[mod] collection of redis functions and lua scripts (initial)

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

docs/conf.py

@@ -139,6 +139,7 @@ intersphinx_mapping = {
     "jinja": ("https://jinja.palletsprojects.com/", None),
     "linuxdoc" : ("https://return42.github.io/linuxdoc/", None),
     "sphinx" : ("https://www.sphinx-doc.org/en/master/", None),
+    "redis": ('https://redis.readthedocs.io/en/stable/', None),
 }
 
 issues_github_path = "searxng/searxng"

docs/src/searx.redislib.rst

@@ -0,0 +1,8 @@
+.. _searx.redis:
+
+=============
+Redis Library
+=============
+
+.. automodule:: searx.redislib
+  :members:

searx/redislib.py

@@ -0,0 +1,213 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+# lint: pylint
+"""A collection of convenient functions and redis/lua scripts.
+
+This code was partial inspired by the `Bullet-Proofing Lua Scripts in RedisPy`_
+article.
+
+.. _Bullet-Proofing Lua Scripts in RedisPy:
+   https://redis.com/blog/bullet-proofing-lua-scripts-in-redispy/
+
+"""
+
+import hmac
+
+from searx import get_setting
+
+PURGE_BY_PREFIX = """
+local prefix = tostring(ARGV[1])
+for i, name in ipairs(redis.call('KEYS', prefix .. '*')) do
+    redis.call('EXPIRE', name, 0)
+end
+"""
+
+
+def purge_by_prefix(client, prefix: str = "SearXNG_"):
+    """Purge all keys with ``prefix`` from database.
+
+    Queries all keys in the database by the given prefix and set expire time to
+    zero.  The default prefix will drop all keys which has been set by SearXNG
+    (drops SearXNG schema entirely from database).
+
+    The implementation is the lua script from string :py:obj:`PURGE_BY_PREFIX`.
+    The lua script uses EXPIRE_ instead of DEL_: if there are a lot keys to
+    delete and/or their values are big, `DEL` could take more time and blocks
+    the command loop while `EXPIRE` turns back immediate.
+
+    :param prefix: prefix of the key to delete (default: ``SearXNG_``)
+    :type name: str
+
+    .. _EXPIRE: https://redis.io/commands/expire/
+    .. _DEL: https://redis.io/commands/del/
+
+    """
+    script = client.register_script(PURGE_BY_PREFIX)
+    script(args=[prefix])
+
+
+def secret_hash(name: str):
+    """Creates a hash of the ``name``.
+
+    Combines argument ``name`` with the ``secret_key`` from :ref:`settings
+    server`.  This function can be used to get a more anonymised name of a Redis
+    KEY.
+
+    :param name: the name to create a secret hash for
+    :type name: str
+    """
+    m = hmac.new(bytes(name, encoding='utf-8'), digestmod='sha256')
+    m.update(bytes(get_setting('server.secret_key'), encoding='utf-8'))
+    return m.hexdigest()
+
+
+INCR_COUNTER = """
+local limit = tonumber(ARGV[1])
+local expire = tonumber(ARGV[2])
+local c_name = KEYS[1]
+
+local c = redis.call('GET', c_name)
+
+if not c then
+    c = redis.call('INCR', c_name)
+    if expire > 0 then
+        redis.call('EXPIRE', c_name, expire)
+    end
+else
+    c = tonumber(c)
+    if limit == 0 or c < limit then
+       c = redis.call('INCR', c_name)
+    end
+end
+return c
+"""
+
+
+def incr_counter(client, name: str, limit: int = 0, expire: int = 0):
+    """Increment a counter and return the new value.
+
+    If counter with redis key ``SearXNG_counter_<name>`` does not exists it is
+    created with initial value 1 returned.  The replacement ``<name>`` is a
+    *secret hash* of the value from argument ``name`` (see
+    :py:func:`secret_hash`).
+
+    The implementation of the redis counter is the lua script from string
+    :py:obj:`INCR_COUNTER`.
+
+    :param name: name of the counter
+    :type name: str
+
+    :param expire: live-time of the counter in seconds (default ``None`` means
+      infinite).
+    :type expire: int / see EXPIRE_
+
+    :param limit: limit where the counter stops to increment (default ``None``)
+    :type limit: int / limit is 2^64 see INCR_
