[PATCH v2 04/24] python/aqmp: add asyncio compatibility wrappers

[John Snow <jsnow@redhat.com>]

Python 3.6 does not have all of the goodies that Python 3.7 does, and I
need to support both. Add some compatibility wrappers needed for this
purpose.

(Note: Python 3.6 is EOL December 2021.)

Signed-off-by: John Snow <jsnow@redhat.com>
---
 python/qemu/aqmp/util.py | 106 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 106 insertions(+)
 create mode 100644 python/qemu/aqmp/util.py

diff --git a/python/qemu/aqmp/util.py b/python/qemu/aqmp/util.py
new file mode 100644
index 00000000000..7e7c2584d2b
--- /dev/null
+++ b/python/qemu/aqmp/util.py
@@ -0,0 +1,106 @@
+"""
+Miscellaneous Utilities
+
+This module provides asyncio utilities and compatibility wrappers for
+Python 3.6 to provide some features that otherwise become available in
+Python 3.7+.
+"""
+
+import asyncio
+import sys
+from typing import (
+    Any,
+    Coroutine,
+    Optional,
+    TypeVar,
+    cast,
+)
+
+
+T = TypeVar('T')
+
+
+# --------------------------
+# Section: Utility Functions
+# --------------------------
+
+
+async def flush(writer: asyncio.StreamWriter) -> None:
+    """
+    Utility function to ensure a StreamWriter is *fully* drained.
+
+    `asyncio.StreamWriter.drain` only promises we will return to below
+    the "high-water mark". This function ensures we flush the entire
+    buffer -- by setting the high water mark to 0 and then calling
+    drain. The flow control limits are restored after the call is
+    completed.
+    """
+    transport = cast(asyncio.WriteTransport, writer.transport)
+
+    # https://github.com/python/typeshed/issues/5779
+    low, high = transport.get_write_buffer_limits()  # type: ignore
+    transport.set_write_buffer_limits(0, 0)
+    try:
+        await writer.drain()
+    finally:
+        transport.set_write_buffer_limits(high, low)
+
+
+# -------------------------------
+# Section: Compatibility Wrappers
+# -------------------------------
+
+
+def create_task(coro: Coroutine[Any, Any, T],
+                loop: Optional[asyncio.AbstractEventLoop] = None
+                ) -> 'asyncio.Future[T]':
+    """
+    Python 3.6-compatible `asyncio.create_task` wrapper.
+
+    :param coro: The coroutine to execute in a task.
+    :param loop: Optionally, the loop to create the task in.
+
+    :return: An `asyncio.Future` object.
+    """
+    if sys.version_info >= (3, 7):
+        if loop is not None:
+            return loop.create_task(coro)
+        return asyncio.create_task(coro)  # pylint: disable=no-member
+
+    # Python 3.6:
+    return asyncio.ensure_future(coro, loop=loop)
+
+
+def is_closing(writer: asyncio.StreamWriter) -> bool:
+    """
+    Python 3.6-compatible `asyncio.StreamWriter.is_closing` wrapper.
+
+    :param writer: The `asyncio.StreamWriter` object.
+    :return: `True` if the writer is closing, or closed.
+    """
+    if sys.version_info >= (3, 7):
+        return writer.is_closing()
+
+    # Python 3.6:
+    transport = writer.transport
+    assert isinstance(transport, asyncio.WriteTransport)
+    return transport.is_closing()
+
+
+async def wait_closed(writer: asyncio.StreamWriter) -> None:
+    """
+    Python 3.6-compatible `asyncio.StreamWriter.wait_closed` wrapper.
+
+    :param writer: The `asyncio.StreamWriter` to wait on.
+    """
+    if sys.version_info >= (3, 7):
+        await writer.wait_closed()
+        return
+
+    # Python 3.6
+    transport = writer.transport
+    assert isinstance(transport, asyncio.WriteTransport)
+
+    while not transport.is_closing():
+        await asyncio.sleep(0)
+    await flush(writer)
-- 
2.31.1