net: when selecting a network device, use natural sort order

The code deciding which interface to choose as the default to request the
IP address through DHCP does not sort the interfaces correctly. On Ubuntu
Xenial images for example, the interfaces are named ens1, ens2, ens3...,
ens11, ... depending on the pci bus address. The python sorting will list
'ens11' before 'ens3' for example despite the fact that 'ens3' should be
before 'ens11'.

This patch address this issue and sort the interface names according to a
human sorting.

Signed-off-by: Marc-Aurèle Brothier <m@brothier.org>

1 files changed, 12 insertions, 1 deletions

diff --git a/cloudinit/net/__init__.py b/cloudinit/net/__init__.py
index 8c6cd057..65accbb0 100644
--- a/cloudinit/net/__init__.py
+++ b/cloudinit/net/__init__.py
@@ -17,6 +17,17 @@ SYS_CLASS_NET = "/sys/class/net/"
 DEFAULT_PRIMARY_INTERFACE = 'eth0'
 
 
+def _natural_sort_key(s, _nsre=re.compile('([0-9]+)')):
+    """Sorting for Humans: natural sort order. Can be use as the key to sort
+    functions.
+    This will sort ['eth0', 'ens3', 'ens10', 'ens12', 'ens8', 'ens0'] as
+    ['ens0', 'ens3', 'ens8', 'ens10', 'ens12', 'eth0'] instead of the simple
+    python way which will produce ['ens0', 'ens10', 'ens12', 'ens3', 'ens8',
+    'eth0']."""
+    return [int(text) if text.isdigit() else text.lower()
+            for text in re.split(_nsre, s)]
+
+
 def sys_dev_path(devname, path=""):
     return SYS_CLASS_NET + devname + "/" + path
 
@@ -169,7 +180,7 @@ def generate_fallback_config():
 
     # if eth0 exists use it above anything else, otherwise get the interface
     # that we can read 'first' (using the sorted defintion of first).
-    names = list(sorted(potential_interfaces))
+    names = list(sorted(potential_interfaces, key=_natural_sort_key))
     if DEFAULT_PRIMARY_INTERFACE in names:
         names.remove(DEFAULT_PRIMARY_INTERFACE)
         names.insert(0, DEFAULT_PRIMARY_INTERFACE)