Synchronize access to the Gem::Specification::LOAD_CACHE Hash

* It's accessed concurrently, notably when installing a C extension such as:
  $ jt ruby --vm.ea --vm.esa -S gem install oily_png

Author: eregon

CHANGELOG.md

@@ -16,6 +16,7 @@ Bug fixes:
 * Procs will now yield to the block in their declaration context even when called with a block argument (#1657).
 * Fixed problems with calling POSIX methods if `Symbol#[]` is redefined (#1665).
 * Fixed sharing of `Array` and `Hash` elements for thread-safety of objects (#1601).
+* Fixed concurrent modifications of `Gem::Specification::LOAD_CACHE` (#1601).
 * Fix `TCPServer#accept` to set `#do_not_reverse_lookup` correctly on the created `TCPSocket`.
 
 Compatibility

lib/mri/rubygems/specification.rb

@@ -111,6 +111,7 @@ 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
 
@@ -759,7 +760,9 @@ def self._all # :nodoc:
   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:
@@ -1154,7 +1157,7 @@ def self._latest_specs(specs, prerelease = false) # :nodoc:
   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
@@ -1169,7 +1172,9 @@ def self.load(file)
 
       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