azure: check for primary interface when performing DHCP

For Savable PPS we rely on a connectivity check to IMDS to determine
if the NIC is primary.  However, in some cases, connectivity may be
delayed and we incorrectly assume the NIC is not primary.

Instead of relying on connectivity, check the DHCP-provided route
configuration for the presence of Wireserver and/or IMDS IPs.

- Return bool from _setup_ephemeral_networking() indicating primary
NIC or not.

- Remove _check_if_nic_is_primary() used in Savable PPS.

- Add a relevant diagnostic to assist debugging failures when a
secondary NIC is chosen.

- Remove some tests that are redundant.

Signed-off-by: Chris Patterson <cpatterson@microsoft.com>

cloudinit/sources/DataSourceAzure.py

@@ -19,6 +19,7 @@
 
 from cloudinit import net, sources, ssh_util, subp, util
 from cloudinit.event import EventScope, EventType
+from cloudinit.net import device_driver
 from cloudinit.net.dhcp import (
     NoDHCPLeaseError,
     NoDHCPLeaseInterfaceError,
@@ -371,7 +372,7 @@ def _setup_ephemeral_networking(
         iface: Optional[str] = None,
         retry_sleep: int = 1,
         timeout_minutes: int = 5,
-    ) -> None:
+    ) -> bool:
         """Setup ephemeral networking.
 
         Keep retrying DHCP up to specified number of minutes.  This does
@@ -381,13 +382,19 @@ def _setup_ephemeral_networking(
         :param timeout_minutes: Number of minutes to keep retrying for.
 
         :raises NoDHCPLeaseError: If unable to obtain DHCP lease.
+
+        :returns: True if NIC is determined to be primary.
         """
         if self._ephemeral_dhcp_ctx is not None:
             raise RuntimeError(
                 "Bringing up networking when already configured."
             )
 
-        LOG.debug("Requested ephemeral networking (iface=%s)", iface)
+        report_diagnostic_event(
+            "Bringing up ephemeral networking with iface=%s: %r"
+            % (iface, net.get_interfaces()),
+            logger_func=LOG.debug,
+        )
         self._ephemeral_dhcp_ctx = EphemeralDHCPv4(
             self.distro,
             iface=iface,
@@ -458,15 +465,46 @@ def _setup_ephemeral_networking(
             if lease is None:
                 self._ephemeral_dhcp_ctx = None
                 raise NoDHCPLeaseError()
+
+            # Ensure iface is set.
+            iface = lease["interface"]
+            self._ephemeral_dhcp_ctx.iface = iface
+
+            # Update wireserver IP from DHCP options.
+            if "unknown-245" in lease:
+                self._wireserver_endpoint = get_ip_from_lease_value(
+                    lease["unknown-245"]
+                )
+
+            driver = device_driver(iface)
+            ephipv4 = self._ephemeral_dhcp_ctx._ephipv4
+            if ephipv4 is None:
+                raise RuntimeError("dhcp context missing ephipv4")
+
+            if not ephipv4.static_routes:
+                # Primary nics must contain routes.
+                primary = False
             else:
-                # Ensure iface is set.
-                self._ephemeral_dhcp_ctx.iface = lease["interface"]
+                routed_networks = [r[0] for r in ephipv4.static_routes]
+                primary = any(
+                    n in routed_networks
+                    for n in ["168.63.129.16/32", "169.254.169.254/32"]
+                )
 
-                # Update wireserver IP from DHCP options.
-                if "unknown-245" in lease:
-                    self._wireserver_endpoint = get_ip_from_lease_value(
-                        lease["unknown-245"]
-                    )
+            report_diagnostic_event(
+                "Obtained DHCP lease on interface %r "
+                "(primary=%r driver=%r router=%r routes=%r lease=%r)"
+                % (
+                    iface,
+                    primary,
+                    driver,
+                    ephipv4.router,
+                    ephipv4.static_routes,
+                    lease,
+                ),
+                logger_func=LOG.debug,
+            )
+            return primary
 
     @azure_ds_telemetry_reporter
     def _teardown_ephemeral_networking(self) -> None:
@@ -1006,32 +1044,6 @@ def _report_ready_for_pps(
         if create_marker:
             self._create_report_ready_marker()
 
-    @azure_ds_telemetry_reporter
-    def _check_if_nic_is_primary(self, ifname: str) -> bool:
-        """Check if a given interface is the primary nic or not."""
-        # For now, only a VM's primary NIC can contact IMDS and WireServer. If
-        # DHCP fails for a NIC, we have no mechanism to determine if the NIC is
-        # primary or secondary. In this case, retry DHCP until successful.
-        self._setup_ephemeral_networking(iface=ifname, timeout_minutes=20)
-
-        # Primary nic detection will be optimized in the future. The fact that
-        # primary nic is being attached first helps here. Otherwise each nic
-        # could add several seconds of delay.
-        imds_md = self.get_metadata_from_imds(report_failure=False)
-        if imds_md:
-            # Only primary NIC will get a response from IMDS.
-            LOG.info("%s is the primary nic", ifname)
-            return True
-
-        # If we are not the primary nic, then clean the dhcp context.
-        LOG.warning(
-            "Failed to fetch IMDS metadata using nic %s. "
-            "Assuming this is not the primary nic.",
-            ifname,
-        )
-        self._teardown_ephemeral_networking()
-        return False
-
     @azure_ds_telemetry_reporter
     def _wait_for_hot_attached_primary_nic(self, nl_sock):
         """Wait until the primary nic for the vm is hot-attached."""
@@ -1073,12 +1085,16 @@ def _wait_for_hot_attached_primary_nic(self, nl_sock):
                 # won't be in primary_nic_found = false state for long.
                 if not primary_nic_found:
                     LOG.info("Checking if %s is the primary nic", ifname)
-                    primary_nic_found = self._check_if_nic_is_primary(ifname)
+                    primary_nic_found = self._setup_ephemeral_networking(
+                        iface=ifname, timeout_minutes=20
+                    )
 
                 # Exit criteria: check if we've discovered primary nic
                 if primary_nic_found:
                     LOG.info("Found primary nic for this VM.")
                     break
+                else:
+                    self._teardown_ephemeral_networking()
 
         except AssertionError as error:
             report_diagnostic_event(str(error), logger_func=LOG.error)