ssl_mode maps to libmariadb's MARIADB_OPT_SKIP_READ_RESPONSE: SSL_MODE_DISABLED is a no-op, other modes corrupt the protocol

[quinnj]

Summary

The ssl_mode connect kwarg does nothing it claims to do: API.MYSQL_OPT_SSL_MODE does not exist in libmariadb. MySQL.jl's mysql_option enum places it at an ordinal that collides with libmariadb's MARIADB_OPT_SKIP_READ_RESPONSE, so:

• ssl_mode = SSL_MODE_DISABLED (0) is a silent no-op — TLS stays on;
• ssl_mode = SSL_MODE_PREFERRED / REQUIRED / VERIFY_CA / VERIFY_IDENTITY (1–4) actually sets MARIADB_OPT_SKIP_READ_RESPONSE = true, telling the client to stop reading server responses — i.e. asking for more TLS quietly corrupts the connection protocol instead.

This is why the reporter in #234 found ssl_mode = MySQL.API.SSL_MODE_DISABLED "did not resolve the issue", and it also means workloads that believe they've disabled TLS are still running it (relevant to the crash in #220, which is TLS-sensitive).

Details

MySQL.jl's enum (src/api/consts.jl#L219-L224) ends:

 MARIADB_OPT_IO_WAIT # 7024
 MYSQL_OPT_SSL_MODE # 7025
end

libmariadb's mysql.h (MariaDB_Connector_C_jll 3.4.9, identical prefix through 7024) continues instead:

 MARIADB_OPT_IO_WAIT, /* 7024 */
 MARIADB_OPT_SKIP_READ_RESPONSE, /* 7025 <- what MySQL.jl's MYSQL_OPT_SSL_MODE hits */
 MARIADB_OPT_RESTRICTED_AUTH,
 ...

There is no MYSQL_OPT_SSL_MODE anywhere in libmariadb — it's a MySQL-Connector/C-only option.

API.setoption sends it as a Ref{Cuint} (src/api/capi.jl#L971-L974, MYSQL_OPT_SSL_MODE ∈ CUINTOPTS), and libmariadb reads one byte of it (mariadb_lib.c v3.4.9, line 3878):

 case MARIADB_OPT_SKIP_READ_RESPONSE:
 OPT_SET_EXTENDED_VALUE_INT(&mysql->options, skip_read_response, *(my_bool *)arg1);
 break;

so on little-endian the enum value 0 → no-op, 1–4 → skip_read_response = true.

Reproduction

Against a stock mysql:8 container (TLS-capable):

using MySQL, DBInterface
conn = DBInterface.connect(MySQL.Connection, "127.0.0.1", "root", "";
 db="test", port=3306, ssl_mode=MySQL.API.SSL_MODE_DISABLED)
first(DBInterface.execute(conn, "SHOW STATUS LIKE 'Ssl_cipher'")).Value
# "TLS_AES_256_GCM_SHA384" <- TLS very much not disabled

Related behavior worth documenting while fixing this

With MariaDB_Connector_C_jll 3.4.x there is no way to force a plaintext connection to a TLS-capable server at all: libmariadb 3.4 sets use_ssl by default, and the only escape hatch (plugins/auth/my_auth.c) is the plaintext fallback for servers that don't offer TLS, gated on tls_allow_invalid_server_cert:

if (mysql->options.use_ssl && !(mysql->server_capabilities & CLIENT_SSL))
{
 if (!mysql->options.extension->tls_allow_invalid_server_cert || ...)
 my_set_error(..., "SSL is required, but the server does not support it");
}

i.e. ssl_verify_server_cert = false (the new default from #235) permits connecting to a no-TLS server, but nothing can turn TLS off when the server supports it. ssl_enforce = false doesn't help either. So true SSL_MODE_DISABLED semantics are simply not implementable on top of Connector/C 3.4.

Suggested fix

Remove the ssl_mode kwarg and the bogus MYSQL_OPT_SSL_MODE enum entry (or make passing it throw an ArgumentError explaining the situation and pointing at ssl_verify_server_cert). Anything is better than the current behavior, where SSL_MODE_DISABLED silently does nothing and SSL_MODE_REQUIRED silently breaks the wire protocol.

Environment: MySQL.jl 1.5.1 (enum unchanged on current main/1.5.2), MariaDB_Connector_C_jll 3.4.9+1, Julia 1.12.6, macOS aarch64, server mysql:8 (8.4.11).