Re: [PATCH] char: fix logging when chardev write fails

["Marc-André Lureau" <marcandre.lureau@redhat.com>]

On Mon, Sep 14, 2020 at 2:31 PM Daniel P. Berrangé <berrange@redhat.com> wrote:
>
> The qemu_chr_write_buffer() method sends data to the chardev backend for
> writing, and then also writes to the log file. In case the chardev
> backend only writes part of the data buffer, we need to make sure we
> only log the same subset. qemu_chr_write_buffer() will be invoked again
> later to write the rest of the buffer.
>
> In the case the chardev backend returns an error though, no further
> attempts to likely to be made to write the data. We must therefore write
> the entire buffer to the log immediately.
>
> An example where this is important is with the socket backend. This will
> return -1 for all writes if no client is currently connected. We still
> wish to write data to the log file when no client is present though.
> This used to work because the chardev would return "len" to pretend it
> had written all data when no client is connected, but this changed to
> return an error in
>
>   commit 271094474b65de1ad7aaf729938de3d9b9d0d36f
>   Author: Dima Stepanov <dimastep@yandex-team.ru>
>   Date: Thu May 28 12:11:18 2020 +0300
>
>     char-socket: return -1 in case of disconnect during tcp_chr_write
>
> and this broke the logging, resulting in all data being discarded when
> no client is present.
>
> Fixes: https://bugs.launchpad.net/qemu/+bug/1893691
> Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>

> ---
>  chardev/char.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
>
> diff --git a/chardev/char.c b/chardev/char.c
> index 77e7ec814f..e30cd025f5 100644
> --- a/chardev/char.c
> +++ b/chardev/char.c
> @@ -138,7 +138,20 @@ static int qemu_chr_write_buffer(Chardev *s,
>          }
>      }
>      if (*offset > 0) {
> +        /*
> +         * If some data was written by backend, we should
> +         * only log what was actually written. This method
> +         * may be invoked again to write the remaining
> +         * method, thus we'll log the remainder at that time.
> +         */
>          qemu_chr_write_log(s, buf, *offset);
> +    } else if (res < 0) {
> +        /*
> +         * If a fatal error was reported by the backend,
> +         * assume this method won't be invoked again with
> +         * this buffer, so log it all right away.
> +         */
> +        qemu_chr_write_log(s, buf, len);
>      }
>      qemu_mutex_unlock(&s->chr_write_lock);
>
> --
> 2.26.2
>