The checker requires Redis

Remove the abstraction in searx.shared.SharedDict.
Implement a basic and dedicated scheduler for the checker using a Redis script.

searx/search/checker/scheduler.py

@@ -0,0 +1,57 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+# lint: pylint
+# pylint: disable=missing-module-docstring
+"""Lame scheduler which use Redis as a source of truth:
+* the Redis key SearXNG_checker_next_call_ts contains the next time the embedded checker should run.
+* to avoid lock, a unique Redis script reads and updates the Redis key SearXNG_checker_next_call_ts.
+* this Redis script returns a list of two elements:
+   * the first one is a boolean. If True, the embedded checker must run now in this worker.
+   * the second element is the delay in second to wait before the next call to the Redis script.
+
+This scheduler is not generic on purpose: if more feature are required, a dedicate scheduler must be used
+(= a better scheduler should not use the web workers)
+"""
+
+import logging
+import time
+import importlib
+from typing import Callable
+
+from searx.shared.redisdb import client as get_redis_client
+from searx.redislib import lua_script_storage
+
+
+logger = logging.getLogger('searx.search.checker')
+
+
+def scheduler_function(start_after_from: int, start_after_to: int, every_from: int, every_to: int, callback: Callable):
+    """Run the checker periodically. The function never returns.
+
+    Parameters:
+    * start_after_from and start_after_to: when to call "callback" for the first on the Redis instance
+    * every_from and every_to: after the first call, how often to call "callback"
+
+    There is no issue:
+    * to call this function is multiple workers
+    * to kill workers at any time as long there is one at least one worker
+    """
+    scheduler_now_script = importlib.resources.read_text(__package__, "scheduler.lua")
+    while True:
+        # ask the Redis script what to do
+        # the script says
+        # * if the checker must run now.
+        # * how to long to way before calling the script again (it can be call earlier, but not later).
+        script = lua_script_storage(get_redis_client(), scheduler_now_script)
+        call_now, wait_time = script(args=[start_after_from, start_after_to, every_from, every_to])
+
+        # does the worker run the checker now?
+        if call_now:
+            # run the checker
+            try:
+                callback()
+            except Exception:  # pylint: disable=broad-except
+                logger.exception("Error calling the embedded checker")
+            # only worker display the wait_time
+            logger.info("Next call to the checker in %s seconds", wait_time)
+        # wait until the next call
+        time.sleep(wait_time)