Allow for reuse of allocated but unassociated Elastic IPs.

library/cloud/ec2_eip

@@ -53,6 +53,11 @@ options:
     required: false
     default: false
     version_added: "1.4"
+  reuse:
+    description:
+      - Reuse an EIP that is not associated to an instance (when available), instead of allocating a new one.
+    required: false
+    default: false
 requirements: [ "boto" ]
 author: Lorin Hochstein <lorin@nimbisservices.com>
 notes:
@@ -198,13 +203,27 @@ def ip_is_associated_with_instance(ec2, public_ip, instance_id, module):
         return False
 
 
-def allocate_address(ec2, domain, module):
+def allocate_address(ec2, domain, module, reuse):
-    """ Allocate a new elastic IP address and return it """
+    """ Allocate a new elastic IP address (when needed) and return it """
     # If we're in check mode, nothing else to do
     if module.check_mode:
         module.exit_json(change=True)
 
-    address = ec2.allocate_address(domain=domain)
+    if reuse:
+      if domain:
+        domain_filter = { 'domain' : domain }
+      else:
+        domain_filter = { 'domain' : 'standard' }
+      all_addresses = ec2.get_all_addresses(filters=domain_filter)
+
+      unassociated_addresses = filter(lambda a: a.instance_id is None, all_addresses)
+      if unassociated_addresses:
+        address = unassociated_addresses[0];
+      else:
+        address = ec2.allocate_address(domain=domain)
+    else:
+      address = ec2.allocate_address(domain=domain)
+
     return address
 
 
@@ -252,6 +271,7 @@ def main():
             ec2_access_key = dict(required=False, aliases=['EC2_ACCESS_KEY']),
             region = dict(required=False, aliases=['ec2_region']),
             in_vpc = dict(required=False, choices=BOOLEANS, default=False),
+            reuse = dict(required=False, choices=BOOLEANS, default=False),
         ),
         supports_check_mode=True
     )
@@ -272,18 +292,19 @@ def main():
     state = module.params.get('state')
     in_vpc = module.params.get('in_vpc')
     domain  = "vpc" if in_vpc else None
+    reuse = module.params.get('reuse');
 
     if state == 'present':
         if public_ip is None:
             if instance_id is None:
-                address = allocate_address(ec2, domain, module)
+                address = allocate_address(ec2, domain, module, reuse)
                 module.exit_json(changed=True, public_ip=address.public_ip)
             else:
                 # Determine if the instance is inside a VPC or not
                 instance = find_instance(ec2, instance_id, module)
                 if instance.vpc_id != None:
                     domain = "vpc"
-                address = allocate_address(ec2, domain, module)
+                address = allocate_address(ec2, domain, module, reuse)
         else:
             address = find_address(ec2, public_ip, module)
         associate_ip_and_instance(ec2, address, instance_id, module)