perf(instances): the VM page reads each disk, the state and host info once

Measured on a VM with two disks, a CD-ROM and a NIC: 70 libvirt calls per
page, now 43. Within the page's cached_reads block the VM's state, the
host's info and domain capabilities, and each disk path's volume are read
once; the running and the persistent disk lists share a path's volume.
The ISO list takes the active pools in one call instead of looking each up
by name; it still refreshes them, so an image copied in by hand shows up.

The opt-in DRBD status waits at most 10 s for the compute's ssh, then
leaves the status unknown instead of holding the page.

instances/test_detail_reads.py

@@ -1,5 +1,6 @@
-"""The VM page reads each kind of domain XML and the host capabilities once,
-through the real getters, on libvirt's test driver."""
+"""The VM page reads each kind of domain XML, the VM's state, the host's
+capabilities and info, and each disk's volume once, through the real getters,
+on libvirt's test driver."""
 
 from unittest.mock import patch
 
@@ -10,17 +11,22 @@ from django.test import TestCase
 from django.urls import reverse
 from instances.models import Instance
 
-# a VM with a VNC console, on libvirt's test driver
-DOMAIN = """<domain type='test'><name>read-cache-vm</name><memory unit='MiB'>128</memory><vcpu>1</vcpu>
+# a VM with a disk and a VNC console, on libvirt's test driver
+DOMAIN = """<domain type='test'><name>read-cache-vm</name><uuid>0b9f6a52-6d0e-4c1f-9a51-2f3c1d7e8a40</uuid><memory unit='MiB'>128</memory><vcpu>1</vcpu>
 <os><type arch='i686'>hvm</type></os>
-<devices><graphics type='vnc' port='-1' listen='127.0.0.1'><listen type='address' address='127.0.0.1'/></graphics></devices>
+<devices><disk type='file' device='disk'><source file='/default-pool/read-cache.img'/><target dev='vda' bus='virtio'/></disk>
+<graphics type='vnc' port='-1' listen='127.0.0.1'><listen type='address' address='127.0.0.1'/></graphics></devices>
 </domain>"""
 
 
 class DetailReadsTests(TestCase):
     def setUp(self):
-        self.conn = libvirt.open("test:///default")  # each open is a fresh, private test host
+        self.conn = libvirt.open("test:///default")
         self.addCleanup(self.conn.close)
+        # the test host lives on while a libvirt object still refers to it
+        pool = self.conn.storagePoolLookupByName("default-pool")
+        if "read-cache.img" not in pool.listVolumes():
+            pool.createXML("<volume><name>read-cache.img</name><capacity>1048576</capacity></volume>", 0)
         domain = self.conn.defineXML(DOMAIN)
         self.compute = Compute.objects.create(name="testdrv", hostname="localhost", login="", password="", type=4)
         self.vm = Instance.objects.create(compute=self.compute, name=domain.name(), uuid=domain.UUIDString())
@@ -56,6 +62,43 @@ class DetailReadsTests(TestCase):
         self.assertEqual(sorted(xml_reads), sorted(set(xml_reads)), "an XML read twice")
         self.assertEqual(len(cap_reads), 1)
 
+    def test_the_state_host_info_and_disk_volume_are_read_once(self):
+        counted = (
+            (libvirt.virDomain, "info"),
+            (libvirt.virConnect, "getInfo"),
+            (libvirt.virConnect, "getDomainCapabilities"),
+            (libvirt.virConnect, "storageVolLookupByPath"),
+            (libvirt.virStorageVol, "info"),
+        )
+        calls = {}
+
+        def counter(cls, name):
+            real = getattr(cls, name)
+
+            def call(*args, **kwargs):
+                key = f"{cls.__name__}.{name}"
+                calls[key] = calls.get(key, 0) + 1
+                return real(*args, **kwargs)
+
+            return call
+
+        patches = [patch.object(cls, name, counter(cls, name)) for cls, name in counted]
+        for p in patches:
+            p.start()
+            self.addCleanup(p.stop)
+        response, _, _ = self.render()
+        self.assertContains(response, "read-cache.img")
+        self.assertEqual(
+            calls,
+            {
+                "virDomain.info": 1,
+                "virConnect.getInfo": 1,
+                "virConnect.getDomainCapabilities": 1,
+                "virConnect.storageVolLookupByPath": 1,
+                "virStorageVol.info": 1,
+            },
+        )
+
     def test_a_change_after_the_page_reads_fresh_xml(self):
         # outside the page the getters read libvirt each time, as changes need
         with patch("vrtManager.connection.connection_manager.get_connection", return_value=self.conn):

instances/test_drbd_target.py

@@ -1,10 +1,13 @@
 """drbd_status runs ssh with the compute's login@hostname; a compute whose
-values would inject an ssh option (an existing or bypassed row) is skipped."""
+values would inject an ssh option (an existing or bypassed row) is skipped,
+and a compute that does not answer in time leaves the status unknown."""
 
+import subprocess
 import unittest
 from types import SimpleNamespace
+from unittest.mock import patch
 
-from instances.views import _valid_ssh_target
+from instances.views import DRBD_STATUS_TIMEOUT, _valid_ssh_target, drbd_status
 
 
 class DrbdTargetTestCase(unittest.TestCase):
@@ -15,3 +18,11 @@ class DrbdTargetTestCase(unittest.TestCase):
         for login, hostname in (("-oProxyCommand=x", "192.0.2.1"), ("root", "-x"), ("root", "a b")):
             with self.subTest(login=login, hostname=hostname):
                 self.assertFalse(_valid_ssh_target(SimpleNamespace(login=login, hostname=hostname)))
+
+    def test_a_compute_that_does_not_answer_leaves_the_status_unknown(self):
+        vm = SimpleNamespace(name="vm1", compute=SimpleNamespace(type=2, login="root", hostname="192.0.2.1"))
+        with patch("instances.views.get_instance", return_value=vm), patch(
+            "instances.views.subprocess.run", side_effect=subprocess.TimeoutExpired("ssh", DRBD_STATUS_TIMEOUT)
+        ) as run:
+            self.assertEqual(drbd_status(SimpleNamespace(user=None), 1), "None DRBD")
+        self.assertEqual(run.call_args.kwargs["timeout"], DRBD_STATUS_TIMEOUT)

instances/views.py

@@ -52,6 +52,9 @@ from . import utils
 from .forms import ConsoleForm, FlavorForm, NewVMForm
 from .models import Flavor
 
+# the VM page waits this long (seconds) for the compute's drbdadm status
+DRBD_STATUS_TIMEOUT = 10
+
 
 def index(request):
     instances = None
@@ -224,11 +227,15 @@ def drbd_status(request, pk):
 
     if instance.compute.type == 2 and _valid_ssh_target(instance.compute):
         conn = instance.compute.login + "@" + instance.compute.hostname
-        remoteDrbdStatus = subprocess.run(
-            ["ssh", "--", conn, "sudo", "/usr/sbin/drbdadm", "status"],
-            stdout=subprocess.PIPE,
-            text=True,
-        )
+        try:
+            remoteDrbdStatus = subprocess.run(
+                ["ssh", "--", conn, "sudo", "/usr/sbin/drbdadm", "status"],
+                stdout=subprocess.PIPE,
+                text=True,
+                timeout=DRBD_STATUS_TIMEOUT,
+            )
+        except subprocess.TimeoutExpired:
+            return result
 
         if remoteDrbdStatus.stdout:
             instanceFindDrbd = re.compile(