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

[Marc-André Lureau]

Hi

On Tue, Jul 26, 2022 at 12:44 PM Daniel P. Berrangé <berrange@redhat.com> wrote:
>
> 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.

The current "-chardev pipe,id=id,path=path" option handling will first
check the presence of unidirectional "path.in" & "path.out" (although
they are opened RDWR...), and fallback on bidirectional "path".

We could allow for the presence of "path.out" alone, although this may
be a behaviour/breaking change:

diff --git a/chardev/char-pipe.c b/chardev/char-pipe.c
index 7db963035e..f78bcd7daf 100644
--- a/chardev/char-pipe.c
+++ b/chardev/char-pipe.c
@@ -137,12 +137,12 @@ static void qemu_chr_open_pipe(Chardev *chr,
     g_free(filename_in);
     g_free(filename_out);
     if (fd_in < 0 || fd_out < 0) {
+        if (fd_out >= 0) {
+            goto out;
+        }
         if (fd_in >= 0) {
             close(fd_in);
         }
-        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);
@@ -152,6 +152,7 @@ static void qemu_chr_open_pipe(Chardev *chr,
             return;
         }
     }
+out:
     qemu_chr_open_fd(chr, fd_in, fd_out);


or we introduce a new "access=write" option, or a new chardev type ?

>
> >
> > 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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>