[rubygems/rubygems] Synchronize access to the Gem::Specification::LOAD_CACHE Hash

* It's accessed concurrently, notably when installing a gem with a C extension.

https://github.com/rubygems/rubygems/commit/543294d7dd

lib/rubygems/specification.rb

@@ -109,6 +109,7 @@ class Gem::Specification < Gem::BasicSpecification
   # rubocop:disable Style/MutableConstant
   LOAD_CACHE = {} # :nodoc:
   # rubocop:enable Style/MutableConstant
+  LOAD_CACHE_MUTEX = Mutex.new
 
   private_constant :LOAD_CACHE if defined? private_constant
 
@@ -753,7 +754,9 @@ class Gem::Specification < Gem::BasicSpecification
   end
 
   def self._clear_load_cache # :nodoc:
-    LOAD_CACHE.clear
+    LOAD_CACHE_MUTEX.synchronize do
+      LOAD_CACHE.clear
+    end
   end
 
   def self.each_gemspec(dirs) # :nodoc:
@@ -1105,7 +1108,7 @@ class Gem::Specification < Gem::BasicSpecification
   def self.load(file)
     return unless file
 
-    _spec = LOAD_CACHE[file]
+    _spec = LOAD_CACHE_MUTEX.synchronize { LOAD_CACHE[file] }
     return _spec if _spec
 
     file = file.dup.untaint
@@ -1120,7 +1123,9 @@ class Gem::Specification < Gem::BasicSpecification
 
       if Gem::Specification === _spec
         _spec.loaded_from = File.expand_path file.to_s
-        LOAD_CACHE[file] = _spec
+        LOAD_CACHE_MUTEX.synchronize do
+          LOAD_CACHE[file] = _spec
+        end
         return _spec
       end