fix(frontend): allow write in rw txn for CI usage, cherry pick 16012 …

…to release-1.8 (#16016)

e2e_test/batch/transaction/read_only.slt

@@ -81,18 +81,22 @@ select * from t;
 2
 3
 
-statement error read-only transaction
+statement ok
 insert into t values (4);
 
+statement ok
+COMMIT;
+
+statement ok
+flush;
+
 query I rowsort
 select * from t;
 ----
 1
 2
 3
-
-statement ok
-COMMIT;
+4
 
 statement ok
 drop table t;

src/frontend/src/handler/transaction.rs

@@ -34,7 +34,8 @@ pub async fn handle_begin(
     modes: Vec<TransactionMode>,
 ) -> Result<RwPgResponse> {
     let HandlerArgs { session, .. } = handler_args;
-    let mut notices = vec![];
+
+    let mut builder = RwPgResponse::builder(stmt_type);
 
     let access_mode = {
         let mut access_mode = None;
@@ -47,9 +48,9 @@ pub async fn handle_begin(
                     // Note: This is for compatibility with some external drivers (like postgres_fdw) that
                     // always start a transaction with an Isolation Level.
                     const MESSAGE: &str = "\
-                    Transaction with given Isolation Level is not supported yet.\n\
-                    For compatibility, this statement will still proceed with RepeatableRead.";
-                    notices.push(MESSAGE);
+                        Transaction with given Isolation Level is not supported yet.\n\
+                        For compatibility, this statement will proceed with RepeatableRead.";
+                    builder = builder.notice(MESSAGE);
                 }
             }
         }
@@ -58,27 +59,20 @@ pub async fn handle_begin(
             Some(TransactionAccessMode::ReadOnly) => AccessMode::ReadOnly,
             Some(TransactionAccessMode::ReadWrite) | None => {
                 // Note: This is for compatibility with some external drivers (like psycopg2) that
-                // issue `BEGIN` implicitly for users.
+                // issue `BEGIN` implicitly for users. Not actually starting a transaction is okay
+                // since `COMMIT` and `ROLLBACK` are no-ops (except for warnings) when there is no
+                // active transaction.
                 const MESSAGE: &str = "\
                     Read-write transaction is not supported yet. Please specify `READ ONLY` to start a read-only transaction.\n\
-                    For compatibility, this statement will still succeed and be executed as Read-only transactions.\n\
-                    The write operations in this transaction will be rejected.";
-                notices.push(MESSAGE);
-                AccessMode::ReadOnly
+                    For compatibility, this statement will still succeed but no transaction is actually started.";
+                builder = builder.notice(MESSAGE);
+                return Ok(builder.into());
             }
         }
     };
 
     session.txn_begin_explicit(access_mode);
-    if notices.is_empty() {
-        Ok(RwPgResponse::empty_result(stmt_type))
-    } else {
-        let mut builder = RwPgResponse::builder(stmt_type);
-        for notice in notices {
-            builder = builder.notice(notice);
-        }
-        Ok(builder.into())
-    }
+    Ok(builder.into())
 }
 
 #[expect(clippy::unused_async)]