Support ssl_mode setting with mariadb client (#475)

According to https://mariadb.com/kb/en/mysql_optionsv/ MariaDB supports
TLS enforcing in its own way. So the idea behind this PR is to keep the
same interface for MariaDB based clients, but behind the scenes handle
it accordingly.(MariaDB gets its own args set, instead of ssl_mode dict
supported by MySQL).

Co-authored-by: Teodor Moroz <teodor.moroz@openvpn.net>
Co-authored-by: Inada Naoki <songofacandy@gmail.com>

MySQLdb/_mysql.c

@@ -380,7 +380,14 @@ static int _mysql_ResultObject_clear(_mysql_ResultObject *self)
     return 0;
 }
 
-#ifdef HAVE_ENUM_MYSQL_OPT_SSL_MODE
+enum {
+    SSLMODE_DISABLED = 1,
+    SSLMODE_PREFERRED = 2,
+    SSLMODE_REQUIRED = 3,
+    SSLMODE_VERIFY_CA = 4,
+    SSLMODE_VERIFY_IDENTITY = 5
+};
+
 static int
 _get_ssl_mode_num(char *ssl_mode)
 {
@@ -395,7 +402,6 @@ _get_ssl_mode_num(char *ssl_mode)
     }
     return -1;
 }
-#endif
 
 static int
 _mysql_ConnectionObject_Initialize(
@@ -429,6 +435,7 @@ _mysql_ConnectionObject_Initialize(
     int read_timeout = 0;
     int write_timeout = 0;
     int compress = -1, named_pipe = -1, local_infile = -1;
+    int ssl_mode_num = SSLMODE_DISABLED;
     char *init_command=NULL,
          *read_default_file=NULL,
          *read_default_group=NULL,
@@ -469,15 +476,10 @@ _mysql_ConnectionObject_Initialize(
         _stringsuck(cipher, value, ssl);
     }
     if (ssl_mode) {
-#ifdef HAVE_ENUM_MYSQL_OPT_SSL_MODE
-        if (_get_ssl_mode_num(ssl_mode) <= 0) {
+        if ((ssl_mode_num = _get_ssl_mode_num(ssl_mode)) <= 0) {
             PyErr_SetString(_mysql_NotSupportedError, "Unknown ssl_mode specification");
             return -1;
         }
-#else
-        PyErr_SetString(_mysql_NotSupportedError, "MySQL client library does not support ssl_mode specification");
-        return -1;
-#endif
     }
 
     conn = mysql_init(&(self->connection));
@@ -487,6 +489,7 @@ _mysql_ConnectionObject_Initialize(
     }
     Py_BEGIN_ALLOW_THREADS ;
     self->open = 1;
+
     if (connect_timeout) {
         unsigned int timeout = connect_timeout;
         mysql_options(&(self->connection), MYSQL_OPT_CONNECT_TIMEOUT,
@@ -521,12 +524,23 @@ _mysql_ConnectionObject_Initialize(
     if (ssl) {
         mysql_ssl_set(&(self->connection), key, cert, ca, capath, cipher);
     }
-#ifdef HAVE_ENUM_MYSQL_OPT_SSL_MODE
     if (ssl_mode) {
-        int ssl_mode_num = _get_ssl_mode_num(ssl_mode);
+#ifdef HAVE_ENUM_MYSQL_OPT_SSL_MODE
         mysql_options(&(self->connection), MYSQL_OPT_SSL_MODE, &ssl_mode_num);
-    }
+#else
+        // MariaDB doesn't support MYSQL_OPT_SSL_MODE.
+        // See https://github.com/PyMySQL/mysqlclient/issues/474
+        // TODO: Does MariaDB supports PREFERRED and VERIFY_CA?
+        // We support only two levels for now.
+        if (sslmode_num >= SSLMODE_REQUIRED) {
+            mysql_optionsv(&(self->connection), MYSQL_OPT_SSL_ENFORCE, (void *)&enforce_tls);
+        }
+        if (sslmode_num >= SSLMODE_VERIFY_CA) {
+            mysql_optionsv(&(self->connection), MYSQL_OPT_SSL_VERIFY_SERVER_CERT, (void *)&enforce_tls);
+        }
 #endif
+    }
+
     if (charset) {
         mysql_options(&(self->connection), MYSQL_SET_CHARSET_NAME, charset);
     }