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/__init__.py

@@ -2,3 +2,5 @@
 
 from .impl import Checker
 from .background import initialize, get_result
+
+__all__ = ('Checker', 'initialize', 'get_result')

searx/search/checker/background.py

@@ -1,26 +1,28 @@
 # SPDX-License-Identifier: AGPL-3.0-or-later
 # lint: pylint
 # pylint: disable=missing-module-docstring
-# pyright: strict
+# pyright: basic
 
 import json
-import random
 import time
 import threading
 import os
 import signal
-from typing import Dict, Union, List, Any, Tuple
+from typing import Dict, Union, List, Any, Tuple, Optional
 from typing_extensions import TypedDict, Literal
 
+import redis.exceptions
+
 from searx import logger, settings, searx_debug
+from searx.shared.redisdb import client as get_redis_client
 from searx.exceptions import SearxSettingsException
 from searx.search.processors import PROCESSORS
 from searx.search.checker import Checker
-from searx.shared import schedule, storage  # pyright: ignore
+from searx.search.checker.scheduler import scheduler_function
 
 
-CHECKER_RESULT = 'CHECKER_RESULT'
-running = threading.Lock()
+REDIS_RESULT_KEY = 'SearXNG_checker_result'
+REDIS_LOCK_KEY = 'SearXNG_checker_lock'
 
 
 CheckerResult = Union['CheckerOk', 'CheckerErr', 'CheckerOther']
@@ -77,20 +79,24 @@ def _get_interval(every: Any, error_msg: str) -> Tuple[int, int]:
     return (every[0], every[1])
 
 
-def _get_every():
-    every = settings.get('checker', {}).get('scheduling', {}).get('every', (300, 1800))
-    return _get_interval(every, 'checker.scheduling.every is not a int or list')
-
-
 def get_result() -> CheckerResult:
-    serialized_result = storage.get_str(CHECKER_RESULT)
-    if serialized_result is not None:
-        return json.loads(serialized_result)
-    return {'status': 'unknown'}
+    client = get_redis_client()
+    if client is None:
+        # without Redis, the checker is disabled
+        return {'status': 'disabled'}
+    serialized_result: Optional[bytes] = client.get(REDIS_RESULT_KEY)
+    if serialized_result is None:
+        # the Redis key does not exist
+        return {'status': 'unknown'}
+    return json.loads(serialized_result)
 
 
 def _set_result(result: CheckerResult):
-    storage.set_str(CHECKER_RESULT, json.dumps(result))
+    client = get_redis_client()
+    if client is None:
+        # without Redis, the function does nothing
+        return
+    client.set(REDIS_RESULT_KEY, json.dumps(result))
 
 
 def _timestamp():
@@ -98,41 +104,29 @@ def _timestamp():
 
 
 def run():
-    if not running.acquire(blocking=False):  # pylint: disable=consider-using-with
-        return
     try:
-        logger.info('Starting checker')
-        result: CheckerOk = {'status': 'ok', 'engines': {}, 'timestamp': _timestamp()}
-        for name, processor in PROCESSORS.items():
-            logger.debug('Checking %s engine', name)
-            checker = Checker(processor)
-            checker.run()
-            if checker.test_results.successful:
-                result['engines'][name] = {'success': True}
-            else:
-                result['engines'][name] = {'success': False, 'errors': checker.test_results.errors}
+        # use a Redis lock to make sure there is no checker running at the same time
+        # (this should not happen, this is a safety measure)
+        with get_redis_client().lock(REDIS_LOCK_KEY, blocking_timeout=60, timeout=3600):
+            logger.info('Starting checker')
+            result: CheckerOk = {'status': 'ok', 'engines': {}, 'timestamp': _timestamp()}
+            for name, processor in PROCESSORS.items():
+                logger.debug('Checking %s engine', name)
+                checker = Checker(processor)
+                checker.run()
+                if checker.test_results.successful:
+                    result['engines'][name] = {'success': True}
+                else:
+                    result['engines'][name] = {'success': False, 'errors': checker.test_results.errors}
 
