STDOUT", io_wrapped_logger) unless ::Puma.const_defined?("STDOUT")
+      io_wrapped_logger = LogStash::IOWrappedLogger.new(logger)
 
       app = LogStash::Api::RackApp.app(logger, agent, http_environment)
 

logstash-core/spec/logstash/webserver_spec.rb

@@ -126,7 +126,7 @@ describe LogStash::WebServer do
   end
 end
 
-describe LogStash::WebServer::IOWrappedLogger do
+describe LogStash::IOWrappedLogger do
   let(:logger)  { spy("logger") }
   let(:message) { "foobar" }