Make sure ssl is enabled if only :sslverify is set

[rhafer]

Previously, when "sslverify: false/true" was the only ssl related
options passed to the constructor, the module skipped the call to
"mysql_ssl_set". It seems however that for some variants for the mysql
client libraries calling "mysql_ssl_set" is the only way to enable SSL
for the client connections. (E.g. the libraries shipped as part of
mariadb 10.1 still lack support for MYSQL_OPT_SSL_ENFORCE and
MYSQL_OPT_SSL_MODE)

This change allows enabling ssl with default values for all other
options by just passing "sslverify: true" or "sslverify: false" to the
constructor. (Depending on whether server certificate verification is
wanted or not)

[sodabrew]

Based on my reading of https://dev.mysql.com/doc/refman/5.7/en/mysql-ssl-set.html if all of the arguments to mysql_ssl_set are NULL, then it is a no-op. I haven't confirmed if the actual client code matches the documentation on this point.

Based on https://dev.mysql.com/doc/refman/5.7/en/c-api-encrypted-connections.html the way to enforce SSL is to call mysql_options(... MYSQL_OPT_SSL_MODE ... SSL_MODE_REQUIRED).

[rhafer]

Based on my reading of https://dev.mysql.com/doc/refman/5.7/en/mysql-ssl-set.html if all of the arguments to mysql_ssl_set are NULL, then it is a no-op. I haven't confirmed if the actual client code matches the documentation on this point.

This doesn't seem to be true for mariadb 10.1:
https://github.com/MariaDB/server/blob/10.1/sql-common/client.c#L1683

Even if it's called with all arguments NULL it still sets mysql->options.use_ssl= TRUE;

Based on https://dev.mysql.com/doc/refman/5.7/en/c-api-encrypted-connections.html the way to enforce SSL is to call mysql_options(... MYSQL_OPT_SSL_MODE ... SSL_MODE_REQUIRED).

Unfortunately mariadb 10.1 (which AFAIK is still officially maintained) doesn't have those options.

[sodabrew]

Makes sense now! Thanks for linking to the relevant line of code in the driver.

[jhesketh]

lgtm;

[rhafer]

@sodabrew Hi. Is there any chance to get this merged?

[sodabrew]

This will turn on ssl even if the only flag is sslverify: false which doesn't quite make sense to me.

[sodabrew]

I think given #879 that ssl_set should be called if any value of :ssl_mode is set, too.