Ability to hot-attach NICs to preprovisioned VMs before reprovisioning (

#613)

Adds the ability to run the Azure preprovisioned VMs as NIC-less and
then hot-attach them when assigned for reprovision.

The NIC on the preprovisioned VM is hot-detached as soon as it reports
ready and goes into wait for one or more interfaces to be hot-attached.
Once they are attached, cloud-init gets the expected number of NICs (in
case there are more than one) that will be attached from IMDS and waits
until all of them are attached. After all the NICs are attached,
reprovision proceeds as usual.

cloudinit/distros/networking.py

@@ -2,6 +2,7 @@
 import logging
 import os
 
+from cloudinit import subp
 from cloudinit import net, util
 
 
@@ -175,6 +176,10 @@ def wait_for_physdevs(
         if strict:
             raise RuntimeError(msg)
 
+    @abc.abstractmethod
+    def try_set_link_up(self, devname: DeviceName) -> bool:
+        """Try setting the link to up explicitly and return if it is up."""
+
 
 class BSDNetworking(Networking):
     """Implementation of networking functionality shared across BSDs."""
@@ -185,6 +190,9 @@ def is_physical(self, devname: DeviceName) -> bool:
     def settle(self, *, exists=None) -> None:
         """BSD has no equivalent to `udevadm settle`; noop."""
 
+    def try_set_link_up(self, devname: DeviceName) -> bool:
+        raise NotImplementedError()
+
 
 class LinuxNetworking(Networking):
     """Implementation of networking functionality common to Linux distros."""
@@ -214,3 +222,10 @@ def settle(self, *, exists=None) -> None:
         if exists is not None:
             exists = net.sys_dev_path(exists)
         util.udevadm_settle(exists=exists)
+
+    def try_set_link_up(self, devname: DeviceName) -> bool:
+        """Try setting the link to up explicitly and return if it is up.
+           Not guaranteed to bring the interface up. The caller is expected to
+           add wait times before retrying."""
+        subp.subp(['ip', 'link', 'set', devname, 'up'])
+        return self.is_up(devname)

cloudinit/distros/tests/test_networking.py

@@ -30,6 +30,9 @@ def is_physical(self, *args, **kwargs):
         def settle(self, *args, **kwargs):
             raise NotImplementedError
 
+        def try_set_link_up(self, *args, **kwargs):
+            raise NotImplementedError
+
     error = AssertionError("Unexpectedly used /sys in generic networking code")
     with mock.patch(
         "cloudinit.net.get_sys_class_path", side_effect=error,
@@ -74,6 +77,34 @@ def test_returns_true_if_device_is_physical(self, sys_class_net):
         assert LinuxNetworking().is_physical(devname)
 
 
+class TestBSDNetworkingTrySetLinkUp:
+    def test_raises_notimplementederror(self):
+        with pytest.raises(NotImplementedError):
+            BSDNetworking().try_set_link_up("eth0")
+
+
+@mock.patch("cloudinit.net.is_up")
+@mock.patch("cloudinit.distros.networking.subp.subp")
+class TestLinuxNetworkingTrySetLinkUp:
+    def test_calls_subp_return_true(self, m_subp, m_is_up):
+        devname = "eth0"
+        m_is_up.return_value = True
+        is_success = LinuxNetworking().try_set_link_up(devname)
+
+        assert (mock.call(['ip', 'link', 'set', devname, 'up']) ==
+                m_subp.call_args_list[-1])
+        assert is_success
+
+    def test_calls_subp_return_false(self, m_subp, m_is_up):
+        devname = "eth0"
+        m_is_up.return_value = False
+        is_success = LinuxNetworking().try_set_link_up(devname)
+
+        assert (mock.call(['ip', 'link', 'set', devname, 'up']) ==
+                m_subp.call_args_list[-1])
+        assert not is_success
+
+
 class TestBSDNetworkingSettle:
     def test_settle_doesnt_error(self):
         # This also implicitly tests that it doesn't use subp.subp