serial: Ensure close callback is always executed

This change fixes SerialIoHandler::Close() to ensure that the callback
is always executed even if there isn't a file handle to close. Failing
to do so will trigger a DCHECK in the Mojo bindings layer and could
cause clients of the Mojo interface to stall waiting for a response.

Bug: 884928
Change-Id: I272cc1987627232dc2143380071a07b7c58fb2cc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2354775
Auto-Submit: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Matt Reynolds <mattreynolds@chromium.org>
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#798115}

services/device/serial/serial_io_handler.cc

@@ -174,6 +174,8 @@ void SerialIoHandler::Close(base::OnceClosure callback) {
         {base::MayBlock(), base::TaskShutdownBehavior::CONTINUE_ON_SHUTDOWN},
         base::BindOnce(&SerialIoHandler::DoClose, std::move(file_)),
         std::move(callback));
+  } else {
+    std::move(callback).Run();
   }
 }
 

services/device/serial/serial_port_impl_unittest.cc

@@ -214,4 +214,14 @@ TEST_F(SerialPortImplTest, Drain) {
   loop.Run();
 }
 
+TEST_F(SerialPortImplTest, Close) {
+  mojo::Remote<mojom::SerialPort> serial_port;
+  mojo::SelfOwnedReceiverRef<mojom::SerialPortConnectionWatcher> watcher;
+  CreatePort(&serial_port, &watcher);
+
+  base::RunLoop loop;
+  serial_port->Close(loop.QuitClosure());
+  loop.Run();
+}
+
 }  // namespace device