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/federated.test

@@ -17,7 +17,7 @@ create table t1 (a int);
 --replace_result $MASTER_MYPORT MASTER_PORT
 eval create table fed (a int) engine=Federated CONNECTION='mysql://root@127.0.0.1:$MASTER_MYPORT/test/t1';
 drop table t1;
---error 1146,1431
+--error ER_NO_SUCH_TABLE,ER_GET_ERRMSG,ER_FOREIGN_DATA_SOURCE_DOESNT_EXIST
 select * from fed;
 drop table fed;
 

mysql-test/suite/federated/federatedx_mdev39196.result

@@ -0,0 +1,53 @@
+connect  master,127.0.0.1,root,,test,$MASTER_MYPORT,;
+connect  slave,127.0.0.1,root,,test,$SLAVE_MYPORT,;
+connection master;
+CREATE DATABASE federated;
+connection slave;
+CREATE DATABASE federated;
+#
+# MDEV-39196: INFORMATION_SCHEMA query must succeed with a
+# warning when a FederatedX remote table is unreachable.
+#
+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;
+CREATE TABLE t1 (
+id INT NOT NULL,
+name VARCHAR(64)
+) ENGINE=FEDERATED
+CONNECTION='mysql://root@127.0.0.1:SLAVE_PORT/federated/t1';
+# 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.
+connection slave;
+USE federated;
+DROP TABLE t1;
+connection master;
+# 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	1
+Warnings:
+Warning	1430	FederatedX: Table 't1' is inaccessible: 1146 : Remote table does not exist
+# Warning must be present.
+SHOW WARNINGS;
+Level	Code	Message
+Warning	1430	FederatedX: Table 't1' is inaccessible: 1146 : Remote table does not exist
+# Cleanup.
+connection master;
+DROP TABLE IF EXISTS federated.t1;
+DROP DATABASE IF EXISTS federated;
+connection slave;
+DROP TABLE IF EXISTS federated.t1;
+DROP DATABASE IF EXISTS federated;

mysql-test/suite/federated/federatedx_mdev39196.test

@@ -0,0 +1,46 @@
+--source include/not_embedded.inc
+--source have_federatedx.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;
+USE federated;
+CREATE TABLE t1 (
+  id INT NOT NULL,
+  name VARCHAR(64)
+) ENGINE=MyISAM;
+INSERT INTO t1 VALUES (1, 'foo');
+
+connection master;
+USE federated;
+--replace_result $SLAVE_MYPORT SLAVE_PORT
+eval CREATE TABLE t1 (
+  id INT NOT NULL,
+  name VARCHAR(64)
+) ENGINE=FEDERATED
+CONNECTION='mysql://root@127.0.0.1:$SLAVE_MYPORT/federated/t1';
+
+--echo # Verify the federated table works before dropping remote table.
+SELECT * FROM federated.t1;
+
+--echo # Drop the remote table to simulate unreachable/missing table.
+connection slave;
+USE federated;
+DROP TABLE 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.
+--source include/federated_cleanup.inc

storage/federatedx/federatedx_io_mysql.cc

@@ -68,6 +68,8 @@ class federatedx_io_mysql :public federatedx_io
   DYNAMIC_ARRAY savepoints;
   bool requested_autocommit;
   bool actual_autocommit;
+  int stored_error_code;
+  char stored_error_msg[MYSQL_ERRMSG_SIZE];
 
   int actual_query(const char *buffer, size_t length);
   bool test_all_restrict() const;
@@ -134,12 +136,14 @@ federatedx_io *instantiate_io_mysql(MEM_ROOT *server_root,
 
 federatedx_io_mysql::federatedx_io_mysql(FEDERATEDX_SERVER *aserver)
   : federatedx_io(aserver),
-    requested_autocommit(TRUE), actual_autocommit(TRUE)
+    requested_autocommit(TRUE), actual_autocommit(TRUE),
+    stored_error_code(0)
 {
   DBUG_ENTER("federatedx_io_mysql::federatedx_io_mysql");
 
   bzero(&mysql, sizeof(MYSQL));
   bzero(&savepoints, sizeof(DYNAMIC_ARRAY));
+  stored_error_msg[0]= 0;
 
   my_init_dynamic_array(PSI_INSTRUMENT_ME, &savepoints, sizeof(SAVEPT), 16, 16, MYF(0));
 
@@ -483,12 +487,19 @@ my_ulonglong federatedx_io_mysql::last_insert_id() const
 
 int federatedx_io_mysql::error_code()
 {
+  if (stored_error_code)
+    return stored_error_code;
   return mysql_errno(&mysql);
 }
 
 
 const char *federatedx_io_mysql::error_str()
 {
+  if (stored_error_code)
+  {
+    DBUG_ASSERT(stored_error_msg[0] != 0);
+    return stored_error_msg;
+  }
   return mysql_error(&mysql);
 }
 
@@ -583,7 +594,11 @@ bool federatedx_io_mysql::table_metadata(ha_statistics *stats,
     goto error;
 
   if (!(row= fetch_row(result)))
+  {
+    stored_error_code= 0;
+    stored_error_msg[0]= 0;
     goto error;
+  }
 
   /*
     deleted is set in ha_federatedx::info
@@ -618,8 +633,13 @@ bool federatedx_io_mysql::table_metadata(ha_statistics *stats,
 error:
   if (!mysql_errno(&mysql))
   {
-    mysql.net.last_errno= ER_NO_SUCH_TABLE;
-    strmake_buf(mysql.net.last_error, "Remote table does not exist");
+    stored_error_code= ER_NO_SUCH_TABLE;
+    strmake_buf(stored_error_msg, "Remote table does not exist");
+  }
+  else
+  {
+    stored_error_code= mysql_errno(&mysql);
+    strmake_buf(stored_error_msg, mysql_error(&mysql));
   }
   free_result(result);
   return 1;

storage/federatedx/ha_federatedx.cc

@@ -3130,12 +3130,48 @@ 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());
+    if ((flag & HA_STATUS_VARIABLE) && !(flag & HA_STATUS_NO_LOCK))
+    {
+      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;
+    }
+    else
+    {
+      my_printf_error((*iop)->error_code(),
+                      ": %d : %s", MYF(0),
+                      (*iop)->error_code(), (*iop)->error_str());
+      error_code= (*iop)->error_code();
+    }
   }
   else if (remote_error_number != -1 /* error already reported */)
   {
     error_code= remote_error_number;
+    /*
+     * Downgrade remote errors to warnings only when called from the
+     * INFORMATION_SCHEMA table scan path.
+     *
+     * INFORMATION_SCHEMA scans (sql_show.cc, fill_schema_table_by_open)
+     * call ha_federatedx::info() with HA_STATUS_VARIABLE set but WITHOUT
+     * HA_STATUS_NO_LOCK. All other callers that trigger the error path
+     * (e.g. direct SELECT, DELETE, SHOW TABLE STATUS) pass
+     * HA_STATUS_NO_LOCK alongside HA_STATUS_VARIABLE.
+     *
+     * This combination is therefore used as an indirect signal that we are
+     * inside an I_S scan, where aborting with a hard error would break the
+     * entire query rather than just skipping the inaccessible table.
+     *
+     * NOTE: This is an indirect inference based on current caller conventions.
+     * If a future caller passes HA_STATUS_VARIABLE without HA_STATUS_NO_LOCK
+     * for a non-I_S purpose, it would unintentionally receive
+     * warning-downgrade behavior. A cleaner long-term solution would be an
+     * explicit flag or context distinguishing I_S scans from direct access.
+     */
     my_error(error_code, MYF(0), ER_THD(thd, error_code));
   }
 fail: