ephemeral: Handle link up failure for both ipv4 and ipv6

EphemeralIPv{4,6} failure is not always an error, therefore do not log
this event as an error in the context manager. Allow callsites to
determine log level.

Fixes canonicalGH-4540

cloudinit/net/ephemeral.py

@@ -107,10 +107,6 @@ def __enter__(self):
             elif self.router:
                 self._bringup_router()
         except subp.ProcessExecutionError:
-            LOG.error(
-                "Error bringing up EphemeralIPv4Network. "
-                "Datasource setup cannot continue"
-            )
             self.__exit__(None, None, None)
             raise
 
@@ -274,11 +270,9 @@ def __exit__(self, excp_type, excp_value, excp_traceback):
 
     def clean_network(self):
         """Exit _ephipv4 context to teardown of ip configuration performed."""
-        if self.lease:
-            self.lease = None
-        if not self._ephipv4:
-            return
-        self._ephipv4.__exit__(None, None, None)
+        self.lease = None
+        if self._ephipv4:
+            self._ephipv4.__exit__(None, None, None)
 
     def obtain_lease(self):
         """Perform dhcp discovery in a sandboxed environment if possible.
@@ -350,7 +344,13 @@ def get_first_option_value(
 
 
 class EphemeralIPNetwork:
-    """Marries together IPv4 and IPv6 ephemeral context managers"""
+    """Combined ephemeral context manager for IPv4 and IPv6
+
+    Either ipv4 or ipv6 ephemeral network may fail to initialize, but if either
+    succeeds, then this context manager will not raise exception. This allows
+    either ipv4 or ipv6 ephemeral network to succeed, but requires that error
+    handling for networks unavailable be done within the context.
+    """
 
     def __init__(
         self,
@@ -367,21 +367,51 @@ def __init__(
         self.distro = distro
 
     def __enter__(self):
-        # ipv6 dualstack might succeed when dhcp4 fails
-        # therefore catch exception unless only v4 is used
+        if not (self.ipv4 or self.ipv6):
+            # no ephemeral network requested, but this object still needs to
+            # function as a context manager
+            return self
         try:
+            exceptions = []
+            ephemeral_obtained = False
             if self.ipv4:
-                self.stack.enter_context(
-                    EphemeralDHCPv4(self.distro, self.interface)
-                )
+                try:
+                    self.stack.enter_context(
+                        EphemeralDHCPv4(
+                            self.distro,
+                            self.interface,
+                        )
+                    )
+                    ephemeral_obtained = True
+                except subp.ProcessExecutionError as e:
+                    LOG.info("Failed to bring up EphemeralIPv4Network.")
+                    exceptions.append(e)
+
             if self.ipv6:
-                self.stack.enter_context(
-                    EphemeralIPv6Network(self.distro, self.interface)
+                try:
+                    self.stack.enter_context(
+                        EphemeralIPv6Network(
+                            self.distro,
+                            self.interface,
+                        )
+                    )
+                    ephemeral_obtained = True
+                except subp.ProcessExecutionError as e:
+                    LOG.info("Failed to bring up EphemeralIPv6Network.")
+                    exceptions.append(e)
+
+            if not ephemeral_obtained:
+                # Ephemeral network setup failed in linkup for both ipv4 and
+                # ipv6. Raise only the first exception found.
+                LOG.error(
+                    "Failed to bring up EphemeralIPNetwork. "
+                    "Datasource setup cannot continue"
                 )
-        # v6 link local might be usable
-        # caller may want to log network state
+                raise exceptions[0]
         except NoDHCPLeaseError as e:
             if self.ipv6:
+                # ipv6 dualstack might succeed when dhcp4 fails, so catch
+                # NoDHCPLeaseError unless only v4 is used
                 self.state_msg = "using link-local ipv6"
             else:
                 raise e