perf(computes): the host's info is read once per request

The overview read the host's info four times, now once (12 libvirt calls).
A connection object keeps it next to the capabilities, and the VM page and
VM stats take it from there; the VM page's cached_reads block holds only
the VM's own data. Free memory is still read each time.

vrtManager/test_host_reads.py

@@ -0,0 +1,99 @@
+"""A wvmConnect reads the host's info and capabilities once, and its domain
+capabilities once per emulator, arch, machine and domain type: the create
+page's getters parse the capabilities some 60 times, the overview reads the
+info four times. An object lives for one request."""
+
+import unittest
+
+from django.conf import settings
+
+if not settings.configured:
+    settings.configure(MAC_OUI="52:54:10")
+
+from vrtManager.connection import wvmConnect
+from vrtManager.hostdetails import wvmHostDetails
+
+CAPABILITIES = """<capabilities><host><cpu><arch>x86_64</arch></cpu></host>
+<guest><os_type>hvm</os_type><arch name='x86_64'><emulator>/usr/bin/qemu-kvm</emulator>
+<machine>pc-q35-9.0</machine><machine canonical='pc-q35-9.0'>q35</machine><machine canonical='pc-i440fx-9.0'>pc</machine>
+<domain type='qemu'/><domain type='kvm'/></arch></guest></capabilities>"""
+
+
+class FakeConn:
+    def __init__(self):
+        self.reads = []
+
+    def getCapabilities(self):
+        self.reads.append("capabilities")
+        return CAPABILITIES
+
+    def getInfo(self):
+        self.reads.append("info")
+        return ["x86_64", 2048, 4, 0, 1, 2, 2, 1]
+
+    def getMemoryStats(self, cell, flags):
+        self.reads.append("memory")
+        return {"buffers": 0, "free": 1048576, "cached": 0}
+
+    def getHostname(self):
+        return "host"
+
+    def getSysinfo(self, flags):
+        return "<sysinfo/>"
+
+    def getURI(self):
+        return "qemu:///system"
+
+    def getDomainCapabilities(self, emulator, arch, machine, virttype):
+        self.reads.append((emulator, arch, machine, virttype))
+        return "<domainCapabilities/>"
+
+
+def connection(cls=wvmConnect):
+    conn = cls.__new__(cls)  # no libvirt connection: only these calls
+    conn.wvm = FakeConn()
+    return conn
+
+
+class CapabilitiesOnceTestCase(unittest.TestCase):
+    def test_the_capabilities_are_read_once_for_all_getters(self):
+        conn = connection()
+        for _ in range(2):
+            conn.get_emulator("x86_64")
+            conn.get_machine_types("x86_64")
+            conn.get_hypervisors_domain_types()
+            conn.get_cap_xml()
+        self.assertEqual(conn.wvm.reads, ["capabilities"])
+
+    def test_domain_capabilities_are_read_once_per_machine(self):
+        conn = connection()
+        for _ in range(2):
+            conn.get_dom_cap_xml("x86_64", "q35")
+            conn.get_dom_cap_xml("x86_64", "pc")
+            conn.get_dom_cap_xml("x86_64", "unknown")  # falls back to pc
+        self.assertEqual(
+            conn.wvm.reads,
+            ["capabilities", ("/usr/bin/qemu-kvm", "x86_64", "q35", "kvm"), ("/usr/bin/qemu-kvm", "x86_64", "pc", "kvm")],
+        )
+
+    def test_the_overview_reads_the_info_once_and_the_free_memory_each_time(self):
+        conn = connection(wvmHostDetails)
+        conn.get_node_info()
+        conn.get_memory_usage()
+        self.assertEqual(conn.get_memory_usage()["total"], 2048 * 1048576)
+        self.assertEqual(conn.wvm.reads, ["info", "memory", "memory"])
+
+    def test_two_objects_do_not_share_their_reads(self):
+        a, b = connection(), connection()
+        a.get_cap_xml()
+        b.get_cap_xml()
+        self.assertEqual((a.wvm.reads, b.wvm.reads), (["capabilities"], ["capabilities"]))
+
+    def test_a_failed_read_is_not_kept(self):
+        conn = connection()
+        conn.wvm.getCapabilities = lambda: (_ for _ in ()).throw(RuntimeError("libvirt error"))
+        with self.assertRaises(RuntimeError):
+            conn.get_cap_xml()
+        conn.wvm = FakeConn()
+        conn.get_cap_xml()
+        self.assertEqual(conn.wvm.reads, ["capabilities"])