Console empty string bug

[alexandruCarp]

Fixes #31 .
As suggested, I tried to add a check for command's return:

if let Err(err) = S::command(DRIVER_NUM, command::WRITE, s.len() as u32, 0)
                .to_result::<(), ErrorCode>()
            {
                return Err(err);
            }

But that didn't solve the problem, and from the console capsule's code it seems like that command will never return an error:

1 => {
                        // putstr
                        let len = arg1;
                        self.send_new(appid, app, kernel_data, len)
                    }

fn send_new(
        &self,
        app_id: ProcessId,
        app: &mut App,
        kernel_data: &GrantKernelData,
        len: usize,
    ) -> Result<(), ErrorCode> {
        app.write_len = kernel_data
            .get_readonly_processbuffer(ro_allow::WRITE)
            .map_or(0, |write| write.len())
            .min(len);
        app.write_remaining = app.write_len;
        self.send(app_id, app, kernel_data);
        Ok(())
    }

I've also added a guard clause in the write function of the Console API:

        if s.len() == 0 {
            return Ok(());
        }

This solves the problem, and it seems right for the write function to do nothing at all if there isn't anything to be written.

[alexandruCarp]

I found this note in the documentation of WRITE command:

Additional notes: A process may call this command with a write size of 0 to cancel a write transaction, if one is ongoing. Unless an error occurs, this will generate a write transaction completed event, regardless of whether or not a write transaction was already in progress.

However, it does not return an error and it does not generate an event (the process wouldn't remain yielded otherwise). Am I missing something?

[alexandruCarp]

Posted issue tock/tock#3433.