Close open connections from parent after fork

DRb::DRbConn keeps a global pool of open connections which is shared by
child processes when they are forked from a parent.  If this parent
executes a DRb call prior to forking a child process the child picks up
this open connection and uses it which can cause replies from the server
to go to the wrong DRb client.

There is a long standing ruby bug https://bugs.ruby-lang.org/issues/2718
which describes the issue and has reproducer code attached.

Fixes https://bugzilla.redhat.com/show_bug.cgi?id=1385038

app/models/miq_worker.rb

@@ -314,6 +314,7 @@ def self.before_fork
   def self.after_fork
     close_pg_sockets_inherited_from_parent
     DRb.stop_service
+    close_drb_pool_connections
     renice(Process.pid)
   end
 
@@ -330,6 +331,23 @@ def self.close_pg_sockets_inherited_from_parent
     end
   end
 
+  # Close all open DRb connections so that connections in the parent's memory space
+  # which is shared due to forking the child process do not pollute the child's DRb
+  # connection pool.  This can lead to errors when the children connect to a server
+  # and get an incorrect response back.
+  #
+  # ref: https://bugs.ruby-lang.org/issues/2718
+  def self.close_drb_pool_connections
+    require 'drb'
+
+    # HACK: DRb doesn't provide an interface to close open pool connections.
+    #
+    # Once that is added this should be replaced.
+    DRb::DRbConn.instance_variable_get(:@mutex).synchronize do
+      DRb::DRbConn.instance_variable_get(:@pool).each(&:close)
+    end
+  end
+
   # Overriding queue_name as now some queue names can be
   # arrays of names for some workers not just a singular name.
   # We use JSON.parse as the array of names is stored as a string.