ddl: Reorganize Partition failure/rollback tests and fixes

pkg/ddl/BUILD.bazel

@@ -251,7 +251,6 @@ go_test(
         "placement_policy_test.go",
         "placement_sql_test.go",
         "primary_key_handle_test.go",
-        "reorg_partition_test.go",
         "repair_table_test.go",
         "restart_test.go",
         "rollingback_test.go",
@@ -337,7 +336,6 @@ go_test(
         "//pkg/util/domainutil",
         "//pkg/util/gcutil",
         "//pkg/util/generic",
-        "//pkg/util/mathutil",
         "//pkg/util/mock",
         "//pkg/util/sem",
         "//pkg/util/sqlexec",

pkg/ddl/index.go

@@ -1574,6 +1574,14 @@ func (w *addIndexTxnWorker) checkHandleExists(idxInfo *model.IndexInfo, key kv.K
 	if hasBeenBackFilled {
 		return nil
 	}
+	if idxInfo.Global {
+		// 'handle' comes from reading directly from a partition, without partition id,
+		// so we can only compare the handle part of the key.
+		if ph, ok := h.(kv.PartitionHandle); ok && ph.Handle.Equal(handle) {
+			// table row has been back-filled already, OK to add the index entry
+			return nil
+		}
+	}
 	return ddlutil.GenKeyExistsErr(key, value, idxInfo, tblInfo)
 }