ec2: Allow Ec2 to run in init-local using dhclient in a sandbox.

This branch is a prerequisite for IPv6 support in AWS by allowing Ec2
datasource to query the metadata source version 2016-09-02 about whether
or not it needs to configure IPv6 on interfaces. If version 2016-09-02
is not present, fallback to the min_metadata_version of 2009-04-04. The
DataSourceEc2Local not run on FreeBSD because dhclient in doesn't
support the -sf flag allowing us to run dhclient without filesystem
side-effects.

To query AWS' metadata address @ 169.254.169.254, the instance must have
a dhcp-allocated address configured. Configuring IPv4 link-local
addresses result in timeouts from the metadata service. We introduced a
DataSourceEc2Local subclass which will perform a sandboxed dhclient
discovery which obtains an authorized IP address on eth0 and crawl
metadata about full instance network configuration.

Since ec2 IPv6 metadata is not sufficient in itself to tell us all the
ipv6 knownledge we need, it only be used as a boolean to tell us which
nics need IPv6. Cloud-init will then configure desired interfaces to
DHCPv6 versus DHCPv4.

Performance side note: Shifting the dhcp work into init-local for Ec2
actually gets us 1 second faster deployments by skipping init-network
phase of alternate datasource checks because Ec2Local is configured in
an ealier boot stage. In 3 test runs prior to this change: cloud-init
runs were 5.5 seconds, with the change we now average 4.6 seconds.

This efficiency could be even further improved if we avoiding dhcp
discovery in order to talk to the metadata service from an AWS
authorized dhcp address if there were some way to advertize the dhcp
configuration via DMI/SMBIOS or system environment variables.

Inspecting time costs of the dhclient setup/teardown in 3 live runs the
time cost for the dhcp setup round trip on AWS is:
test 1: 76 milliseconds
         dhcp discovery + metadata: 0.347 seconds
         metadata alone: 0.271 seconds
test 2: 88 milliseconds
         dhcp discovery + metadata: 0.388 seconds
         metadata alone: 0.300 seconds
test 3: 75 milliseconds
         dhcp discovery + metadata: 0.366 seconds
         metadata alone: 0.291 seconds

LP: #1709772

1 files changed, 20 insertions, 15 deletions

diff --git a/cloudinit/net/__init__.py b/cloudinit/net/__init__.py
index 1ff8fae0..a1b0db10 100644
--- a/cloudinit/net/__init__.py
+++ b/cloudinit/net/__init__.py
@@ -175,13 +175,8 @@ def is_disabled_cfg(cfg):
     return cfg.get('config') == "disabled"
 
 
-def generate_fallback_config(blacklist_drivers=None, config_driver=None):
-    """Determine which attached net dev is most likely to have a connection and
-       generate network state to run dhcp on that interface"""
-
-    if not config_driver:
-        config_driver = False
-
+def find_fallback_nic(blacklist_drivers=None):
+    """Return the name of the 'fallback' network device."""
     if not blacklist_drivers:
         blacklist_drivers = []
 
@@ -233,15 +228,24 @@ def generate_fallback_config(blacklist_drivers=None, config_driver=None):
     if DEFAULT_PRIMARY_INTERFACE in names:
         names.remove(DEFAULT_PRIMARY_INTERFACE)
         names.insert(0, DEFAULT_PRIMARY_INTERFACE)
-    target_name = None
-    target_mac = None
+
+    # pick the first that has a mac-address
     for name in names:
-        mac = read_sys_net_safe(name, 'address')
-        if mac:
-            target_name = name
-            target_mac = mac
-            break
-    if target_mac and target_name:
+        if read_sys_net_safe(name, 'address'):
+            return name
+    return None
+
+
+def generate_fallback_config(blacklist_drivers=None, config_driver=None):
+    """Determine which attached net dev is most likely to have a connection and
+       generate network state to run dhcp on that interface"""
+
+    if not config_driver:
+        config_driver = False
+
+    target_name = find_fallback_nic(blacklist_drivers=blacklist_drivers)
+    if target_name:
+        target_mac = read_sys_net_safe(target_name, 'address')
         nconf = {'config': [], 'version': 1}
         cfg = {'type': 'physical', 'name': target_name,
                'mac_address': target_mac, 'subnets': [{'type': 'dhcp'}]}
@@ -585,6 +589,7 @@ class EphemeralIPv4Network(object):
             self._bringup_router()
 
     def __exit__(self, excp_type, excp_value, excp_traceback):
+        """Teardown anything we set up."""
         for cmd in self.cleanup_cmds:
             util.subp(cmd, capture=True)