On Thu, Jul 01, 2021 at 12:12:53AM -0400, John Snow wrote:
> GitLab: https://gitlab.com/jsnow/qemu/-/commits/python-async-qmp-aqmp
> CI: https://gitlab.com/jsnow/qemu/-/pipelines/330003554
> Docs: https://people.redhat.com/~jsnow/sphinx/html/qemu.aqmp.html
> Based-on: <20210701020921.1679468-1-jsnow@redhat.com>
> [PULL 00/15] Python patches
>
> Hi!
>
> This patch series adds an Asynchronous QMP package to the Python
> library. It offers a few improvements over the previous library:
>
> - out-of-band support
> - true asynchronous event support
> - avoids undocumented interfaces abusing non-blocking sockets
>
> This library serves as the basis for a new qmp-shell program that will
> offer improved reconnection support, true asynchronous display of
> events, VM and job status update notifiers, and so on.
>
> My intent is to eventually publish this library directly to PyPI as a
> standalone package. I would like to phase out our usage of the old QMP
> library over time; eventually replacing it entirely with this one.
>
> This series looks big by line count, but it's *mostly*
> docstrings. Seriously!
>
> This package has *no* external dependencies whatsoever.
>
> Notes & Design
> ==============
>
> Here are some notes on the design of how the library works, to serve as
> a primer for review; however I also **highly recommend** browsing the
> generated Sphinx documentation for this series.
>
> Here's that link again:
> https://people.redhat.com/~jsnow/sphinx/html/qemu.aqmp.html
>
> The core machinery is split between the AsyncProtocol and QMP
> classes. AsyncProtocol provides the generic machinery, while QMP
> provides the QMP-specific details.
>
> The design uses two independent coroutines that act as the "bottom
> half", a writer task and a reader task. These tasks run for the duration
> of the connection and independently send and receive messages,
> respectively.
>
> A third task, disconnect, is scheduled asynchronously whenever an
> unrecoverable error occurs and facilitates coalescing of the other two
> tasks.
>
> This diagram for how execute() operates may be helpful for understanding
> how AsyncProtocol is laid out. The arrows indicate the direction of a
> QMP message; the long horizontal dash indicates the separation between
> the upper and lower half of the event loop. The queue mechanisms between
> both dashes serve as the intermediaries between the upper and lower
> half.
>
> +---------+
> | caller |
> +---------+
> ^ |
> | v
> +---------+
> +---------------> |execute()| -----------+
> | +---------+ |
> | |
> [-----------------------------------------------------------]
> | |
> | v
> +----+------+ +----------------+ +------+-------+
> | ExecQueue | | EventListeners | |Outbound Queue|
> +----+------+ +----+-----------+ +------+-------+
> ^ ^ |
> | | |
> [-----------------------------------------------------------]
> | | |
> | | v
> +--+----------------+---+ +-----------+-----------+
> | Reader Task/Coroutine | | Writer Task/Coroutine |
> +-----------+-----------+ +-----------+-----------+
> ^ |
> | v
> +-----+------+ +-----+------+
> |StreamReader| |StreamWriter|
> +------------+ +------------+
>
> The caller will invoke execute(), which in turn will deposit a message
> in the outbound send queue. This will wake up the writer task, which
> well send the message over the wire.
>
> The execute() method will then yield to wait for a reply delivered to an
> execution queue created solely for that execute statement.
>
> When a message arrives, the Reader task will unblock and route the
> message either to the EventListener subsystem, or place it in the
> appropriate pending execution queue.
>
> Once a message is placed in the pending execution queue, execute() will
> unblock and the execution will conclude, returning the result of the RPC
> call to the caller.
>
> Ugly Bits
> =========
>
> - MultiException is ... wonky. I am still working out how to avoid needing it.
> See patch 04/20 for details here, or see
> https://people.redhat.com/~jsnow/sphinx/html/qemu.aqmp.error.html
>
> Patch 06/20 also goes into details of the ugliness; see
> AsyncProtocol._results or view the same information here:
> https://people.redhat.com/~jsnow/sphinx/html/_modules/qemu/aqmp/protocol.html#AsyncProtocol._results
>
> - There are quite a few lingering questions I have over the design of the
> EventListener subsystem; I wrote about those ugly bits in excruciating detail
> in patch 14/20.
