[PATCH v4 0/8] 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, namely VNC and local
TTY emulator. 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.

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

v3 .. v4:

* Rebase on latest chardev changes
* Add unit tests which test corner cases:
   * Inability to remove mux with active frontend
   * Inability to add more chardevs to a mux than `MUX_MAX`
   * Inability to mix mux-fe and mux-be for the same chardev

v2 .. v3:

* Split frontend and backend multiplexer implementations and
  move them to separate files: char-mux-fe.c and char-mux-be.c

v1 .. v2:

* 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 (8):
  chardev/char: rename `MuxChardev` struct to `MuxFeChardev`
  chardev/char: rename `char-mux.c` to `char-mux-fe.c`
  chardev/char: move away mux suspend/resume calls
  chardev/char: rename frontend mux calls
  chardev/char: introduce `mux-be-id=ID` option
  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                     |  25 ++-
 chardev/char-mux-be.c                 | 290 ++++++++++++++++++++++++
 chardev/{char-mux.c => char-mux-fe.c} | 157 +++++--------
 chardev/char.c                        | 133 +++++++++--
 chardev/chardev-internal.h            |  55 ++++-
 chardev/meson.build                   |   3 +-
 include/chardev/char.h                |   8 +-
 qapi/char.json                        |  31 ++-
 qemu-options.hx                       |  78 +++++--
 system/vl.c                           |   4 +-
 tests/unit/test-char.c                | 306 +++++++++++++++++++++++++-
 11 files changed, 922 insertions(+), 168 deletions(-)
 create mode 100644 chardev/char-mux-be.c
 rename chardev/{char-mux.c => char-mux-fe.c} (71%)

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

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2.34.1