[#3109,#27616] YSQL: Ensure DDL Atomicity is enabled for transactional DDL

Summary:
Transactional DDL introduces the ability to run DDLs within a transaction block and interleaving them with DML operations. Without transactional ddl, DDLs run in their own transaction and auto-commit at the end of the statement even if they are executed in an explicit transaction block.

With txn ddl, users can also rollback the DDLs via the `ROLLBACK` or `ABORT` statement. This feature is supported by using the earlier introduced DDL atomicity feature. In DDL atomicity, we track the transaction associated with a DDL and upon txn completion, we roll-forward / rollback depending on the transaction outcome (commit or abort). Transactional DDL utilizes the atomicity infra for the rollback feature as it is just the lifetime of the transaction (txn block) which is different vs autonomous ddl transactions (statement level).

So transactional DDL depends on DDL atomicity to function correctly. If DDL atomicity is disabled, the rollback feature will not work at all and we'll lose the atomicity guarantees for DDLs.

This revision adds a validator for the flag `ysql_yb_ddl_transaction_block_enabled` which ensures that DDL atomicity is enabled when enabling transactional
DDL.

Test Plan:
- Success

```
bin/yugabyted destroy && bin/yugabyted start --ui=false --tserver_flags="ysql_yb_ddl_transaction_block_enabled=true"
bin/yugabyted destroy && bin/yugabyted start --ui=false --tserver_flags="ysql_yb_ddl_transaction_block_enabled=false,ysql_yb_ddl_rollback_enabled=false" --master_flags="ysql_yb_ddl_transaction_block_enabled=false,ysql_yb_ddl_rollback_enabled=false"
```

- Failure

```
bin/yugabyted destroy && bin/yugabyted start --ui=false --tserver_flags="ysql_yb_ddl_transaction_block_enabled=true,ysql_yb_ddl_rollback_enabled=false" --master_flags="ysql_yb_ddl_transaction_block_enabled=true,ysql_yb_ddl_rollback_enabled=false"
```

Reviewers: hsunder, patnaik.balivada, pjain, myang, bkolagani

Reviewed By: patnaik.balivada

Subscribers: ybase

Differential Revision: https://phorge.dev.yugabyte.com/D44827

Author: dr0pdb

src/yb/common/common_flags.cc

@@ -177,7 +177,8 @@ constexpr bool kEnableDdlTransactionBlocks = false;
 #endif
 DEFINE_NON_RUNTIME_PG_FLAG(bool, yb_ddl_transaction_block_enabled, kEnableDdlTransactionBlocks,
     "If true, DDL operations in YSQL will execute within the active transaction"
-    "block instead of their separate transactions.");
+    "block instead of their separate transactions. Ensure DDL atomicity is "
+    "enabled via ysql_yb_enable_ddl_atomicity_infra and ysql_yb_ddl_rollback_enabled flags.");
 
 DEFINE_NON_RUNTIME_PG_FLAG(bool, yb_disable_ddl_transaction_block_for_read_committed, false,
     "If true, DDL operations in READ COMMITTED mode will be executed in a separate DDL transaction "
@@ -230,6 +231,9 @@ DEFINE_validator(enable_object_locking_for_table_locks,
 DEFINE_validator(ysql_enable_db_catalog_version_mode,
     FLAG_REQUIRED_BY_FLAG_VALIDATOR(enable_object_locking_for_table_locks));
 DEFINE_validator(ysql_yb_ddl_transaction_block_enabled,
+    FLAG_DELAYED_COND_VALIDATOR(
+        (!_value || FLAGS_ysql_yb_ddl_rollback_enabled),
+        "ysql_yb_ddl_rollback_enabled must be enabled"),
     FLAG_REQUIRED_BY_FLAG_VALIDATOR(enable_object_locking_for_table_locks));
 DEFINE_validator(refresh_waiter_timeout_ms,
     FLAG_REQUIRED_NONZERO_BY_FLAG_VALIDATOR(enable_object_locking_for_table_locks));

src/yb/common/common_util.cc

@@ -105,6 +105,8 @@ int GetInitialNumTabletsPerTable(TableType table_type, size_t tserver_count) {
 }
 
 bool YsqlDdlRollbackEnabled() {
+  // Also change the validator for ysql_yb_ddl_transaction_block_enabled flag in common_flag.cc if
+  // changing this logic.
   return FLAGS_ysql_yb_enable_ddl_atomicity_infra && FLAGS_ysql_yb_ddl_rollback_enabled;
 }