[PATCH v9 0/8] Introduce 'yank' oob qmp command to recover from hanging qemu

[Lukas Straub <lukasstraub2@web.de>]

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Hello Everyone,
I finally found time again to work on this, so here is v9 with the new qmp api.
We still need ACKs from NBD and chardev maintainers.

Changes:

v9:
 -rebase onto master
 -implemented new qmp api as proposed by Markus

v8:
 -add Reviewed-by and Acked-by tags
 -rebase onto master
  -minor change to migration
  -convert to meson
 -change "Since:" to 5.2
 -varios code style fixes (Markus Armbruster)
 -point to oob restrictions in comment to yank_register_function
  (Markus Armbruster)
 -improve qmp documentation (Markus Armbruster)
 -document oob suitability of qio_channel and io_shutdown (Markus Armbruster)

v7:
 -yank_register_instance now returns error via Error **errp instead of aborting
 -dropped "chardev/char.c: Check for duplicate id before  creating chardev"

v6:
 -add Reviewed-by and Acked-by tags
 -rebase on master
 -lots of changes in nbd due to rebase
 -only take maintainership of util/yank.c and include/qemu/yank.h (Daniel P. Berrangé)
 -fix a crash discovered by the newly added chardev test
 -fix the test itself

v5:
 -move yank.c to util/
 -move yank.h to include/qemu/
 -add license to yank.h
 -use const char*
 -nbd: use atomic_store_release and atomic_load_aqcuire
 -io-channel: ensure thread-safety and document it
 -add myself as maintainer for yank

v4:
 -fix build errors...

v3:
 -don't touch softmmu/vl.c, use __contructor__ attribute instead (Paolo Bonzini)
 -fix build errors
 -rewrite migration patch so it actually passes all tests

v2:
 -don't touch io/ code anymore
 -always register yank functions
 -'yank' now takes a list of instances to yank
 -'query-yank' returns a list of yankable instances

Overview:
Hello Everyone,
In many cases, if qemu has a network connection (qmp, migration, chardev, etc.)
to some other server and that server dies or hangs, qemu hangs too.
These patches introduce the new 'yank' out-of-band qmp command to recover from
these kinds of hangs. The different subsystems register callbacks which get
executed with the yank command. For example the callback can shutdown() a
socket. This is intended for the colo use-case, but it can be used for other
things too of course.

Regards,
Lukas Straub

Lukas Straub (8):
  Introduce yank feature
  block/nbd.c: Add yank feature
  chardev/char-socket.c: Add yank feature
  migration: Add yank feature
  io/channel-tls.c: make qio_channel_tls_shutdown thread-safe
  io: Document qmp oob suitability of qio_channel_shutdown and
    io_shutdown
  MAINTAINERS: Add myself as maintainer for yank feature
  tests/test-char.c: Wait for the chardev to connect in
    char_socket_client_dupid_test

 MAINTAINERS                   |   6 +
 block/nbd.c                   | 154 ++++++++++++++----------
 chardev/char-socket.c         |  35 ++++++
 include/io/channel.h          |   5 +-
 include/qemu/yank.h           |  95 +++++++++++++++
 io/channel-tls.c              |   6 +-
 migration/channel.c           |  13 +++
 migration/migration.c         |  25 ++++
 migration/multifd.c           |  10 ++
 migration/qemu-file-channel.c |   7 ++
 migration/savevm.c            |   6 +
 qapi/misc.json                | 106 +++++++++++++++++
 tests/test-char.c             |   1 +
 util/meson.build              |   1 +
 util/yank.c                   | 213 ++++++++++++++++++++++++++++++++++
 15 files changed, 619 insertions(+), 64 deletions(-)
 create mode 100644 include/qemu/yank.h
 create mode 100644 util/yank.c

--
2.20.1

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