Hi,
HMP and QMP commands are handled synchronously in qemu today. But
there are benefits allowing the command handler to re-enter the main
loop if the command cannot be handled synchronously, or if it is
long-lasting. Some bugs such as rhbz#1230527 are difficult to solve
without it.
The common solution is to use a pair of command+event in this case.
But this approach has a number of issues:
- you can't "fix" an existing command: you need a new API, and ad-hoc
documentation for that command+signal association, and old/broken
command deprecation
- since the reply event is broadcasted and 'id' is used for matching the
request, it may conflict with other clients request 'id' space
- it is arguably less efficient and elegant (weird API, useless return
in most cases, broadcast reply, no cancelling on disconnect etc)
The following series implements an async command solution instead. By
introducing a session context and a command return handler, it can:
- defer the return, allowing the mainloop to reenter
- return only to the caller (instead of broadcast events for reply)
- optionnally allow cancellation when the client is gone
- track on-going qapi command(s) per client/session
and without introduction of new QMP APIs or client visible change.
Existing qemu commands can be gradually replaced by async:true
variants when needed, while carefully reviewing the concurrency
aspects. The async:true commands marshaller helpers are splitted in
half, the calling and return functions. The command is called with a
QmpReturn context, that can return immediately or later, using the
generated return helper, which allows for a step-by-step conversion.
The screendump command is converted to an async:true version to solve
rhbz#1230527. The command shows basic cancellation (this could be
extended if needed). It could be further improved to do asynchronous
IO writes as well.
v5:
- rebased
v4:
- rebased, mostly adapting to new OOB code
(there was not much feedback in v3 for the async command part,
but preliminary patches got merged!)
- drop the RFC status
v3:
- complete rework, dropping the asynchronous commands visibility from
the protocol side entirely (until there is a real need for it)
- rebased, with a few preliminary cleanup patches
- teach asynchronous commands to HMP
v2:
- documentation fixes and improvements
- fix calling async commands sync without id
- fix bad hmp monitor assert
- add a few extra asserts
- add async with no-id failure and screendump test
Marc-André Lureau (20):
qmp: constify QmpCommand and list
json-lexer: make it safe to call destroy multiple times
qmp: add QmpSession
QmpSession: add a return callback
QmpSession: add json parser and use it in qga
monitor: use qmp session to parse json feed
qga: simplify dispatch_return_cb
QmpSession: introduce QmpReturn
qmp: simplify qmp_return_error()
QmpSession: keep a queue of pending commands
QmpSession: return orderly
qmp: introduce asynchronous command type
scripts: learn 'async' qapi commands
qmp: add qmp_return_is_cancelled()
monitor: add qmp_return_get_monitor()
console: add graphic_hw_update_done()
console: make screendump asynchronous
monitor: start making qmp_human_monitor_command() asynchronous
monitor: teach HMP about asynchronous commands
hmp: call the asynchronous QMP screendump to fix outdated/glitches
hmp-commands.hx | 3 +-
hw/display/qxl-render.c | 9 +-
hw/display/qxl.c | 1 +
include/monitor/monitor.h | 3 +
include/qapi/qmp/dispatch.h | 89 +++++++++-
include/qapi/qmp/json-parser.h | 7 +-
include/ui/console.h | 4 +
monitor/hmp-cmds.c | 6 +-
monitor/hmp.c | 110 +++++++++++-
monitor/misc.c | 46 +----
monitor/monitor-internal.h | 12 +-
monitor/monitor.c | 2 +-
monitor/qmp.c | 79 ++++-----
qapi/misc.json | 3 +-
qapi/qmp-dispatch.c | 214 +++++++++++++++++++-----
qapi/qmp-registry.c | 33 +++-
qapi/ui.json | 3 +-
qga/commands.c | 2 +-
qga/main.c | 51 ++----
qobject/json-lexer.c | 5 +-
qobject/json-streamer.c | 3 +-
scripts/qapi/commands.py | 151 ++++++++++++++---
scripts/qapi/common.py | 15 +-
scripts/qapi/doc.py | 3 +-
scripts/qapi/introspect.py | 3 +-
tests/qapi-schema/qapi-schema-test.json | 5 +
tests/qapi-schema/qapi-schema-test.out | 8 +
tests/qapi-schema/test-qapi.py | 8 +-
tests/test-qmp-cmds.c | 206 +++++++++++++++++++----
ui/console.c | 100 +++++++++--
30 files changed, 908 insertions(+), 276 deletions(-)
--
2.22.0.428.g6d5b264208