Add support for MySQL UNIX Socket connections

[NormB]

Summary

This PR adds support for MySQL UNIX Socket connections.

Details

OpenSIPS traditionally communicates with MySQL using the network stack. Often times OpenSIPS runs within a Docker container. When MySQL also runs within a Docker container on the same server a performance improvement can be achieved by removing the network stack communication and replacing it with UNIX sockets.

Solution

MySQL supports UNIX socket communication and its C API exposes the mysql_real_connect() function contains a "unix_socket" parameter. Prior to this PR, the "unix_socket" parameter is hard-coded to NULL indicating that is is not used.

This PR enhances the db_url parsing logic to add the ability to specify a UNIX Socket named path. When a UNIX Socket is specified, the "host" and "port" parameters are no longer parsed by the db_url parsing logic. Note that when a UNIX Socket is specified, the MySQL documentation indicates that the "host" must be set to NULL or "localhost". This PR sets the "host" to "localhost" when a UNIX Socket is specified.

The db_url parsing will recognize the following db_url modparam as shown in this example:

modparam("usrloc", "db_url", "mysql://opensips:opensipsrw@unix(/var/run/mysqld/mysqld.sock)/opensips)

The UNIX Socket is defined in the same place that previously would have been the "host:port". The rest of the db_url syntax remains unchanged.

To use a MySQL UNIX Socket in a Docker containerized environment, the following example settings can be made:

Update the MySQL my.cnf file to use a UNIX Socket:

[mysqld]
socket=/var/run/mysqld/mysqld.sock

Update docker-compose.yml to share the UNIX Socket between the MySQL and OpenSIPS services:

volumes:
shared-sockets:
driver: local

services:
mysql:
volumes:
- shared-sockets:/var/run/mysqld

opensips:
volumes:
- shared-sockets:/var/run/mysqld

Define the UNIX Socket in opensips.cfg:

modparam("usrloc", "db_url", "mysql://opensips:opensipsrw@unix(/var/run/mysqld/mysqld.sock)/opensips)

Compatibility

This is a new feature so there are no backward compatibility issues.

The db_url parsing has been enhanced to support the UNIX Socket specification. If not specified, the existing db_url parsing logic will be used and remains unchanged.

Closing issues

[liviuchircu]

Good work, @NormB! I will merge right away -- it remains to be seen whether this introduces any (small) breakage in the regular DB URL parser, but it doesn't seem to be the case. Worst case, if there is a breakage, we should also add a few unit tests under a new db/test/ directory, to be sure we lock in the expected parsing behavior instead of just fixing the breakage.

[NormB]

db_url_tests1.patch
db_url_tests2.patch

These two patches implement some db_url parsing tests.

The tests include typical MySQL URLs with and without UNIX Socket settings.

To make the tests pass, I have added reasonable "default" values when they are not able to be determined from parsing the URL. These defaults have added a bit of code/logic to db_id.c.

I have a couple of comments:

1. Specifying default values instead of db_url parsing failures are much more user friendly than what we have today. Others may not hold the same opinion.
2. The tests are pretty simplistic. They start with a fully formed URL and then the URL is trimmed from the right at important "delimiter" locations.
3. Many more tests are possible, for example specifying all the values, except for a password (ie, a value from the middle of the URI). Yes, MySQL users without passwords are legal (not wise, but legal).
4. So, I don't know where to move forward with this. On one hand, I could submit a PR, but the logic updates to db_id.c around default values is something I think you and the rest of the team should come to an agreement on first.