Reset Async queue on fork

lib/concurrent-ruby/concurrent/async.rb

@@ -309,6 +309,7 @@ def initialize(delegate)
         @delegate = delegate
         @queue = []
         @executor = Concurrent.global_io_executor
+        @ruby_pid = $$
       end
 
       # Delegates method calls to the wrapped object.
@@ -326,6 +327,7 @@ def method_missing(method, *args, &block)
 
         ivar = Concurrent::IVar.new
         synchronize do
+          reset_if_forked
           @queue.push [ivar, method, args, block]
           @executor.post { perform } if @queue.length == 1
         end
@@ -361,6 +363,13 @@ def perform
           end
         end
       end
+
+      def reset_if_forked
+        if $$ != @ruby_pid
+          @queue.clear
+          @ruby_pid = $$
+        end
+      end
     end
     private_constant :AsyncDelegator
 

spec/concurrent/async_spec.rb

@@ -296,5 +296,18 @@ def gather(seconds, first, *rest)
         expect(object.bucket).to eq [:a, :b, :c, :d]
       end
     end
+
+    context 'fork safety' do
+      it 'does not hang when forked' do
+        skip "Platform does not support fork" unless Process.respond_to?(:fork)
+        object = Class.new {
+          include Concurrent::Async
+          def foo; end
+        }.new
+        object.async.foo
+        _, status = Process.waitpid2(fork {object.await.foo})
+        expect(status.exitstatus).to eq 0
+      end
+    end
   end
 end