Do not remove closed streams immediately (#120)

Closed streams need to remain in the Connection @streams list so that
frames received for closed streams do not cause a protocol error.
They are cleaned up if closed for 15 seconds when another stream is
closed.

lib/http/2/connection.rb

@@ -656,15 +656,18 @@ def activate_stream(id: nil, **args)
       # permitted to open.
       stream.once(:active) { @active_stream_count += 1 }
       stream.once(:close) do
-        @streams.delete id
         @active_stream_count -= 1
 
         # Store a reference to the closed stream, such that we can respond
         # to any in-flight frames while close is registered on both sides.
         # References to such streams will be purged whenever another stream
         # is closed, with a minimum of 15s RTT time window.
-        @streams_recently_closed.delete_if { |_, v| (Time.now - v) > 15 }
         @streams_recently_closed[id] = Time.now
+        to_delete = @streams_recently_closed.select { |_, v| (Time.now - v) > 15 }
+        to_delete.each do |stream_id|
+          @streams.delete stream_id
+          @streams_recently_closed.delete stream_id
+        end
       end
 
       stream.on(:promise, &method(:promise)) if self.is_a? Server

spec/connection_spec.rb

@@ -543,6 +543,20 @@
       end.to raise_error(ProtocolError)
     end
 
+    it 'should not raise an error on frame for a closed stream ID' do
+      srv = Server.new
+      srv << CONNECTION_PREFACE_MAGIC
+
+      stream = srv.new_stream
+      stream.send HEADERS.dup
+      stream.send DATA.dup
+      stream.close
+
+      expect do
+        srv << f.generate(RST_STREAM.dup.merge(stream: stream.id))
+      end.to_not raise_error
+    end
+
     it 'should send GOAWAY frame on connection error' do
       stream = @conn.new_stream