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