Make GTG::TransTable thread safe.

From now on only the `[]=` method is allowed to modify the internal states hashes.

Author: thedarkone

actionpack/lib/action_dispatch/journey/gtg/transition_table.rb

@@ -9,8 +9,8 @@ class TransitionTable # :nodoc:
         attr_reader :memos
 
         def initialize
-          @regexp_states = Hash.new { |h,k| h[k] = {} }
-          @string_states = Hash.new { |h,k| h[k] = {} }
+          @regexp_states = {}
+          @string_states = {}
           @accepting     = {}
           @memos         = Hash.new { |h,k| h[k] = [] }
         end
@@ -111,14 +111,8 @@ def visualizer(paths, title = 'FSM')
         end
 
         def []=(from, to, sym)
-          case sym
-          when String
-            @string_states[from][sym] = to
-          when Regexp
-            @regexp_states[from][sym] = to
-          else
-            raise ArgumentError, 'unknown symbol: %s' % sym.class
-          end
+          to_mappings = states_hash_for(sym)[from] ||= {}
+          to_mappings[sym] = to
         end
 
         def states
@@ -137,18 +131,35 @@ def transitions
 
         private
 
+          def states_hash_for(sym)
+            case sym
+            when String
+              @string_states
+            when Regexp
+              @regexp_states
+            else
+              raise ArgumentError, 'unknown symbol: %s' % sym.class
+            end
+          end
+
           def move_regexp(t, a)
             return [] if t.empty?
 
             t.map { |s|
-              @regexp_states[s].map { |re, v| re === a ? v : nil }
+              if states = @regexp_states[s]
+                states.map { |re, v| re === a ? v : nil }
+              end
             }.flatten.compact.uniq
           end
 
           def move_string(t, a)
             return [] if t.empty?
 
-            t.map { |s| @string_states[s][a] }.compact
+            t.map do |s|
+              if states = @string_states[s]
+                states[a]
+              end
+            end.compact
           end
       end
     end