release 5.1.0: add compatibility change for `tidb_enable_noop_functio…

…ns` (#6222)

releases/release-5.1.0.md

@@ -63,6 +63,7 @@ In v5.1, the key new features or improvements are as follows:
 - Avoid executing statements like `alter table ... modify column` or `alter table ... change column` during the TiDB rolling upgrade.
 - Since v5.1, setting the replica of system tables, when building TiFlash replicas for each table, is no longer supported. Before upgrading the cluster, you need to clear the relevant system table replicas; otherwise, the upgrade will fail.
 - Deprecate the `--sort-dir` parameter in the `cdc cli changefeed` command of TiCDC. Instead, you can set `--sort-dir` in the `cdc server` command. [#1795](https://github.com/pingcap/ticdc/pull/1795)
+- After upgrading to TiDB 5.1, if TiDB returns the "function READ ONLY has only noop implementation" error, you can let TiDB ignore this error by setting the value of [`tidb_enable_noop_functions`](/system-variables.md#tidb_enable_noop_functions-new-in-v40) to `ON`. This is because the `read_only` variable in MySQL does not yet take effect in TiDB (which is a 'noop' behavior in TiDB). Therefore, even if this variable is set in TiDB, you can still write data into the TiDB cluster.
 
 ## New features