Re: [PATCH v2] char-pty: add support for the terminal size

[Maximilian Immanuel Brandtner <maxbr@linux.ibm.com>]

On Wed, 2025-09-17 at 14:31 +0100, Daniel P. Berrangé wrote:
> On Wed, Sep 17, 2025 at 03:09:50PM +0200, Filip Hejsek wrote:
> > 
> > 
> > On September 17, 2025 11:39:55 AM GMT+02:00, Maximilian Immanuel
> > Brandtner <maxbr@linux.ibm.com> wrote:
> > > On Tue, 2025-09-16 at 00:02 +0200, Filip Hejsek wrote:
> > > > On Mon, 2025-09-15 at 18:34 +0200, Maximilian Immanuel
> > > > Brandtner
> > > > wrote:
> > > > > Update the terminal size upon SIGWINCH delivery.
> > > > > 
> > > > > Signed-off-by: Maximilian Immanuel Brandtner
> > > > > <maxbr@linux.ibm.com>
> > > > 
> > > > I don't think this will work, because SIGWINCH is only
> > > > delivered for
> > > > the process' controling terminal. Unfortunately I don't think
> > > > there
> > > > is
> > > > any way to get size notifications for arbitrary terminal.
> > > 
> > > In that case there are two solutions:
> > > 1. make qemu the controlling process of the pty (this has the
> > > disadvantage of QEMU being quit when the pty is closed)
> > 
> > A bigger disadvantage is that a process can only have one
> > controlling terminal, and a terminal can only be the controlling
> > terminal for a single session (and only sends signals to the
> > foreground process group of that session). It would require forking
> > a process for each pty, and I don't even know if the master end can
> > have its own session.
> > 
> > > 2. create a timer polling every eg 100ms to check if the winsize
> > > has
> > > changed
> > > 
> > > I would go with the second approach then
> > 
> > Me too, the timer is a bit unfortunate, but it's probably the less
> > bad option.
> 
> I don't think we want a timer polling for an situation that will very
> rarely arise.  We already add the 'chardev_resize' QMP command, which
> is
> a good enough way to kick QEMU to re-read the size.
> 
> With regards,
> Daniel

This approach would only work with libvirt and not generic pty
applications though. Perhaps a bool poll_resize could be added to the
struct Chardev which is disabled, as soon as the chardev_resize QMP
command is used to avoid race conditions.