Re: [PATCH] RFC: char: deprecate usage of bidirectional pipe

["Daniel P. Berrangé" <berrange@redhat.com>]

On Tue, Jul 26, 2022 at 12:32:32PM +0400, marcandre.lureau@redhat.com wrote:
> From: Marc-André Lureau <marcandre.lureau@redhat.com>
> 
> As Ed Swierk explained back in 2006:
> https://lists.nongnu.org/archive/html/qemu-devel/2006-12/msg00160.html
> 
> "When qemu writes into the pipe, it immediately reads back what it just
> wrote and treats it as a monitor command, endlessly breathing its own
> exhaust."
> 
> This is similarly confusing when using the chardev with a serial device,
> as reported in https://bugzilla.redhat.com/show_bug.cgi?id=2106975.
> 
> It seems we have kept the support for bidirectional pipes for historical
> reasons and odd systems, however it's not documented in qemu -chardev
> options. I suggest to stop supporting it, for portability reasons.

Hmm, I always assumed that in this scenario the pipe was operating
in output-only mode. Obviously not the case with the code as it
exists, but perhaps this would be useful ?  eg its good as a serial
console logging mechanism at least.

> 
> Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
> ---
>  docs/about/deprecated.rst | 6 ++++++
>  chardev/char-pipe.c       | 4 ++++
>  2 files changed, 10 insertions(+)
> 
> diff --git a/docs/about/deprecated.rst b/docs/about/deprecated.rst
> index 7ee26626d5cf..dd5ca30d527b 100644
> --- a/docs/about/deprecated.rst
> +++ b/docs/about/deprecated.rst
> @@ -45,6 +45,12 @@ transmit audio through the VNC protocol.
>  ``tty`` and ``parport`` are aliases that will be removed. Instead, the
>  actual backend names ``serial`` and ``parallel`` should be used.
>  
> +``-chardev pipe`` support for bidirectional pipes (since 7.1)
> +''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
> +
> +For portability reasons, the support for bidirectional ``pipe`` will
> +be removed. Instead, use ``file.in`` & ``file.out`` unidirectional pipes.
> +
>  Short-form boolean options (since 6.0)
>  ''''''''''''''''''''''''''''''''''''''
>  
> diff --git a/chardev/char-pipe.c b/chardev/char-pipe.c
> index 66d3b8509183..7db963035e7d 100644
> --- a/chardev/char-pipe.c
> +++ b/chardev/char-pipe.c
> @@ -27,6 +27,7 @@
>  #include "qemu/main-loop.h"
>  #include "qemu/module.h"
>  #include "qemu/option.h"
> +#include "qemu/error-report.h"
>  #include "chardev/char.h"
>  
>  #ifdef _WIN32
> @@ -142,6 +143,9 @@ static void qemu_chr_open_pipe(Chardev *chr,
>          if (fd_out >= 0) {
>              close(fd_out);
>          }
> +        warn_report("Support for bidirectional pipe is deprecated,");
> +        warn_report("please use portable one-way pipes instead (%s.in & %s.out).",
> +                    filename, filename);
>          TFR(fd_in = fd_out = qemu_open_old(filename, O_RDWR | O_BINARY));
>          if (fd_in < 0) {
>              error_setg_file_open(errp, errno, filename);
> -- 
> 2.37.0.rc0
> 

With regards,
Daniel
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