sys var: add tidb_enable_shared_lock_upgrade

pessimistic-transaction.md

@@ -111,7 +111,11 @@ Pessimistic transactions in TiDB behave similarly to those in MySQL. See the min
 
 2. TiDB does not support `SELECT LOCK IN SHARE MODE`.
 
-    When `SELECT LOCK IN SHARE MODE` is executed, it has the same effect as that without the lock, so the read or write operation of other transactions is not blocked.
+    TiDB does not support `SELECT LOCK IN SHARE MODE` by default.
+
+    You can enable [`tidb_enable_noop_functions`](/system-variables.md#tidb_enable_noop_functions-new-in-v40) to be compatible with the `SELECT LOCK IN SHARE MODE` statement. The effect of the statement is the same as when there is no lock, and it will not block the read and write operations of other transactions.
+
+    Starting from v8.3.0, TiDB supports using the [`tidb_enable_shared_lock_upgrade`](/system-variables.md#tidb_enable_shared_lock_upgrade-new-in-v830) system variable to enable the `SELECT LOCK IN SHARE MODE` statement to add locks. However, note that the lock added at this time is not a true shared lock, but consistent with `SELECT FOR UPDATE`, which actually adds an exclusive lock. You can enable this variable when you want to be compatible with the `SELECT LOCK IN SHARE MODE` syntax while blocking writes to prevent data from being modified by parallel write transactions during reads. This variable takes effect regardless of the configuration of [`tidb_enable_noop_functions`](/system-variables.md#tidb_enable_noop_functions-new-in-v40).
 
 3. DDL may result in failure of the pessimistic transaction commit.
 

system-variables.md

@@ -2567,6 +2567,16 @@ mysql> SELECT job_info FROM mysql.analyze_jobs ORDER BY end_time DESC LIMIT 1;
 - Value options: `OFF`, `ON`
 - This variable controls whether TiDB enables chunk objects cache. If the value is `ON`, TiDB prefers to use the cached chunk object and only requests from the system if the requested object is not in the cache. If the value is `OFF`, TiDB requests chunk objects from the system directly.
 
+### tidb_enable_shared_lock_upgrade <span class="version-mark">New in v8.3.0</span>
+
+- Scope: SESSION | GLOBAL
+- Persists to cluster: Yes
+- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): No
+- Type: Boolean
+- Default value: `OFF`
+- This variable controls whether to enable the function of upgrading shared locks to exclusive locks. TiDB does not support `SELECT LOCK IN SHARE MODE` by default. When the variable value is `ON`, TiDB will try to upgrade the `SELECT LOCK IN SHARE MODE` statement to `SELECT FOR UPDATE` and add a pessimistic lock. The default value of this variable is `OFF`, which means that the function of upgrading shared locks to exclusive locks is disabled.
+- This variable takes effect on the `SELECT LOCK IN SHARE MODE` statement regardless of the configuration of [`tidb_enable_noop_functions`](/system-variables.md#tidb_enable_noop_functions-new-in-v40). 
+
 ### tidb_enable_slow_log
 
 > **Note:**