Add exclusive option to authorized_keys

This option allows the module to ensure that ONLY the specified keys
exist in the authorized_keys file. All others will be removed. This is
quite useful when rotating keys and ensuring no other key will be
accepted.

lib/ansible/modules/system/authorized_key.py

@@ -70,6 +70,15 @@ options:
     required: false
     default: null
     version_added: "1.4"
+  exclusive:
+    description:
+      - Whether to remove all other non-specified keys from the
+        authorized_keys file. Multiple keys can be specified in a single
+        key= string value by separating them by newlines.
+    required: false
+    choices: [ yes", "no" ]
+    default: "no"
+    version_added: "1.9"
 description:
     - "Adds or removes authorized keys for particular user accounts"
 author: Brad Olson
@@ -101,6 +110,9 @@ EXAMPLES = '''
                   key="{{ lookup('file', '/home/charlie/.ssh/id_rsa.pub') }}"
                   key_options='no-port-forwarding,host="10.0.1.1"'
 
+# Set up authorized_keys exclusively with one key
+- authorized_keys: user=root key=public_keys/doe-jane state=present
+                   exclusive=yes
 '''
 
 # Makes sure the public key line is present or absent in the user's .ssh/authorized_keys.
@@ -336,6 +348,7 @@ def enforce_state(module, params):
     manage_dir  = params.get("manage_dir", True)
     state       = params.get("state", "present")
     key_options = params.get("key_options", None)
+    exclusive   = params.get("exclusive", False)
     error_msg   = "Error getting key from: %s"
 
     # if the key is a url, request it and use it as key source
@@ -357,6 +370,10 @@ def enforce_state(module, params):
     params["keyfile"] = keyfile(module, user, do_write, path, manage_dir)
     existing_keys = readkeys(module, params["keyfile"])
 
+    # Add a place holder for keys that should exist in the state=present and
+    # exclusive=true case
+    keys_to_exist = []
+
     # Check our new keys, if any of them exist we'll continue.
     for new_key in key:
         parsed_new_key = parsekey(module, new_key)
@@ -386,6 +403,7 @@ def enforce_state(module, params):
 
         # handle idempotent state=present
         if state=="present":
+            keys_to_exist.append(parsed_new_key[0])
             if len(non_matching_keys) > 0:
                 for non_matching_key in non_matching_keys:
                     if non_matching_key[0] in existing_keys:
@@ -402,6 +420,13 @@ def enforce_state(module, params):
             del existing_keys[parsed_new_key[0]]
             do_write = True
 
+    # remove all other keys to honor exclusive
+    if state == "present" and exclusive:
+        to_remove = frozenset(existing_keys).difference(keys_to_exist)
+        for key in to_remove:
+            del existing_keys[key]
+            do_write = True
+
     if do_write:
         if module.check_mode:
             module.exit_json(changed=True)
@@ -424,6 +449,7 @@ def main():
            state       = dict(default='present', choices=['absent','present']),
            key_options = dict(required=False, type='str'),
            unique      = dict(default=False, type='bool'),
+           exclusive   = dict(default=False, type='bool'),
         ),
         supports_check_mode=True
     )