stages: allow data sources to override network config source order

Currently, if a platform provides any network configuration via the
"cmdline" method (i.e. network-data=... on the kernel command line,
ip=... on the kernel command line, or iBFT config via /run/net-*.conf),
the value of the data source's network_config property is completely
ignored.

This means that on platforms that use iSCSI boot (such as Oracle Compute
Infrastructure), there is no way for the data source to configure any
network interfaces other than those that have already been configured by
the initramfs.

This change allows data sources to specify the order in which network
configuration sources are considered.  Data sources that opt to use this
mechanism will be expected to consume the command line network data and
integrate it themselves.

(The generic merging of network configuration sources was considered,
but we concluded that the single use case we have presently (a) didn't
warrant the increased complexity, and (b) didn't give us a broad enough
view to be sure that our generic implementation would be sufficiently
generic.  This change in no way precludes a merging strategy in future.)

1 files changed, 16 insertions, 0 deletions

diff --git a/cloudinit/sources/__init__.py b/cloudinit/sources/__init__.py
index e6966b31..9d249366 100644
--- a/cloudinit/sources/__init__.py
+++ b/cloudinit/sources/__init__.py
@@ -66,6 +66,13 @@ CLOUD_ID_REGION_PREFIX_MAP = {
     'china': ('azure-china', lambda c: c == 'azure'),  # only change azure
 }
 
+# NetworkConfigSource represents the canonical list of network config sources
+# that cloud-init knows about.  (Python 2.7 lacks PEP 435, so use a singleton
+# namedtuple as an enum; see https://stackoverflow.com/a/6971002)
+_NETCFG_SOURCE_NAMES = ('cmdline', 'ds', 'system_cfg', 'fallback')
+NetworkConfigSource = namedtuple('NetworkConfigSource',
+                                 _NETCFG_SOURCE_NAMES)(*_NETCFG_SOURCE_NAMES)
+
 
 class DataSourceNotFoundException(Exception):
     pass
@@ -153,6 +160,15 @@ class DataSource(object):
     # Track the discovered fallback nic for use in configuration generation.
     _fallback_interface = None
 
+    # The network configuration sources that should be considered for this data
+    # source.  (The first source in this list that provides network
+    # configuration will be used without considering any that follow.)  This
+    # should always be a subset of the members of NetworkConfigSource with no
+    # duplicate entries.
+    network_config_sources = (NetworkConfigSource.cmdline,
+                              NetworkConfigSource.system_cfg,
+                              NetworkConfigSource.ds)
+
     # read_url_params
     url_max_wait = -1   # max_wait < 0 means do not wait
     url_timeout = 10    # timeout for each metadata url read attempt