-        _set_result(result)
-        logger.info('Check done')
+            _set_result(result)
+            logger.info('Check done')
+    except redis.exceptions.LockError:
+        _set_result({'status': 'error', 'timestamp': _timestamp()})
+        logger.exception('Error while running the checker')
     except Exception:  # pylint: disable=broad-except
         _set_result({'status': 'error', 'timestamp': _timestamp()})
         logger.exception('Error while running the checker')
-    finally:
-        running.release()
-
-
-def _run_with_delay():
-    every = _get_every()
-    delay = random.randint(0, every[1] - every[0])
-    logger.debug('Start checker in %i seconds', delay)
-    time.sleep(delay)
-    run()
-
-
-def _start_scheduling():
-    every = _get_every()
-    if schedule(every[0], _run_with_delay):
-        run()
 
 
 def _signal_handler(_signum: int, _frame: Any):
@@ -147,27 +141,31 @@ def initialize():
         logger.info('Send SIGUSR1 signal to pid %i to start the checker', os.getpid())
         signal.signal(signal.SIGUSR1, _signal_handler)
 
-    # disabled by default
-    _set_result({'status': 'disabled'})
-
     # special case when debug is activate
-    if searx_debug and settings.get('checker', {}).get('off_when_debug', True):
+    if searx_debug and settings['checker']['off_when_debug']:
         logger.info('debug mode: checker is disabled')
         return
 
     # check value of checker.scheduling.every now
-    scheduling = settings.get('checker', {}).get('scheduling', None)
+    scheduling = settings['checker']['scheduling']
     if scheduling is None or not scheduling:
         logger.info('Checker scheduler is disabled')
         return
 
-    #
-    _set_result({'status': 'unknown'})
+    # make sure there is a Redis connection
+    if get_redis_client() is None:
+        logger.error('The checker requires Redis')
+        return
 
-    start_after = scheduling.get('start_after', (300, 1800))
-    start_after = _get_interval(start_after, 'checker.scheduling.start_after is not a int or list')
-    delay = random.randint(start_after[0], start_after[1])
-    logger.info('Start checker in %i seconds', delay)
-    t = threading.Timer(delay, _start_scheduling)
+    # start the background scheduler
+    every_range = _get_interval(scheduling.get('every', (300, 1800)), 'checker.scheduling.every is not a int or list')
+    start_after_range = _get_interval(
+        scheduling.get('start_after', (300, 1800)), 'checker.scheduling.start_after is not a int or list'
+    )
+    t = threading.Thread(
+        target=scheduler_function,
+        args=(start_after_range[0], start_after_range[1], every_range[0], every_range[1], run),
+        name='checker_scheduler',
+    )
     t.daemon = True
     t.start()

searx/search/checker/scheduler.lua

@@ -0,0 +1,36 @@
+-- SPDX-License-Identifier: AGPL-3.0-or-later
+--
+-- This script is not a string in scheduler.py, so editors can provide syntax highlighting.
+
+-- The Redis KEY is defined here and not in Python on purpose:
+-- only this LUA script can read and update this key to avoid lock and concurrency issues.
+local redis_key = 'SearXNG_checker_next_call_ts'
+
+local now = redis.call('TIME')[1]
+local start_after_from = ARGV[1]
+local start_after_to = ARGV[2]
+local every_from = ARGV[3]
+local every_to = ARGV[4]
+
+local next_call_ts = redis.call('GET', redis_key)
+
+if (next_call_ts == false or next_call_ts == nil) then
+    -- the scheduler has never run on this Redis instance, so:
+    -- 1/ the scheduler does not run now
+    -- 2/ the next call is a random time between start_after_from and start_after_to
+    local delay = start_after_from + math.random(start_after_to - start_after_from)
+    redis.call('SET', redis_key, now + delay)
+    return { false, delay }
+end
+
+-- next_call_ts is defined
+-- --> if now is lower than next_call_ts then we don't run the embedded checker
+-- --> if now is higher then we update next_call_ts and ask to run the embedded checker now.
+local call_now = next_call_ts <= now
+if call_now then
+    -- the checker runs now, define the timestamp of the next call:
+    -- this is a random delay between every_from and every_to
+    local periodic_delay = every_from + math.random(every_to - every_from)
+    next_call_ts = redis.call('INCRBY', redis_key, periodic_delay)
+end
+return { call_now, next_call_ts - now }

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)