MDEV-38716: Part 2

The last patch was incorrect - it did not consider columns with both

DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP

and these records would be re-written with new timestamps.

The fix is to add the ON UPDATE based default_fields back to the
destination table after the actual data copy.

Reviewed-by: TODO
Signed-off-by: Brandon Nesterenko <[email protected]>

Author: bnestere

sql/sql_table.cc

@@ -12720,16 +12720,6 @@ copy_data_between_tables(THD *thd, TABLE *from, TABLE *to,
           goto err;
         (copy_end++)->set(*ptr,def->field,0);
       }
-
-      /*
-        Ensure that TIMESTAMP or DATETIME fields with ON UPDATE clauses
-        have their default_fields copied, so they are executed after ALTER.
-      */
-      if (dfield_ptr && def->field->has_update_default_function())
-      {
-        *(dfield_ptr++)= *ptr;
-        ++to->s->default_fields;
-      }
     }
     else
     {
@@ -12746,13 +12736,14 @@ copy_data_between_tables(THD *thd, TABLE *from, TABLE *to,
       }
     }
   }
+
+  /*
+    Mark the end of the default field list. Note that this is a temporary
+    state; fields with ON UPDATE defaults will be added back to this list
+    after the data copy is complete.
+  */
   if (dfield_ptr)
-  {
-    if (dfield_ptr == to->default_field)
-      to->default_field= NULL; // No default fields left
-    else
-      *dfield_ptr= NULL; // Mark end of default field pointers
-  }
+    *dfield_ptr= NULL;
 
   if (order)
   {
@@ -13031,6 +13022,43 @@ copy_data_between_tables(THD *thd, TABLE *from, TABLE *to,
   cleanup_done= 1;
   to->file->extra(HA_EXTRA_NO_IGNORE_DUP_KEY);
 
+  /*
+    To prevent this data copy from re-writing previously set default values,
+    this function previously has overridden to->default_fields to only account
+    for new fields added to the table (i.e those which should get new default
+    values). Now that the data copy is done, the to->default_field list is
+    missing fields that have ON UPDATE clauses (i.e. TIMESTAMP and DATETIME).
+    So restore these fields to the default_fields list.
+
+    Notes:
+     * The code is similar to the dfield_ptr modification earlier in the
+       function
+     * This must be done before online alter execution, as it unpacks row
+       data, which requires the table's default_fields to be set up
+     * The following DBUG_ASSERT ensures either there are no default
+       fields for the table, or the dfield_ptr points to the end of the
+       default field list (which is NULL). This ensures that if we are
+       adding back default fields, we are not overwriting any existing
+       fields.
+  */
+  it.rewind();
+  DBUG_ASSERT(!dfield_ptr || !(*dfield_ptr));
+  for (Field **ptr=to->field ; *ptr ; ptr++)
+  {
+    def=it++;
+    if (def->field && dfield_ptr && def->field->has_update_default_function())
+    {
+      *(dfield_ptr++)= *ptr;
+    }
+  }
+  if (dfield_ptr)
+  {
+    if (dfield_ptr == to->default_field)
+      to->default_field= NULL; // No default fields left
+    else
+      *dfield_ptr= NULL; // Mark end of default field pointers
+  }
+
 #ifdef HAVE_REPLICATION
   if (online && error < 0)
   {