cc_growpart: respect /etc/growroot-disabled

cloud-initramfs-growroot is an initramfs module in cloud-initramfs-tools
that resizes the root partition before the root is pivoted over.

growroot was used in Ubuntu up to and including 12.10.  The file
/etc/growroot-disabled on the root filesystem was the only way of
disabling the growing of the root partition.

In cloud-init 0.7.2 cloud-init began resizing the root partition
as growpart gained the ability to utilize 'ptupdate' in kernels > 3.8.
This was a big improvement as now the user could disable or enable
the growing of the root partition via user-data.

In order to let users disable growing of / very simplistically cloud-init
will now respect the presense of /etc/growroot-disabled unless config
specifically tells it to ignore that file.

ChangeLog

@@ -26,6 +26,7 @@
  - config/cloud.cfg: add 'sudo' to list groups for the default user 
    (LP: #1228228)
  - documentation fix for use of 'mkpasswd' [Eric Nordlund]
+ - respect /etc/growroot-disabled file (LP: #1234331)
 0.7.2:
  - add a debian watch file
  - add 'sudo' entry to ubuntu's default user (LP: #1080717)

cloudinit/config/cc_growpart.py

@@ -32,6 +32,7 @@ frequency = PER_ALWAYS
 DEFAULT_CONFIG = {
     'mode': 'auto',
     'devices': ['/'],
+    'ignore_growroot_disabled': False,
 }
 
 
@@ -251,6 +252,12 @@ def handle(_name, cfg, _cloud, log, _args):
         log.debug("growpart disabled: mode=%s" % mode)
         return
 
+    if util.is_false(mycfg.get('ignore_growroot_disabled', False)):
+        if os.path.isfile("/etc/growroot-disabled"):
+            log.debug("growpart disabled: /etc/growroot-disabled exists")
+            log.debug("use ignore_growroot_disabled to ignore")
+            return
+
     devices = util.get_cfg_option_list(cfg, "devices", ["/"])
     if not len(devices):
         log.debug("growpart: empty device list")

doc/examples/cloud-config-growpart.txt

@@ -19,6 +19,15 @@
 #   examples:
 #     devices: [/, /dev/vdb1]
 #
+# ignore_growroot_disabled:
+#   a boolean, default is false.
+#   if the file /etc/growroot-disabled exists, then cloud-init will not grow
+#   the root partition.  This is to allow a single file to disable both
+#   cloud-initramfs-growroot and cloud-init's growroot support.
+#
+#   true indicates that /etc/growroot-disabled should be ignored
+#
 growpart:
   mode: auto
   devices: ['/']
+  ignore_growroot_disabled: false