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.result

@@ -0,0 +1,22 @@
+#
+# MDEV-39196: INFORMATION_SCHEMA query must succeed with a
+# warning when a FederatedX remote table is unreachable.
+#
+# Verify the federated table works before dropping remote table.
+SELECT * FROM federated.t1;
+id	name
+1	foo
+# Drop the remote table to simulate unreachable/missing table.
+# INFORMATION_SCHEMA query must succeed and issue a warning.
+SELECT TABLE_NAME, TABLE_ROWS
+  FROM information_schema.TABLES
+  WHERE TABLE_SCHEMA = 'federated'
+    AND TABLE_NAME = 't1';
+TABLE_NAME	TABLE_ROWS
+t1	NULL
+# Warning must be present.
+SHOW WARNINGS;
+Level	Code	Message
+Warning	1296	FederatedX: Table 't1' is inaccessible: 1146 : Table 'federated.t1' doesn't exist
+# Cleanup.
+DROP TABLE IF EXISTS federated.t1;

mysql-test/suite/federated/federatedx_mdev39196.test

@@ -0,0 +1,46 @@
+# MDEV-39196: SELECT from information_schema fails when FederatedX
+# loses underlying table.
+--source include/not_embedded.inc
+--source include/federated.inc
+
+--echo #
+--echo # MDEV-39196: INFORMATION_SCHEMA query must succeed with a
+--echo # warning when a FederatedX remote table is unreachable.
+--echo #
+
+--connection slave
+CREATE TABLE federated.t1 (
+  id INT NOT NULL,
+  name VARCHAR(64)
+) ENGINE=MyISAM;
+INSERT INTO federated.t1 VALUES (1, 'foo');
+
+--connection master
+--replace_result $SLAVE_MYPORT SLAVE_PORT
+eval CREATE TABLE federated.t1 (
+  id INT NOT NULL,
+  name VARCHAR(64)
+) ENGINE=FEDERATED
+  CONNECTION='mysql://root@127.0.0.1:$SLAVE_MYPORT/federated/t1';
+
+--echo # Verify the federated table works before dropping remote table.
+SELECT * FROM federated.t1;
+
+--echo # Drop the remote table to simulate unreachable/missing table.
+--connection slave
+DROP TABLE federated.t1;
+
+--connection master
+--echo # INFORMATION_SCHEMA query must succeed and issue a warning.
+SELECT TABLE_NAME, TABLE_ROWS
+  FROM information_schema.TABLES
+  WHERE TABLE_SCHEMA = 'federated'
+    AND TABLE_NAME = 't1';
+
+--echo # Warning must be present.
+SHOW WARNINGS;
+
+--echo # Cleanup.
+DROP TABLE IF EXISTS federated.t1;
+
+--source include/federated_cleanup.inc

storage/federatedx/ha_federatedx.cc

@@ -3130,8 +3130,34 @@ 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());
+    uint remote_err= (uint)(*iop)->error_code();
+    /*
+      Only downgrade to a warning for errors that mean the remote server
+      or table is temporarily unreachable (connection failure, table
+      dropped on remote side). All other errors including access-denied
+      must remain hard errors so callers receive the correct errno.
+    */
+    switch (remote_err)
+    {
+    case 2002: /* CR_CONNECTION_ERROR  - can't connect via socket    */
+    case 2003: /* CR_CONN_HOST_ERROR   - can't connect to host       */
+    case 2005: /* CR_UNKNOWN_HOST      - unknown host                */
+    case 2006: /* CR_SERVER_GONE_ERROR - server has gone away        */
+    case 2013: /* CR_SERVER_LOST       - lost connection during query */
+    case 1146: /* ER_NO_SUCH_TABLE     - remote table dropped        */
+      push_warning_printf(thd, Sql_condition::WARN_LEVEL_WARN,
+                          ER_QUERY_ON_FOREIGN_DATA_SOURCE,
+                          "FederatedX: Table '%s' is inaccessible: "
+                          "%d : %s",
+                          share->table_name,
+                          (*iop)->error_code(),
+                          (*iop)->error_str());
+      error_code= 0;
+      break;
+    default:
+      error_code= ER_QUERY_ON_FOREIGN_DATA_SOURCE;
+      break;
+    }
   }
   else if (remote_error_number != -1 /* error already reported */)
   {