introduce SynchronizedArray to avoid race conditions on $LOADED_FEATURES and $LOAD_PATH

Author: Nicolas Laurent

src/main/java/org/truffleruby/core/CoreLibrary.java

@@ -978,6 +978,7 @@ public boolean isTruffleBootMainMethod(SharedMethodInfo info) {
             "/core/truffle/platform.rb",
             "/core/string.rb",
             "/core/random.rb",
+            "/core/truffle/synchronized_array.rb",
             "/core/truffle/kernel_operations.rb",
             "/core/truffle/exception_operations.rb",
             "/core/truffle/feature_loader.rb",

src/main/ruby/truffleruby/core/truffle/kernel_operations.rb

@@ -31,8 +31,8 @@ def self.define_read_only_global(name, getter)
       define_hooked_variable(name, getter, setter)
     end
 
-    LOAD_PATH = []
-    LOADED_FEATURES = []
+    LOAD_PATH = Truffle::SynchronizedArray.new
+    LOADED_FEATURES = Truffle::SynchronizedArray.new
 
     define_read_only_global(:$LOAD_PATH, -> { LOAD_PATH })
     define_read_only_global(:$LOADED_FEATURES, -> { LOADED_FEATURES })