syncthing_folder: Always share a folder with the current device

There is no need to require that the device being configured is
explictly mentioned in its configuration. We can get it through
the status endpoint and automatically add it to the set of devices
the folder is shared with.

library/storage/syncthing/syncthing_folder.py

@@ -165,6 +165,10 @@ def get_data_from_rest_api(module, resource):
 def get_config(module):
     return get_data_from_rest_api(module, 'system/config')
 
+# Fetch Syncthing status
+def get_status(module):
+    return get_data_from_rest_api(module, 'system/status')
+
 # Get the device name -> device ID mapping.
 def get_devices_mapping(config):
     return {
@@ -197,23 +201,28 @@ def post_config(module, config, result):
         module.fail_json(**result)
 
 # Returns an object of a new folder
-def create_folder(params, current_device_ids, devices_mapping):
-    wanted_device_ids = []
+def create_folder(params, self_id, current_device_ids, devices_mapping):
+    # We need the current device ID as per the Syncthing API.
+    # If missing, Syncthing will add it alright, but we don't want to give
+    # the false idea that this configuration is different just because of that.
+    wanted_device_ids = {self_id}
     for device_name_or_id in params['devices']:
         if device_name_or_id in devices_mapping:
-            wanted_device_ids.append(devices_mapping[device_name_or_id])
+            wanted_device_ids.add(devices_mapping[device_name_or_id])
         else:
             # Purposefully do not validate we already know this device ID or
             # name as per previous behavior.  This will need to be fixed.
-            wanted_device_ids.append(device_name_or_id)
+            wanted_device_ids.add(device_name_or_id)
 
-    # Collect wanted devices to share folder with.
-    # Note that the sequence ordering matters, so we stick with lists
-    # instead of sets.
+    # Keep the original ordering if collections are equivalent.
+    # Again, for idempotency reasons.
     device_ids = (
-        current_device_ids if set(current_device_ids) == set(wanted_device_ids)
-        else wanted_device_ids
+        current_device_ids
+        if set(current_device_ids) == wanted_device_ids
+        else sorted(wanted_device_ids)
     )
+
+    # Sort the device IDs to keep idem-potency
     devices = [
         {
             'deviceID': device_id,
@@ -300,6 +309,7 @@ def run_module():
         module.params['api_key'] = get_key_from_filesystem(module)
 
     config = get_config(module)
+    self_id = get_status(module)['myID']
     devices_mapping = get_devices_mapping(config)
     if module.params['state'] == 'absent':
         # Remove folder from list, if found
@@ -314,7 +324,7 @@ def run_module():
             [d['deviceID'] for d in folder_config['devices']] if folder_config else []
         )
         folder_config_wanted = create_folder(
-            module.params, folder_config_devices, devices_mapping
+            module.params, self_id, folder_config_devices, devices_mapping
         )
 
         if folder_config is None: