Fix handling of read_nonblock.

ext/io/event/selector/uring.c

@@ -707,6 +707,20 @@ VALUE IO_Event_Selector_URing_io_read(VALUE self, VALUE fiber, VALUE io, VALUE b
 	off_t from = io_seekable(descriptor);
 
 	size_t maximum_size = size - offset;
+
+	// Are we performing a non-blocking read?
+	if (!length) {
+		// If the (maximum) length is zero, that indicates we just want to read whatever is available without blocking.
+		// If we schedule this read into the URing, it will block until data is available, rather than returning immediately.
+		int state = IO_Event_Selector_nonblock_set(descriptor);
+
+		int result = read(descriptor, (char*)base+offset, maximum_size);
+		int error = errno;
+
+		IO_Event_Selector_nonblock_restore(descriptor, state);
+		return rb_fiber_scheduler_io_result(result, errno);
+	}
+
 	while (maximum_size) {
 		int result = io_read(selector, fiber, descriptor, (char*)base+offset, maximum_size, from);
 
@@ -1093,7 +1107,7 @@ unsigned select_process_completions(struct IO_Event_Selector_URing *selector) {
 		}
 	}
 
-	if (DEBUG && completed > 0) fprintf(stderr, "select_process_completions(completed=%d)\n", completed);
+	if (DEBUG && completed > 0) fprintf(stderr, "select_process_completions: completed=%d\n", completed);
 
 	return completed;
 }

test/io/event/selector/buffered_io.rb

@@ -77,6 +77,46 @@
 			writer.transfer
 			selector.select(0)
 		end
+
+		it "can perform non-blocking read" do
+			skip_if_ruby_platform(/mswin|mingw|cygwin/)
+
+			buffer = IO::Buffer.new(64)
+			result = nil
+
+			output.puts "Hello World\n"
+			output.close
+
+			reader = Fiber.new do
+				result = selector.io_read(Fiber.current, input, buffer, 0)
+			end
+
+			reader.transfer
+			selector.select(0)
+
+			expect(buffer.get_string(0, 12)).to be == "Hello World\n"
+		end
+
+		# Whether the given error code indicates that the operation should be retried.
+		def be_again?
+			(be == -Errno::EAGAIN::Errno).or(be == -Errno::EWOULDBLOCK::Errno)
+		end
+
+		it "can perform non-blocking read with empty input" do
+			skip_if_ruby_platform(/mswin|mingw|cygwin/)
+
+			buffer = IO::Buffer.new(64)
+			result = nil
+
+			reader = Fiber.new do
+				result = selector.io_read(Fiber.current, input, buffer, 0)
+			end
+
+			reader.transfer
+			selector.select(0)
+
+			expect(result).to be_again?
+		end
 	end
 end