Add support for running a command on VPN disconnection

When using VPN, I typically run a handful of SSH sessions to different
servers. For convenience/stability/performance, I run those sessions
over SSH ControlMaster connections.

When I jump off the VPN, those master connections are left hanging and so
are any SSH sessions using them. Even if I reconnect VPN, the master
connection and its sessions are still non-responsive. And if I try to
start a new SSH session, that will simply reuse the stale connection and
will also hang with no progress or indication of what's wrong.

My solution is to kill the stale master connections (also kills any
lingering SSH sessions, and allows new sessions to establish a new,
working, ControlMaster connection). I want this to happen automatically
when I disconnect from VPN.

This patch allows me to configure a command to be run automatically
when disconnecting from VPN.

openconnect_sso/app.py

@@ -77,6 +77,8 @@ def run(args):
     except KeyboardInterrupt:
         logger.warn("CTRL-C pressed, exiting")
         return 0
+    finally:
+        handle_disconnect(cfg.on_disconnect)
 
 
 def configure_logger(logger, level):
@@ -179,3 +181,9 @@ def run_openconnect(auth_info, host, proxy, args):
 
     logger.debug("Starting OpenConnect", command_line=command_line)
     return subprocess.run(command_line).returncode
+
+
+def handle_disconnect(command):
+    if command:
+        logger.info("Running command on disconnect", command_line=command)
+        return subprocess.run(command, timeout=5, shell=True).returncode

openconnect_sso/cli.py

@@ -79,6 +79,12 @@ def create_argparser():
         default="shown",
     )
 
+    parser.add_argument(
+        "--on-disconnect",
+        help="Command to run when disconnecting from VPN server",
+        default="",
+    )
+
     parser.add_argument(
         "-V", "--version", action="version", version=f"%(prog)s {__version__}"
     )

openconnect_sso/config.py

@@ -115,6 +115,7 @@ class Config(ConfigNode):
             n: [AutoFillRule.from_dict(r) for r in rule] for n, rule in rules.items()
         },
     )
+    on_disconnect = attr.ib(converter=str, default="")
 
 
 class DisplayMode(enum.Enum):