data: engine descriptions: use SQLite instead of JSON

To reduce memory usage, use a SQLite database to store the engine descriptions.
A dump of the database is stored in Git to facilitate maintenance,
especially the pull requests made automatically every month.

Related to
* https://github.com/searxng/searxng/discussions/2633
* https://github.com/searxng/searxng/pull/3443

searx/data/__init__.py

@@ -13,15 +13,20 @@ __all__ = [
     'WIKIDATA_UNITS',
     'EXTERNAL_BANGS',
     'OSM_KEYS_TAGS',
-    'ENGINE_DESCRIPTIONS',
     'LOCALES',
     'ahmia_blacklist_loader',
+    'fetch_engine_descriptions',
 ]
 
 import json
+import sqlite3
+from contextlib import contextmanager
+from typing import Dict, Generator, List
 from pathlib import Path
 
+
 data_dir = Path(__file__).parent
+data_connection_local = {}
 
 
 def _load(filename):
@@ -29,6 +34,41 @@ def _load(filename):
         return json.load(f)
 
 
+@contextmanager
+def sql_connection(filename: str) -> Generator[sqlite3.Connection, None, None]:
+    """Return a read only SQLite connection to filename.
+    The filename is relative to searx/data
+
+    Multiple calls to this function in the same thread,
+    already return the same connection.
+    """
+    dict_id = filename
+    connection = data_connection_local.get(dict_id)
+    if connection is None:
+        data_filename = str(data_dir / filename)
+        # open database in read only mode and allow to share between threads
+        # https://www.sqlite.org/faq.html#q6
+        # see https://ricardoanderegg.com/posts/python-sqlite-thread-safety/
+        # and https://docs.python.org/3/library/sqlite3.html#sqlite3.threadsafety
+        #     sqlite3.threadsafety is hard coded to 1
+        # the only reliable way to check if multithreading is supported is to run this query
+        # SELECT * FROM pragma_compile_options WHERE compile_options LIKE 'THREADSAFE=%'
+        # but THREADSAFE=1 on Linux anyway
+        data_connection = sqlite3.connect(f'file:{data_filename}?mode=ro', uri=True, check_same_thread=False)
+        # 512KB of cache instead of 2MB (512KB / 4KB = 128, 4KB is the default page size)
+        # https://www.sqlite.org/pragma.html#pragma_cache_size
+        data_connection.execute("PRAGMA cache_size = 128;")
+        data_connection_local[dict_id] = data_connection
+    yield data_connection
+
+
+def fetch_engine_descriptions(language) -> Dict[str, List[str]]:
+    """Return engine description and source for each engine name."""
+    with sql_connection("engine_descriptions.db") as conn:
+        res = conn.execute("SELECT engine, description, source FROM engine_descriptions WHERE language=?", (language,))
+        return {result[0]: [result[1], result[2]] for result in res.fetchall()}
+
+
 def ahmia_blacklist_loader():
     """Load data from `ahmia_blacklist.txt` and return a list of MD5 values of onion
     names.  The MD5 values are fetched by::
@@ -48,6 +88,5 @@ EXTERNAL_URLS = _load('external_urls.json')
 WIKIDATA_UNITS = _load('wikidata_units.json')
 EXTERNAL_BANGS = _load('external_bangs.json')
 OSM_KEYS_TAGS = _load('osm_keys_tags.json')
-ENGINE_DESCRIPTIONS = _load('engine_descriptions.json')
 ENGINE_TRAITS = _load('engine_traits.json')
 LOCALES = _load('locales.json')

searx/data/dumps/README.rst

@@ -0,0 +1,3 @@
+Dumps of the SQLite files in ``searx.data``.
+
+These files are not used by SearXNG, they are here for reference.