[scripts][loop_execute] Implement user adaptation as done in execute

This allows to have the data generated with the correct user id
if provided in the configuration and doesn't match the current
user running the container.

beat/backend/python/scripts/loop_execute.py

@@ -61,6 +61,8 @@ import logging
 import os
 import sys
 import docopt
+import simplejson as json
+import subprocess  # nosec
 
 import zmq
 
@@ -137,12 +139,50 @@ def main(arguments=None):
 
     # If necessary, change to another user (with less privileges, but has access
     # to the databases)
-    try:
 
-        # Check the dir
-        if not os.path.exists(args["<dir>"]):
-            raise IOError("Running directory `%s' not found" % args["<dir>"])
+    # Check the dir
+    if not os.path.exists(args["<dir>"]):
+        raise IOError("Running directory `%s' not found" % args["<dir>"])
+
+    # Load the configuration
+    with open(os.path.join(args["<dir>"], "configuration.json"), "r") as f:
+        cfg = json.load(f)
+
+    user_id = cfg["uid"]
+
+    # Create a new user with less privileges (if necessary)
+    if os.getuid() != user_id:
+        retcode = subprocess.call(  # nosec
+            [
+                "adduser",
+                "--uid",
+                str(user_id),
+                "--no-create-home",
+                "--disabled-password",
+                "--disabled-login",
+                "--gecos",
+                '""',
+                "-q",
+                "beat-nobody",
+            ]
+        )
+        if retcode != 0:
+            msg = "Failed to create a user with the UID {}".format(user_id)
+            message_handler.send_error(msg, "sys")
+            message_handler.destroy()
+            return 1
+
+        # Change to the user with less privileges
+        try:
+            os.setgid(user_id)
+            os.setuid(user_id)
+        except Exception as e:
+            msg = "Failed to change to user id {}: {}".format(cfg["uid"], e)
+            message_handler.send_error(msg, "sys")
+            message_handler.destroy()
+            return 1
 
+    try:
         # Sets up the execution
         try:
             loop_executor = LoopExecutor(