Variable references like @@GLOBAL.sql_mode cause syntax errors

[brandonpayton]

An example query is:

SELECT @@session.max_allowed_packet

Within WordPress Playground this is leading to an error message like:

SQLSTATE[HY000]: General error: 1 unrecognized token: "@"

Related to WordPress/wordpress-playground#1272

[brandonpayton]

Plan: Follow up with a PR containing a failing test and then work on a fix.

[brandonpayton]

Some notes from @adamziel:

In the SQLite plugin we should either:

• Inspect each consumed token to see if it's a SQL variable, like @@session.max_allowed_packet, and then substitute it for something SQLite understand. Upside: It would work for all queries. Downside: Consuming a token could potentially consume three, if @@session, ., and max_allowed_packet are all separate tokens. Because of that, it could be better to...
• Do a second pass over parsed queries (or maybe just SELECT queries?) to substitute these variables with values. This could potentially use regexps or strcspn as strings should be replaced with :parameters at that stage, although that requires double checking if @ signs can be legally used outside of strings and variable names.
• Is there even a third solution?

In v1 we could use a hardcoded mapping of variable -> value. In v2 we could create a SQLite database table and source the data from there, also supporting SET.

(from this GH comment)

Random thought:
Would there be any value in allowing the default values of these variables to be filterable in WP?

If so, we'd need to be careful not to override values assigned via SET.

[brandonpayton]

Notes about @@ references in MariaDB SQL parsing

To help figure out whether there is any additional SQL syntax that includes @@, I ran the preprocessor to remove inline comments from MariaDB SQL parsing to reduce the number of references:

for FILE in $(ls ../sql); do echo "$FILE"; gcc-13 -fpreprocessed -dD -E "../sql/$FILE" > "$FILE"; done

And the following references remained under the mariadb/sql dir:

sys_vars.cc:       DEPRECATED("'@@debug_dbug'"));
sys_vars.cc:       DEPRECATED("'@@mrr_buffer_size'"));
sys_vars.cc:   "@@skip_replication=1 on the master. Default REPLICATE (no events are "
sys_vars.cc:   "@@skip_replication=1 will be filtered on the master and never be sent to "
sys_vars.cc:       "storage_engine", "Alias for @@default_storage_engine. Deprecated",
sys_vars.cc:       "@@skip_replication flag set. Such events will not be replicated by "
sys_vars.cc:       DEPRECATED("'@@wsrep_sync_wait=1'"));
log_event.cc:    my_b_printf(file,"SET @@session.pseudo_thread_id=%lu%s\n",
log_event.cc:                       "@@session.foreign_key_checks", &need_comma);
log_event.cc:                       "@@session.sql_auto_is_null", &need_comma);
log_event.cc:                       "@@session.unique_checks", &need_comma);
log_event.cc:                       "@@session.autocommit", &need_comma);
log_event.cc:                       "@@session.check_constraint_checks", &need_comma);
log_event.cc:    my_b_printf(file,"SET @@session.sql_mode=%s%s\n",
log_event.cc:    my_b_printf(file,"SET @@session.auto_increment_increment=%lu, @@session.auto_increment_offset=%lu%s\n",
log_event.cc:                "@@session.character_set_client=%d,"
log_event.cc:                "@@session.collation_connection=%d,"
log_event.cc:                "@@session.collation_server=%d"
log_event.cc:      my_b_printf(file,"SET @@session.time_zone='%s'%s\n",
log_event.cc:    my_b_printf(file, "SET @@session.lc_time_names=%d%s\n",
log_event.cc:      my_b_printf(file, "SET @@session.collation_database=%d%s\n",
log_event.cc:      my_b_printf(file, "SET @@session.collation_database=DEFAULT%s\n",
log_event.cc:    my_b_printf(&cache,"%sSET @@session.pseudo_thread_id=%lu%s\n",
log_event.cc:                  "/*!100101 SET @@session.skip_parallel_replication=%u*/%s\n",
log_event.cc:      my_b_printf(&cache, "/*!100001 SET @@session.gtid_domain_id=%u*/%s\n",
log_event.cc:      my_b_printf(&cache, "/*!100001 SET @@session.server_id=%u*/%s\n",
log_event.cc:      my_b_printf(&cache, "/*!100001 SET @@session.gtid_seq_no=%s*/%s\n",
log_event.cc:  my_b_printf(&cache, "SET @@RAND_SEED1=%s, @@RAND_SEED2=%s%s\n",
set_var.cc:    strxnmov(buf1, sizeof(buf1)-1, "@@", name.str, 0);
item_func.cc:    str->append(STRING_WITH_LEN("@@"));
item_func.cc:  return mark_unsupported_function("@@", var->name.str, arg, VCOL_SESSION_FUNC);
sql_table.cc:    memcpy(to + length, "@@@", 4);
sql_parse.cc:    end= strxmov(buff, "@@session.", var_name, NullS);
sql_mode.cc:                          "Expression depends on the @@%s value %s",
slave.cc:                          STRING_WITH_LEN("SELECT @@GLOBAL.COLLATION_SERVER")) &&
slave.cc:    if (!mysql_real_query(mysql, STRING_WITH_LEN("SELECT @@GLOBAL.TIME_ZONE")) &&
slave.cc:    const char query[]= "SET @master_binlog_checksum= @@global.binlog_checksum";
slave.cc:                   "Setting master-side filtering of @@skip_replication failed "
slave.cc:                            "of @@skip_replication events."));
slave.cc:          "with the @@skip_replication flag.";
slave.cc:                         STRING_WITH_LEN("SELECT @@GLOBAL.gtid_domain_id")) ||
slave.cc:                   "Get master @@GLOBAL.gtid_domain_id failed with error: %s",
slave.cc:          "it tries to SELECT @@GLOBAL.gtid_domain_id.";

Planning to review these matches later.