[fix] sqlitedb: create DB Schema (DDL) during app initialization (hardening) (#6187)

The initialization of the DB schema ("base schema") has so far been done on
demand, which causes race conditions with competing threads and processes.

The DDL statements for creating the "base schema" are now executed as part of
the initialization of the app.

Further improvements were made to harden the database applications:

- Wikidata & Radio-Browser engine perform their initialization only once (so far
  the initialization was carried out in each thread/process).

- If multiple processes try to set DB's WAL mode when opening the DB at the same
  time, this usually leads to another race condition, which is now also caught.

Related:

- https://github.com/searxng/searxng/issues/6181#issuecomment-4586705

Closes: #6181

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

searx/engines/wikidata.py

@@ -7,6 +7,7 @@ Some implementations are shared from :ref:`wikipedia engine`.
 
 import typing as t
 
+import os
 from hashlib import md5
 from urllib.parse import urlencode, unquote
 from json import loads
@@ -827,7 +828,19 @@ def debug_explain_wikidata_query(query: str, method: str = "GET"):
 def init(_):
     global CACHE  # pylint: disable=global-statement
     CACHE = EngineCache("wikidata")
-    init_wikidata_properties()
+
+    # In an environment with competing processes, the initial loading of the
+    # cache is required only once.
+    eng_state: str | None = CACHE.get("eng_state")
+    if not eng_state or not eng_state.startswith("STATE:"):
+        CACHE.set("eng_state", f"STATE: being initialized by PID {os.getpid()}")
+        try:
+            init_wikidata_properties()
+        except Exception:
+            CACHE.set("eng_state", f"ERROR: initialization by PID {os.getpid()} failed.")
+            raise
+    else:
+        logger.debug(eng_state)
 
 
 def init_wikidata_properties():