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[PATCH v2 00/24] python: introduce Asynchronous QMP package

From: John Snow
Subject: [PATCH v2 00/24] python: introduce Asynchronous QMP package
Date: Fri, 16 Jul 2021 20:32:29 -0400

GitLab: https://gitlab.com/jsnow/qemu/-/commits/python-async-qmp-aqmp
CI: https://gitlab.com/jsnow/qemu/-/pipelines/338508045
Docs: https://people.redhat.com/~jsnow/sphinx/html/qemu.aqmp.html


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
- unit tests!
- documentation!

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:

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,

A third task, disconnect, is scheduled asynchronously whenever an
unrecoverable error occurs and facilitates coalescing of the other two

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

                       | 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.

Patch Layout

Patches 1-4   add tiny pre-requisites, utilities, etc.
Patches 5-12  add a generic async message-based protocol class,
              AsyncProtocol. They are split fairly small and should
              be reasonably self-contained.
Patches 13-15 check in more QMP-centric components.
Patches 16-21 add qmp_client.py, with a new 'QMPClient()' class.
              They're split into reasonably tiny pieces here.
Patches 22-23 add a few finishing touches, they are small patches.
Patch 24      adds unit tests. They're maybe a little messy yet, but
              they've been quite helpful to me so far. Coverage of
              protocol.py is at about 86%.

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. I have all of iotests
  passing locally, but I am still seeing some failures on gitlab CI I
  need to diagnose, possibly a race condition somewhere.

- A QMP server class; to facilitate writing of unit tests. It's done,
  but needs some polish and tests.

- More unit tests for qmp_client.py, qmp_server.py and other modules.



Renamed classes/methods:

- Renamed qmp_protocol.py to qmp_client.py
- Renamed 'QMP' class to 'QMPClient'
- Renamed _begin_new_session() to _establish_session()
- Split _establish_connection() out from _new_session().
- Removed _results() method


- Suppress duplicate Exceptions when attempting to drain the
- Delay initialization of asyncio.Queue and asyncio.Event variables to
  _new_session or later -- they must not be created outside of the loop,
  even if they are not async functions.
- Rework runstate_changed events to guarantee visibility of events to
- Improve connect()/accept() cleanup to work with
  asyncio.CancelledError, asyncio.TimeoutError
- No-argument form of Message() now succeeds properly.
- flush utility will correctly yield when data is below the "high water
  mark", giving the stream a chance to actually flush.
- Increase read buffer size to accommodate query-qmp-schema (Thanks

Ugly bits from V1 removed:

- Remove tertiary filtering from EventListener (for now), accompanying
  documentation removed from events.py
- Use asyncio.wait() instead of custom wait_task_done()
- MultiException is removed in favor of just raising the first Exception
  that occurs in the bottom half; other Exceptions if any are logged


- QMPClient now allows ID-less execution statements via the _raw()
- Add tests that grant ~86% coverage of protocol.py to the avocado test
- Removed 'force' parameter from _bh_disconnect; the disconnection
  routine determines for itself if we are in the error pathway or not
  instead now.  This removes any chance of duplicate calls to
  _schedule_disconnect accidentally dropping the 'force' setting.

Debugging/Testing changes:

- Add debug: bool parameter to asyncio_run utility wrapper
- Improve error messages for '@require' decorator
- Add debugging message for state change events
- Avoid flushing the StreamWriter if we don't have one (This
  circumstance only arises in testing, but it's helpful.)
- Improved __repr__ method for AsyncProtocol, and removed __str__
  method.  enforcing eval(__repr__(x)) == x does not make sense for
- Misc logging message changes
- Add a suite of fancy Task debugging utilities.
- Most tracebacks now log at the DEBUG level instead of
  CRITICAL/ERROR/WARNING; In those error cases, a one-line summary is
  logged instead.

Misc. aesthetic changes:

- Misc docstring fixes, whitespace, etc.
- Reordered the definition of some methods to try and keep similar
  methods near each other (Moved _cleanup near _bh_disconnect in

~ Shucks Howdy, Gee Golly!

John Snow (24):
  python/aqmp: add asynchronous QMP (AQMP) subpackage
  python/aqmp: add error classes
  python/pylint: Add exception for TypeVar names ('T')
  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 utility helpers
  python/aqmp: add logging to AsyncProtocol
  python/aqmp: add AsyncProtocol.accept() method
  python/aqmp: add configurable read buffer limit
  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/pylint: disable too-many-function-args
  python/aqmp: add QMP protocol support
  python/pylint: disable no-member check
  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/aqmp: add AsyncProtocol unit tests

 python/qemu/aqmp/__init__.py   |  58 +++
 python/qemu/aqmp/error.py      |  50 ++
 python/qemu/aqmp/events.py     | 706 ++++++++++++++++++++++++++
 python/qemu/aqmp/message.py    | 209 ++++++++
 python/qemu/aqmp/models.py     | 133 +++++
 python/qemu/aqmp/protocol.py   | 882 +++++++++++++++++++++++++++++++++
 python/qemu/aqmp/py.typed      |   0
 python/qemu/aqmp/qmp_client.py | 621 +++++++++++++++++++++++
 python/qemu/aqmp/util.py       | 207 ++++++++
 python/setup.cfg               |   5 +-
 python/tests/null_proto.py     |  67 +++
 python/tests/protocol.py       | 458 +++++++++++++++++
 12 files changed, 3395 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_client.py
 create mode 100644 python/qemu/aqmp/util.py
 create mode 100644 python/tests/null_proto.py
 create mode 100644 python/tests/protocol.py


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