+
+    :return: value of the incremented counter
+    :type return: int
+
+    .. _EXPIRE: https://redis.io/commands/expire/
+    .. _INCR: https://redis.io/commands/incr/
+
+    A simple demo of a counter with expire time and limit::
+
+      >>> for i in range(6):
+      ...   i, incr_counter(client, "foo", 3, 5) # max 3, duration 5 sec
+      ...   time.sleep(1) # from the third call on max has been reached
+      ...
+      (0, 1)
+      (1, 2)
+      (2, 3)
+      (3, 3)
+      (4, 3)
+      (5, 1)
+
+    """
+    script = client.register_script(INCR_COUNTER)
+    name = "SearXNG_counter_" + secret_hash(name)
+    c = script(args=[limit, expire], keys=[name])
+    return c
+
+
+def drop_counter(client, name):
+    """Drop counter with redis key ``SearXNG_counter_<name>``
+
+    The replacement ``<name>`` is a *secret hash* of the value from argument
+    ``name`` (see :py:func:`incr_counter` and :py:func:`incr_sliding_window`).
+    """
+    name = "SearXNG_counter_" + secret_hash(name)
+    client.delete(name)
+
+
+INCR_SLIDING_WINDOW = """
+local expire = tonumber(ARGV[1])
+local name = KEYS[1]
+local current_time = redis.call('TIME')
+
+redis.call('ZREMRANGEBYSCORE', name, 0, current_time[1] - expire)
+redis.call('ZADD', name, current_time[1], current_time[1] .. current_time[2])
+local result = redis.call('ZCOUNT', name, 0, current_time[1] + 1)
+redis.call('EXPIRE', name, expire)
+return result
+"""
+
+
+def incr_sliding_window(client, name: str, duration: int):
+    """Increment a sliding-window counter and return the new value.
+
+    If counter with redis key ``SearXNG_counter_<name>`` does not exists it is
+    created with initial value 1 returned.  The replacement ``<name>`` is a
+    *secret hash* of the value from argument ``name`` (see
+    :py:func:`secret_hash`).
+
+    :param name: name of the counter
+    :type name: str
+
+    :param duration: live-time of the sliding window in seconds
+    :typeduration: int
+
+    :return: value of the incremented counter
+    :type return: int
+
+    The implementation of the redis counter is the lua script from string
+    :py:obj:`INCR_SLIDING_WINDOW`.  The lua script uses `sorted sets in Redis`_
+    to implement a sliding window for the redis key ``SearXNG_counter_<name>``
+    (ZADD_).  The current TIME_ is used to score the items in the sorted set and
+    the time window is moved by removing items with a score lower current time
+    minus *duration* time (ZREMRANGEBYSCORE_).
+
+    The EXPIRE_ time (the duration of the sliding window) is refreshed on each
+    call (incrementation) and if there is no call in this duration, the sorted
+    set expires from the redis DB.
+
+    The return value is the amount of items in the sorted set (ZCOUNT_), what
+    means the number of calls in the sliding window.
+
+    .. _Sorted sets in Redis:
+       https://redis.com/ebook/part-1-getting-started/chapter-1-getting-to-know-redis/1-2-what-redis-data-structures-look-like/1-2-5-sorted-sets-in-redis/
+    .. _TIME: https://redis.io/commands/time/
+    .. _ZADD: https://redis.io/commands/zadd/
+    .. _EXPIRE: https://redis.io/commands/expire/
+    .. _ZREMRANGEBYSCORE: https://redis.io/commands/zremrangebyscore/
+    .. _ZCOUNT: https://redis.io/commands/zcount/
+
+    A simple demo of the sliding window::
+
+      >>> for i in range(5):
+      ...   incr_sliding_window(client, "foo", 3) # duration 3 sec
+      ...   time.sleep(1) # from the third call (second) on the window is moved
+      ...
+      1
+      2
+      3
+      3
+      3
+      >>> time.sleep(3)  # wait until expire
+      >>> incr_sliding_window(client, "foo", 3)
+      1
+
+    """
+    script = client.register_script(INCR_SLIDING_WINDOW)
+    name = "SearXNG_counter_" + secret_hash(name)
+    c = script(args=[duration], keys=[name])
+    return c