perf(computes): the create page reads the host's capabilities once

Measured on the create page: 751 ms and 72 libvirt calls, now 123 ms and
13. Its getters parsed the host's capabilities some 60 times, each read
from libvirt. A connection object (one per request) now reads them once,
and the domain capabilities once per emulator, arch, machine and domain
type. The VM page's cached_reads block no longer needs its own copy.

vrtManager/test_capabilities_once.py

@@ -0,0 +1,73 @@
+"""A wvmConnect reads the host's capabilities once, and its domain
+capabilities once per emulator, arch, machine and domain type: the create
+page's getters parse them some 60 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
+
+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 getDomainCapabilities(self, emulator, arch, machine, virttype):
+        self.reads.append((emulator, arch, machine, virttype))
+        return "<domainCapabilities/>"
+
+
+def connection():
+    conn = wvmConnect.__new__(wvmConnect)  # 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_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"])