fix(instances): the IP search asks a guest agent with a timeout

The IP search and the VM report read each running VM's addresses and
OS from its guest agent with interfaceAddresses() and guestInfo(),
which wait for a hung agent indefinitely. The search answered by its
deadline, but the thread stayed blocked, counted against the search
thread limit and held up the worker's exit.

They now use the 10 s agent command the VM page uses, shared as
vrtManager.instance.agent_command() and agent_interfaces(). With the
agent frozen, a search answers in 5 s from the leases and the ARP
table, counts the VM unread, and its thread ends about 5 s later.

vrtManager/instance.py

@@ -362,6 +362,29 @@ class wvmInstances(wvmConnect):
             ) from error
 
 
+def agent_command(dom, command, arguments=None):
+    """The answer of one guest agent command that waits AGENT_TIMEOUT seconds
+    at most: libvirt's own calls for it wait for a hung agent indefinitely,
+    and the domain's agent timeout is shared by every agent call."""
+    request = {"execute": command}
+    if arguments is not None:
+        request["arguments"] = arguments
+    return json.loads(qemuAgentCommand(dom, json.dumps(request), AGENT_TIMEOUT, 0))["return"]
+
+
+def agent_interfaces(dom):
+    """The guest's interfaces from its agent, in interfaceAddresses()'s form,
+    which waits for a hung agent indefinitely."""
+    interfaces = {}
+    for iface in agent_command(dom, "guest-network-get-interfaces"):
+        addrs = []
+        for address in iface.get("ip-addresses", []):
+            kind = 0 if address["ip-address-type"] == "ipv4" else 1
+            addrs.append({"type": kind, "addr": address["ip-address"], "prefix": address["prefix"]})
+        interfaces[iface["name"]] = {"hwaddr": iface.get("hardware-address"), "addrs": addrs}
+    return interfaces
+
+
 class wvmInstance(wvmConnect):
     # set by cached_reads(): libvirt answers reused within the block
     _read_cache = None
@@ -374,9 +397,9 @@ class wvmInstance(wvmConnect):
         self.instance = self.wvm.lookupByUUIDString(uuid) if uuid else self.get_instance(vname)
 
     def osinfo(self):
-        info = self._agent_command("guest-get-osinfo")
-        info.update(self._agent_command("guest-get-timezone"))
-        info.update(self._agent_command("guest-get-host-name"))
+        info = agent_command(self.instance, "guest-get-osinfo")
+        info.update(agent_command(self.instance, "guest-get-timezone"))
+        info.update(agent_command(self.instance, "guest-get-host-name"))
         return info
 
     def start(self, flags=0):
@@ -575,18 +598,6 @@ class wvmInstance(wvmConnect):
             return self.instance.interfaceAddresses(source)
         return {}
 
-    def _agent_interfaces(self):
-        """The guest's interfaces from its agent, in interfaceAddresses()'s
-        form, which waits for a hung agent indefinitely."""
-        interfaces = {}
-        for iface in self._agent_command("guest-network-get-interfaces"):
-            addrs = []
-            for address in iface.get("ip-addresses", []):
-                kind = 0 if address["ip-address-type"] == "ipv4" else 1
-                addrs.append({"type": kind, "addr": address["ip-address"], "prefix": address["prefix"]})
-            interfaces[iface["name"]] = {"hwaddr": iface.get("hardware-address"), "addrs": addrs}
-        return interfaces
-
     def refresh_interface_addresses(self):
         self._ip_cache = {"qemuga": {}, "arp": {}}
 
@@ -595,7 +606,7 @@ class wvmInstance(wvmConnect):
 
         if self.is_agent_ready():
             with contextlib.suppress(libvirtError):
-                self._ip_cache["qemuga"] = self._agent_interfaces()
+                self._ip_cache["qemuga"] = agent_interfaces(self.instance)
 
         self._ip_cache["arp"] = self._get_interface_addresses(VIR_DOMAIN_INTERFACE_ADDRESSES_SRC_ARP)
 
@@ -2509,26 +2520,16 @@ class wvmInstance(wvmConnect):
 
         return util.get_xml_path(self._XMLDesc(0), func=_get_agent)
 
-    def _agent_command(self, command, arguments=None):
-        """The answer of one guest agent command that waits AGENT_TIMEOUT
-        seconds at most: libvirt's own calls for it wait for a hung agent
-        indefinitely, and the domain's agent timeout is shared by every agent
-        call."""
-        request = {"execute": command}
-        if arguments is not None:
-            request["arguments"] = arguments
-        return json.loads(qemuAgentCommand(self.instance, json.dumps(request), AGENT_TIMEOUT, 0))["return"]
-
     def set_user_password(self, user, crypted):
         """Set a user's password in the running guest through its agent;
         crypted is a crypt(3) hash."""
         password = base64.b64encode(crypted.encode()).decode()
-        self._agent_command("guest-set-user-password", {"username": user, "password": password, "crypted": True})
+        agent_command(self.instance, "guest-set-user-password", {"username": user, "password": password, "crypted": True})
 
     def add_authorized_key(self, user, key):
         """Add an SSH public key to the user's authorized keys in the running
         guest through its agent."""
-        self._agent_command("guest-ssh-add-authorized-keys", {"username": user, "keys": [key], "reset": False})
+        agent_command(self.instance, "guest-ssh-add-authorized-keys", {"username": user, "keys": [key], "reset": False})
 
     def is_agent_ready(self):
         """