>
> You can view them formatted nicely here:
> https://people.redhat.com/~jsnow/sphinx/html/qemu.aqmp.events.html#experimental-interfaces-design-issues
>
> Patch Layout
> ============
>
> Patches 1-2 are tiny pylint configuration changes.
> Patches 3-5 begin to check in Async QMP components, they are small.
> Patches 6-11 add a generic async message-based protocol class,
> AsyncProto. They are split as small as I could
> reasonably split them.
> Patches 12-14 check in more QMP-specific components.
> Patches 15-18 add qmp_protocol.py, the new 'QMP' class. They're split as
> far down as I could, I hope they're easy to review.
> Patches 19-20 add a few finishing touches, they are small patches.
>
> Future Work
> ===========
>
> These items are in progress:
>
> - A Synchronous QMP wrapper that allows this library to be easily used
> from non-async code; this will also allow me to prove it works well by
> demoing its replacement throughout iotests.
>
> - A QMP server class; to facilitate writing of unit tests.
>
> - Unit tests. Real unit tests.
>
> If you made it this far in the cover letter, congrats :)
>
> John Snow (20):
> python/pylint: Add exception for TypeVar names ('T')
> python/pylint: disable too-many-function-args
> python/aqmp: add asynchronous QMP (AQMP) subpackage
> python/aqmp: add error classes
> python/aqmp: add asyncio compatibility wrappers
> python/aqmp: add generic async message-based protocol support
> python/aqmp: add runstate state machine to AsyncProtocol
> python/aqmp: add logging to AsyncProtocol
> python/aqmp: add AsyncProtocol.accept() method
> python/aqmp: add _cb_inbound and _cb_inbound logging hooks
> python/aqmp: add AsyncProtocol._readline() method
> python/aqmp: add QMP Message format
> python/aqmp: add well-known QMP object models
> python/aqmp: add QMP event support
> python/aqmp: add QMP protocol support
> python/aqmp: Add message routing to QMP protocol
> python/aqmp: add execute() interfaces
> python/aqmp: add _raw() execution interface
> python/aqmp: add asyncio_run compatibility wrapper
> python/aqmp: add scary message
>
> python/qemu/aqmp/__init__.py | 61 +++
> python/qemu/aqmp/error.py | 97 ++++
> python/qemu/aqmp/events.py | 878 +++++++++++++++++++++++++++++++
> python/qemu/aqmp/message.py | 207 ++++++++
> python/qemu/aqmp/models.py | 133 +++++
> python/qemu/aqmp/protocol.py | 851 ++++++++++++++++++++++++++++++
> python/qemu/aqmp/py.typed | 0
> python/qemu/aqmp/qmp_protocol.py | 565 ++++++++++++++++++++
> python/qemu/aqmp/util.py | 183 +++++++
> python/setup.cfg | 4 +-
> 10 files changed, 2978 insertions(+), 1 deletion(-)
> create mode 100644 python/qemu/aqmp/__init__.py
> create mode 100644 python/qemu/aqmp/error.py
> create mode 100644 python/qemu/aqmp/events.py
> create mode 100644 python/qemu/aqmp/message.py
> create mode 100644 python/qemu/aqmp/models.py
> create mode 100644 python/qemu/aqmp/protocol.py
> create mode 100644 python/qemu/aqmp/py.typed
> create mode 100644 python/qemu/aqmp/qmp_protocol.py
> create mode 100644 python/qemu/aqmp/util.py
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Brave of you, thanks :)
I am working on a V2 presently, with a bunch of avocado unit tests, which has revealed a few small bugs and inconsistencies. This version also adds a mock QMPServer class alongside the client, for the purposes of testing. I think once those pieces are done I will be quite happy to check it into the tree as an "alpha" version, though I'd still be very happy to get more feedback on the EventListener piece from anyone willing to browse the "parts I don't like about this" section of the docs: