Use a different BufferStack per thread

That way, we can ensure that the BufferStack and the Buffers themselves
including their nested fields can not be accessed by different threads,
providing some thread safety for non malicious users.

lib/rackstash/logger.rb

@@ -3,6 +3,7 @@
 # This software may be modified and distributed under the terms
 # of the MIT license. See the LICENSE.txt file for details.
 
+require 'concurrent'
 require 'forwardable'
 
 require 'rackstash/buffer_stack'
@@ -51,6 +52,8 @@ module Rackstash
       @level = DEBUG
       @progname = PROGNAME
       @formatter = Formatter.new
+
+      @buffer_stack = Concurrent::ThreadLocalVar.new
     end
 
     # Set the base log level as either one of the {SEVERITIES} or a
@@ -239,7 +242,7 @@ module Rackstash
     private
 
     def buffer_stack
-      @buffer_stack ||= Rackstash::BufferStack.new(@sink)
+      @buffer_stack.value ||= BufferStack.new(@sink)
     end
 
     def buffer

spec/rackstash/logger_spec.rb

@@ -321,4 +321,18 @@ describe Rackstash::Logger do
       expect(messages.last).to include message: 'Unknown', severity: 5
     end
   end
+
+  context 'with multiple threads' do
+    it 'maintains thread-local stacks' do
+      first_stack = logger.send(:buffer_stack)
+      expect(first_stack).to be_a Rackstash::BufferStack
+
+      Thread.new do
+        second_stack = logger.send(:buffer_stack)
+        expect(second_stack).to be_a Rackstash::BufferStack
+
+        expect(second_stack).to_not eql first_stack
+      end.join
+    end
+  end
 end