[release-22.0] VDiff: Handle the case where a workflow's table has been dropped on the source (#18985)

go/vt/vttablet/tabletmanager/vdiff/table_differ.go

@@ -944,6 +944,13 @@ func (td *tableDiffer) getSourcePKCols() error {
 		return vterrors.Wrapf(err, "failed to get the schema for table %s from source tablet %s",
 			td.table.Name, topoproto.TabletAliasString(sourceTablet.Tablet.Alias))
 	}
+	if len(sourceSchema.TableDefinitions) == 0 {
+		// The table no longer exists on the source. Any rows that exist on the target will be
+		// reported as extra rows.
+		log.Warningf("The %s table was not found on source tablet %s during VDiff for the %s workflow; any rows on the target will be reported as extra",
+			td.table.Name, topoproto.TabletAliasString(sourceTablet.Tablet.Alias), td.wd.ct.workflow)
+		return nil
+	}
 	sourceTable := sourceSchema.TableDefinitions[0]
 	if len(sourceTable.PrimaryKeyColumns) == 0 {
 		// We use the columns from a PKE if there is one.

go/vt/vttablet/tabletmanager/vdiff/table_differ_test.go

@@ -129,3 +129,35 @@ func TestUpdateTableProgress(t *testing.T) {
 		})
 	}
 }
+
+func TestGetSourcePKCols_TableDroppedOnSource(t *testing.T) {
+	tvde := newTestVDiffEnv(t)
+	defer tvde.close()
+
+	ct := tvde.createController(t, 1)
+
+	table := &tabletmanagerdatapb.TableDefinition{
+		Name:              "dropped_table",
+		Columns:           []string{"c1", "c2"},
+		PrimaryKeyColumns: []string{"c1"},
+		Fields:            sqltypes.MakeTestFields("c1|c2", "int64|varchar"),
+	}
+
+	tvde.tmc.schema = &tabletmanagerdatapb.SchemaDefinition{
+		TableDefinitions: []*tabletmanagerdatapb.TableDefinition{},
+	}
+
+	td := &tableDiffer{
+		wd: &workflowDiffer{
+			ct: ct,
+		},
+		table: table,
+		tablePlan: &tablePlan{
+			table: table,
+		},
+	}
+
+	err := td.getSourcePKCols()
+	require.NoError(t, err)
+	require.Nil(t, td.tablePlan.sourcePkCols)
+}