[PATCH v2 0/5] chardev: implement backend chardev multiplexing

[Roman Penyaev <r.peniaev@gmail.com>]

Mux is a character backend (host side) device, which multiplexes
multiple frontends with one backend device. The following is a
few lines from the QEMU manpage [1]:

  A multiplexer is a "1:N" device, and here the "1" end is your
  specified chardev backend, and the "N" end is the various parts
  of QEMU that can talk to a chardev.

But sadly multiple backends are not supported.

This work implements multiplexing capability of several backend
devices, which opens up an opportunity to use a single frontend
device on the guest, which can be manipulated from several
backend devices.

The motivation is the EVE project [2], where it would be very
convenient to have a virtio console frontend device on the guest that
can be controlled from multiple backend devices. The following is
an example of the QEMU command line:

   -chardev mux-be,id=mux0 \
   -chardev socket,path=/tmp/sock,server=on,wait=off,id=sock0,mux-be-id=mux0 \
   -chardev vc,id=vc0,mux-be-id=mux0 \
   -device virtconsole,chardev=mux0 \
   -vnc 0.0.0.0:0

Which creates 2 backend devices: text virtual console (`vc0`) and a
socket (`sock0`) connected to the single virtio hvc console with the
backend multiplexer (`mux0`) help. `vc0` renders text to an image,
which can be shared over the VNC protocol.  `sock0` is a socket
backend which provides biderectional communication to the virtio hvc
console.

New type of multiplexer `mux-be` actually is an alias for the same
`MuxChardev` struct, which uses same functions as for the original
`mux` type, but supports multiplexing N backends with 1 frontend.

Once QEMU starts VNC client and any TTY emulator can be used to
control a single hvc console, for example these two different
consoles should have similar input and output due the buffer
multiplexing:

   # VNC client
   vncviewer :0

   # TTY emulator
   socat unix-connect:/tmp/sock pty,link=/tmp/pty
   tio /tmp/pty

Difference to the previous version:

* Separate type for the backend multiplexer `mux-be`
* Handle EAGAIN on write to the backend device
* Support of watch of previously failed backend device
* Proper json support of the `mux-be-id` option
* Unit test for the `mux-be` multiplexer

[1] https://www.qemu.org/docs/master/system/qemu-manpage.html#hxtool-6
[2] https://github.com/lf-edge/eve

Roman Penyaev (5):
  chardev/char: introduce `mux-be-id=ID` option and _MUX_BE type
  chardev/char: rename `mux_cnt` to `fe_cnt` for the `MuxChardev`
  chardev/char-mux: implement backend chardev multiplexing
  tests/unit/test-char: add unit test for the `mux-be` multiplexer
  qemu-options.hx: describe multiplexing of several backend devices

 chardev/char-fe.c          |  14 ++-
 chardev/char-mux.c         | 212 +++++++++++++++++++++++++++++++-----
 chardev/char.c             |  57 ++++++++--
 chardev/chardev-internal.h |  33 +++++-
 include/chardev/char.h     |   1 +
 qapi/char.json             |   9 +-
 qemu-options.hx            |  46 +++++++-
 tests/unit/test-char.c     | 217 ++++++++++++++++++++++++++++++++++++-
 8 files changed, 538 insertions(+), 51 deletions(-)

Signed-off-by: Roman Penyaev <r.peniaev@gmail.com>
Cc: "Marc-André Lureau" <marcandre.lureau@redhat.com>
Cc: qemu-devel@nongnu.org

-- 
2.43.0