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 +1,27 @@
+# MDEV-39196: SELECT from information_schema fails when FederatedX
+# loses underlying table.
+#
+# Test that INFORMATION_SCHEMA queries succeed with a warning
+# when a FederatedX remote table is unreachable, instead of
+# returning a hard error.
+#
+# Verify the federated table works before dropping remote table.
+SELECT * FROM t1;
+id	name
+1	foo
+# Drop the remote table to simulate an unreachable/missing table.
+# Now SELECT from INFORMATION_SCHEMA should succeed and push a
+# warning instead of returning a hard error.
+# The query must succeed (not error out).
+SELECT TABLE_NAME, TABLE_ROWS
+  FROM information_schema.TABLES
+  WHERE TABLE_SCHEMA = 'federated';
+TABLE_NAME	TABLE_ROWS
+t1	NULL
+# Confirm a warning was issued for the inaccessible table.
+SHOW WARNINGS;
+Level	Code	Message
+Warning	1296	FederatedX: Table 't1' is inaccessible: 1146 : Table 'federated.t1' doesn't exist
+# Cleanup
+DROP TABLE IF EXISTS t1;
+DROP DATABASE federated;

mysql-test/suite/federated/federatedx_mdev39196.test

@@ -0,0 +1,63 @@
+# MDEV-39196: SELECT from information_schema fails when FederatedX
+# loses underlying table.
+--source include/not_embedded.inc
+--source include/have_federatedx.inc
+
+--echo #
+--echo # Test that INFORMATION_SCHEMA queries succeed with a warning
+--echo # when a FederatedX remote table is unreachable, instead of
+--echo # returning a hard error.
+--echo #
+
+# ---------------------------------------------------------------
+# Setup: two servers via federated suite infrastructure
+# ---------------------------------------------------------------
+--connection slave
+CREATE DATABASE federated;
+
+--connection master
+CREATE DATABASE federated;
+
+--connection slave
+USE federated;
+CREATE TABLE t1 (id INT NOT NULL, name VARCHAR(64)) ENGINE=MyISAM;
+INSERT INTO t1 VALUES (1, 'foo');
+
+--connection master
+USE federated;
+
+--eval CREATE TABLE 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 t1;
+
+--echo # Drop the remote table to simulate an unreachable/missing table.
+--connection slave
+USE federated;
+DROP TABLE t1;
+
+--echo # Now SELECT from INFORMATION_SCHEMA should succeed and push a
+--echo # warning instead of returning a hard error.
+--connection master
+USE federated;
+
+--echo # The query must succeed (not error out).
+--disable_warnings
+SELECT TABLE_NAME, TABLE_ROWS
+  FROM information_schema.TABLES
+  WHERE TABLE_SCHEMA = 'federated';
+--enable_warnings
+
+--echo # Confirm a warning was issued for the inaccessible table.
+SHOW WARNINGS;
+
+--echo # Cleanup
+DROP TABLE IF EXISTS t1;
+DROP DATABASE federated;
+
+--connection slave
+DROP DATABASE IF EXISTS federated;
+
+--connection master

storage/federatedx/ha_federatedx.cc

@@ -3130,13 +3130,41 @@ 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());
-  }
-  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));
+    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
+      errors — must remain hard errors so that callers like
+      federated_server.test that expect ER_DBACCESS_DENIED_ERROR still
+      receive them correctly.
+    */
+    switch (remote_err)
+    {
+      /* CR_* client-side connection errors */
+      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 */
+      /* Server-side: remote table no longer exists */
+      case 1146: /* ER_NO_SUCH_TABLE                                   */
+        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:
+        /*
+          For all other errors (e.g. ER_DBACCESS_DENIED_ERROR = 1044),
+          keep the error so it propagates back to the user.
+        */
+        error_code= ER_QUERY_ON_FOREIGN_DATA_SOURCE;
+        break;
+    }
   }
 fail:
   tmp_txn->release(&tmp_io);