Build: avoid leaking memory from generate_plugins_version (#12763) (#12773)

(cherry picked from commit cc615da)

Author: kares

rakelib/plugins_docs_dependencies.rake

@@ -126,6 +126,7 @@ class PluginVersionWorking
   end
 
   def try_plugin(plugin, successful_dependencies)
+    Bundler::DepProxy.__clear!
     builder = Bundler::Dsl.new
     gemfile = LogStash::Gemfile.new(File.new(LogStash::Environment::GEMFILE_PATH, "r+")).load
     gemfile.update(plugin)
@@ -203,6 +204,29 @@ task :generate_plugins_version do
         end
       end
     end
+    DepProxy.class_eval do
+      # Bundler caches it's dep-proxy objects (which contain Gem::Dependency objects) from all resolutions.
+      # The Hash itself continues to grow between dependency resolutions and hold up a lot of memory, to avoid
+      # the issue we expose a way of clear-ing the cached objects before each plugin resolution.
+      def self.__clear!
+        @proxies.clear
+      end
+    end
+
+    Fetcher::CompactIndex.class_eval do
+      alias_method :__bundle_worker, :bundle_worker
+      # The compact index is built using multiple threads and this is hard-coded atm to 25 threads:
+      #   `Bundler::Worker.new(Bundler.current_ruby.rbx? ? 1 : 25, worker_name, func)`
+      # each thread might built up a Bundler::Source::Rubygems object which retains more than 100MB.
+      # By limiting the worker thread count we make sure not to produce too many of these objects.
+      def bundle_worker(func = nil)
+        __bundle_worker(func).tap do |worker|
+          size = worker.instance_variable_get(:@size)
+          fail("@size = #{size.inspect} is no longer an integer") unless size.is_a?(Integer)
+          worker.instance_variable_set(:@size, 2)
+        end
+      end
+    end
   end
 
   PluginVersionWorking.new.generate