MDEV-39196: SELECT from information schema fails when FederatedX loses underlying table

When a remote table is unavailable, FederatedX was passing a hard error back to the SQL layer, causing INFORMATION_SCHEMA queries to abort entirely.

This patch intercepts the remote error in ha_federatedx::info, downgrades it to a warning using push_warning_printf, and includes the local table name in the warning message so the user knows which table is inaccessible.

Signed-off-by: Anway Durge <124391429+itzanway@users.noreply.github.com>

Author: itzanway

mysql-test/suite/federated/federatedx_mdev39196.test

@@ -0,0 +1,28 @@
+--source include/have_plugin_auth.inc
+--source include/have_federatedx.inc
+
+--disable_warnings
+DROP TABLE IF EXISTS t, fed_t;
+DROP SERVER IF EXISTS fedlink;
+--enable_warnings
+
+--replace_result $MASTER_MYPORT MASTER_MYPORT
+--eval create server fedlink foreign data wrapper mysql options (host "127.0.0.1", database "test", user "root", port $MASTER_MYPORT)
+ 
+CREATE TABLE t AS SELECT 1 AS a;
+CREATE TABLE fed_t (a INT) ENGINE=FEDERATED CONNECTION = 'fedlink/t';
+ 
+--echo # Querying Information Schema before dropping the remote table
+SELECT TABLE_NAME, ENGINE FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_SCHEMA = 'test' AND TABLE_NAME IN ('t', 'fed_t');
+
+DROP TABLE t;
+
+--echo # Querying Information Schema AFTER dropping the remote table (this used to crash)
+SELECT TABLE_NAME, ENGINE FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_SCHEMA = 'test' AND TABLE_NAME IN ('t', 'fed_t');
+
+--echo # Verify that a warning was pushed
+SHOW WARNINGS;
+
+--echo # Cleanup
+DROP TABLE fed_t;
+DROP SERVER fedlink;

storage/federatedx/ha_federatedx.cc

@@ -3139,13 +3139,21 @@ int ha_federatedx::info(uint flag)
 error:
   if (iop && *iop)
   {
-    my_printf_error((*iop)->error_code(), "Received error: %d : %s", MYF(0),
-                    (*iop)->error_code(), (*iop)->error_str());
+    /* Downgrade to warning, include the table name, and continue */
+    push_warning_printf(thd, Sql_condition::WARN_LEVEL_WARN,
+                        ER_QUERY_ON_FOREIGN_DATA_SOURCE,
+                        "Table '%s' is inaccessible. Received error: %d : %s",
+                        share->table_name, (*iop)->error_code(), (*iop)->error_str());
+    error_code= 0;
   }
   else if (remote_error_number != -1 /* error already reported */)
   {
     error_code= remote_error_number;
-    my_error(error_code, MYF(0), ER_THD(thd, error_code));
+    push_warning_printf(thd, Sql_condition::WARN_LEVEL_WARN,
+                        error_code,
+                        "Table '%s' is inaccessible. Error: %s",
+                        share->table_name, ER_THD(thd, error_code));
+    error_code= 0;
   }
 fail:
   tmp_txn->release(&tmp_io);