Add support for volatile keys

[ArnoStiefvater]

What:

Add support for volatile keys. Add timeout in seconds for keys to expire.
Depends on greenbone/gvm-libs#460.

Why:

Can be used in combination with redis settings such as maxmemory and maxmemory-policy to have a more options in handling memory issues in regards to redis.

How:

Stop ospd-openvas if running.
Do a flushall in redis. sudo redis-cli -s /run/redis-openvas/redis.sock FLUSHALL)
Start nasl script which places keys with expire in redis. sudo openvas-nasl -X -i <installpath>/var/lib/openvas/plugins/ -t 127.0.0.1 add_volatile_keys.nasl

include("misc_func.inc"); # for rand_str()

i = 0;
key  = "VOLATILE";
value = rand_str(length: 100000);
for (; i<100; i++) {
    set_kb_item(name: key+i, value:value+i, expire:1000000);
}

sleep(10000);
exit(0);

Check number of keys in db. sudo redis-cli -s /run/redis-openvas/redis.sock -n 1 DBSIZE. Should be 100.
Set maxmemory config set maxmemory 15728640 in redis.
Set maxmemory-policy Config set maxmemory-policy volatile-lru in redis.
Start VT2 which places keys without expire set in other terminal. sudo openvas-nasl -X -i <installpath>/var/lib/openvas/plugins/ -t 127.0.0.1 persistent_keys.nasl.

include("misc_func.inc"); # for rand_str()

i = 0;
key  = "PERSISTENT";
value = rand_str(length: 100000);
for (; i<100; i++ ) {
    set_kb_item(name: key+i, value:value);
}

sleep(10000);

When maxmemory is hit the keys with expire set will get removed by redis to make space for new keys. Check number of keys in db 1 and 2. db 2 should have 100 keys while db 1 should have less.
This way we can make sure which keys can be removed when memory limits are reached.

Checklist:

• Tests
• CHANGELOG Entry

[jjnicola]

LGTM.
Do you think it makes sense to set some default values for maxmemory and maxmemory-policy in config/redis-openvas.conf?
Also, it would be nice to document that there is some kind of support/usage of those redis option in doc/redis_config.md

[ArnoStiefvater]

LGTM.
Do you think it makes sense to set some default values for maxmemory and maxmemory-policy in config/redis-openvas.conf?
Also, it would be nice to document that there is some kind of support/usage of those redis option in doc/redis_config.md

I would not set a default for maxmemory because we have no possibility knowing the limitation of the machines. This has to be decided individually for the machine at hand. maxmemory-policy does not make sense without maxmemory.

Will add documentation in doc/redis_config.md in a different PR.