Re: [PATCH v5 3/4] qmp: Move dispatcher to a coroutine

[Markus Armbruster <armbru@redhat.com>]

Kevin Wolf <kwolf@redhat.com> writes:

> This moves the QMP dispatcher to a coroutine and runs all QMP command
> handlers that declare 'coroutine': true in coroutine context so they
> can avoid blocking the main loop while doing I/O or waiting for other
> events.
>
> For commands that are not declared safe to run in a coroutine, the
> dispatcher drops out of coroutine context by calling the QMP command
> handler from a bottom half.
>
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>

Uh, what about @cur_mon?

@cur_mon points to the current monitor while a command executes.
Initial value is null.  It is set in three places (not counting unit
tests), and all three save, set, do something that may use @cur_mon,
restore:

* monitor_qmp_dispatch(), for use within qmp_dispatch()
* monitor_read(), for use within handle_hmp_command()
* qmp_human_monitor_command(), also for use within handle_hmp_command()

Therefore, @cur_mon is null unless we're running within qmp_dispatch()
or handle_hmp_command().

Example of use: error_report() & friends print "to current monitor if we
have one, else to stderr."  Makes sharing code between HMP and CLI
easier.  Uses @cur_mon under the hood.

@cur_mon is thread-local.

I'm afraid we have to save, clear and restore @cur_mon around a yield.