tools/net/ynl: improve async notification handling

The notification handling in ynl is currently very simple, using sleep()
to wait a period of time and then handling all the buffered messages in
a single batch.

This patch changes the notification handling so that messages are
processed as they are received. This makes it possible to use ynl as a
library that supplies notifications in a timely manner.

- Change check_ntf() to be a generator that yields 1 notification at a
  time and blocks until a notification is available.
- Use the --sleep parameter to set an alarm and exit when it fires.

This means that the CLI has the same interface, but notifications get
printed as they are received:

./tools/net/ynl/cli.py --spec <SPEC> --subscribe <TOPIC> [ --sleep <SECS> ]

Here is an example python snippet that shows how to use ynl as a library
for receiving notifications:

    ynl = YnlFamily(f"{dir}/rt_route.yaml")
    ynl.ntf_subscribe('rtnlgrp-ipv4-route')

    for event in ynl.check_ntf():
        handle(event)

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Tested-by: Kory Maincent <kory.maincent@bootlin.com>
Link: https://patch.msgid.link/20241018093228.25477-1-donald.hunter@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

tools/net/ynl/cli.py

@@ -5,6 +5,7 @@ import argparse
 import json
 import pprint
 import time
+import signal
 
 from lib import YnlFamily, Netlink, NlError
 
@@ -17,6 +18,8 @@ class YnlEncoder(json.JSONEncoder):
             return list(obj)
         return json.JSONEncoder.default(self, obj)
 
+def handle_timeout(sig, frame):
+    exit(0)
 
 def main():
     description = """
@@ -81,7 +84,8 @@ def main():
         ynl.ntf_subscribe(args.ntf)
 
     if args.sleep:
-        time.sleep(args.sleep)
+        signal.signal(signal.SIGALRM, handle_timeout)
+        signal.alarm(args.sleep)
 
     if args.list_ops:
         for op_name, op in ynl.ops.items():
@@ -106,8 +110,8 @@ def main():
         exit(1)
 
     if args.ntf:
-        ynl.check_ntf()
-        output(ynl.async_msg_queue)
+        for msg in ynl.check_ntf():
+            output(msg)
 
 
 if __name__ == "__main__":