MDEV-20639 ASAN SEGV in get_prefix upon modifying base column type with...

MDEV-20639 ASAN SEGV in get_prefix upon modifying base column type with existing indexed virtual column

mysql-test/suite/vcol/r/vcol_sql_mode_timestamp.result

@@ -97,5 +97,22 @@ UPDATE IGNORE t1 SET d = NOW();
 DROP TABLE t1;
 SET sql_mode=DEFAULT;
 #
+# MDEV-20639 ASAN SEGV in get_prefix upon modifying base column type with existing indexed virtual column
+#
+CREATE TABLE t1 (
+a TIMESTAMP,
+b TIMESTAMP AS (a) VIRTUAL,
+KEY (b)
+);
+ALTER TABLE t1 MODIFY a BLOB FIRST;
+Warnings:
+Warning	1901	Function or expression '`a`' cannot be used in the GENERATED ALWAYS AS clause of `b`
+Warning	1105	Expression depends on the @@sql_mode value TIME_ROUND_FRACTIONAL
+Warning	1901	Function or expression '`a`' cannot be used in the GENERATED ALWAYS AS clause of `b`
+Warning	1105	Expression depends on the @@sql_mode value TIME_ROUND_FRACTIONAL
+Warning	1901	Function or expression '`a`' cannot be used in the GENERATED ALWAYS AS clause of `b`
+Warning	1105	Expression depends on the @@sql_mode value TIME_ROUND_FRACTIONAL
+DROP TABLE t1;
+#
 # End of 10.4 tests
 #

mysql-test/suite/vcol/t/vcol_sql_mode_timestamp.test

@@ -106,6 +106,18 @@ UPDATE IGNORE t1 SET d = NOW();
 DROP TABLE t1;
 SET sql_mode=DEFAULT;
 
+--echo #
+--echo # MDEV-20639 ASAN SEGV in get_prefix upon modifying base column type with existing indexed virtual column
+--echo #
+
+CREATE TABLE t1 (
+    a TIMESTAMP,
+    b TIMESTAMP AS (a) VIRTUAL,
+    KEY (b)
+);
+ALTER TABLE t1 MODIFY a BLOB FIRST;
+DROP TABLE t1;
+
 
 --echo #
 --echo # End of 10.4 tests

sql/table.cc

@@ -1152,12 +1152,21 @@ bool parse_vcol_defs(THD *thd, MEM_ROOT *mem_root, TABLE *table,
       vcol= unpack_vcol_info_from_frm(thd, mem_root, table, &expr_str,
                                     &((*field_ptr)->vcol_info), error_reported);
       *(vfield_ptr++)= *field_ptr;
+      DBUG_ASSERT(table->map == 0);
+      /*
+        We need Item_field::const_item() to return false, so
+        datetime_precision() and time_precision() do not try to calculate
+        field values, e.g. val_str().
+        Set table->map to non-zero temporarily.
+      */
+      table->map= 1;
       if (vcol && field_ptr[0]->check_vcol_sql_mode_dependency(thd, mode))
       {
         DBUG_ASSERT(thd->is_error());
         *error_reported= true;
         goto end;
       }
+      table->map= 0;
       break;
     case VCOL_DEFAULT:
       vcol= unpack_vcol_info_from_frm(thd, mem_root, table, &expr_str,