[sr-dev] db_redis shared tcp connection issue

[Andrew Pogrebennyk]

Dear community,
I've been looking at some weirdness in db_redis behavior when it returns
the responses to the queries made by tcp processes in mixed order.
Tested this on various kamailio 5.3 and 5.4 (sipwise spce) and they are
showing interesting pattern.
After restart of kamailio I ran lsof to enumerate all the sockets open in
kamailio children.
There is a connection to db port 6379 which is held by multiple processes
at the same time.

> for i in $(ps auxww | grep kamailio.proxy | grep -v grep | awk '{print
> $2}'); do echo "print file descriptors of $i" && sudo lsof -p $i | grep
> 6379; done > redis_conn.txt

...i see that lsof lists tcp client socket to redis server with same source
TCP port and same inode number in several processes:

  14199,   "TIMER NH",
  14200,  "ctl handler",
  14205,  "Dialog Clean Timer",
  14206,  "JSONRPCS FIFO",
  14210,  "JSONRPCS DATAGRAM",
  14213,  "tcp receiver (generic) child=0",
  14214,  "tcp receiver (generic) child=1",
  14215,  "tcp receiver (generic) child=2",
  14220,  "tcp receiver (generic) child=3",
  14224,  "tcp receiver (generic) child=4",
  14225,  "tcp main process"

The UDP processes are safe (and some timer ones too), because in that lsof
they have unique TCP client port.

That's giving me a lot of headache because UA registrations received by any
of the TCP workers (or IPSec ones for that matter) are
randomly failing, because  if two processes made same query to DB in
parallel it is appearing on the wire with same TCP source port and replies
can be mixed up.

This can be some bug in usage of hiredis, impacting all users of db_redis
module. Is there any relation to the way kamailio is working its TCP
workers, where maybe tcp workers are forked from the main attendant
processes after having opened the DB connection?

P.S. Why I have the above hypothesis: when I log redis queries with
redis-cli monitor at startup of kamailio, I see only that srem_key_lua is
executed against redis in runtime only once from that source port, but then
this connection is shared across multiple processes.

Regards,
Andrew
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