Better window update synchronization.

lib/async/http/protocol/http2/output.rb

@@ -17,7 +17,8 @@ def initialize(stream, body, trailer = nil)
 
 						@task = nil
 
-						@window_updated = Async::Condition.new
+						@guard = ::Mutex.new
+						@window_updated = ::ConditionVariable.new
 					end
 
 					attr :trailer
@@ -33,17 +34,26 @@ def start(parent: Task.current)
 					end
 
 					def window_updated(size)
-						@window_updated.signal
+						@guard.synchronize do
+							@window_updated.signal
+						end
 					end
 
 					def write(chunk)
 						until chunk.empty?
 							maximum_size = @stream.available_frame_size
 
-							while maximum_size <= 0
-								@window_updated.wait
-
-								maximum_size = @stream.available_frame_size
+							# We try to avoid synchronization if possible:
+							if maximum_size <= 0
+								@guard.synchronize do
+									maximum_size = @stream.available_frame_size
+
+									while maximum_size <= 0
+										@window_updated.wait(@guard)
+
+										maximum_size = @stream.available_frame_size
+									end
+								end
 							end
 
 							break unless chunk = send_data(chunk, maximum_size)