add cloud-init-run-module and ec2init/execute.py

cloud-init-run-module handles some boilerplate code for running
items on a 'frequency'.  It has the following usefulness

- a config module can be put into ec2init dir and implement a 'run'
  method that takes a list of arguments and the path to a config file
- it handles invoking module.run() only at a given frequency
  This is similar to karmic's ec2init's "run_once_ever" or run_once_per_ami

execute.py is an example module that executes the arguments given to it

An example usage in an upstart job would be with a 'exec' line like:

   exec cloud-init-run-module once_per_ami clean-core execute rm /var/run/core

The above would then run the command 'rm /var/run/core' only once

1 files changed, 50 insertions, 0 deletions

diff --git a/cloud-init-run-module.py b/cloud-init-run-module.py
new file mode 100755
index 00000000..5a35de5d
--- /dev/null
+++ b/cloud-init-run-module.py
@@ -0,0 +1,50 @@
+#!/usr/bin/python
+
+import sys
+import ec2init
+
+def Usage(out = sys.stdout):
+    out.write("Usage: cloud-init-run-module freq sem-name mod-name [args]")
+    
+def main():
+    # expect to be called with
+    #   <freq> <semaphore-name> <module-name> args
+    if len(sys.argv) < 4:
+        Usage(sys.stderr)
+        sys.exit(1)
+
+    (freq,semname,modname)=sys.argv[1:4]
+    run_args=sys.argv[4:]
+
+    if ec2init.sem_has_run(semname,freq):
+        sys.stderr.write("%s already ran %s\n" % (semname,freq))
+        sys.exit(0)
+
+    try:
+        mod = __import__('ec2init.' + modname)
+        inst = getattr(mod,modname)
+    except:
+        sys.stderr.write("Failed to load module ec2init.%s\n" % modname)
+        sys.exit(1)
+
+    import os
+
+    cfg_path = None
+    cfg_env_name = "CLOUD_CFG"
+    if os.environ.has_key(cfg_env_name):
+        cfg_path = os.environ[cfg_env_name]
+
+    try:
+        if not ec2init.sem_acquire(semname,freq):
+            sys.stderr.write("Failed to acquire lock on %s\n" % semname)
+            sys.exit(1)
+
+        inst.run(run_args,cfg_path)
+    except:
+        ec2init.sem_clear(semname,freq)
+        raise
+
+    sys.exit(0)
+
+if __name__ == '__main__':
+    main()