[client] Fix creating session file

If we run multiple CodeChecker commands (login, store) which uses the same
home directory and the CodeChecker session file (`~/.codechecker.session.json`)
does not exist yet the following scenario is possible:
- The first command recognizes that this file doesn't exist yet so it will open
  this file for writing.
- Before the first command writes data to the file the second command will be run.
  It recognizes that the file exists, so it will read up the file. It expects that
  the file is a valid json file but it is empty. So the `token_dict` variable
  will be an empty dict, we try to get the `tokens` member of this empty
  dictionary (`self.__tokens`) which will be `None`, and when we will try to
  iterate over this directory (which is `None` in our case) the CodeChecker will
  raise an exception.

To solve this problem we will use `portalocker` after we opened the file to
lock it, so only one process will be able to read/write this file simultaneously.
Also we will handle the use case when this file is still empty.

web/client/codechecker_client/credential_manager.py

@@ -98,22 +98,28 @@ def __init__(self):
         self.token_file = get_session_file()
         LOG.info("Checking for local valid sessions.")
 
-        if os.path.exists(self.token_file):
-            token_dict = load_json_or_empty(self.token_file, {},
-                                            "user authentication")
-            check_file_owner_rw(self.token_file)
-
-            self.__tokens = token_dict.get('tokens')
-            LOG.debug("Found session information for these hosts:")
-            for k, _ in self.__tokens.items():
-                LOG.debug("  %s", k)
-        else:
-            with open(self.token_file, 'w',
-                      encoding="utf-8", errors="ignore") as f:
+        with open(self.token_file, 'a+',
+                  encoding="utf-8", errors="ignore") as f:
+            f.seek(0)
+
+            portalocker.lock(f, portalocker.LOCK_EX)
+
+            try:
+                token_dict = json.loads(f.read())
+
+                check_file_owner_rw(self.token_file)
+
+                self.__tokens = token_dict.get('tokens', {})
+                LOG.debug("Found session information for these hosts:")
+                for k, _ in self.__tokens.items():
+                    LOG.debug("  %s", k)
+            except json.JSONDecodeError:
                 json.dump({'tokens': {}}, f)
-            os.chmod(self.token_file, stat.S_IRUSR | stat.S_IWUSR)
+                os.chmod(self.token_file, stat.S_IRUSR | stat.S_IWUSR)
+
+                self.__tokens = {}
 
-            self.__tokens = {}
+            portalocker.unlock(f)
 
     def is_autologin_enabled(self):
         return self.__autologin