Fix batch size check to only apply to atomic batches with tests

The verify_batch_size() check was applied to all batches before determining mutate_atomic, causing unnecessary failures for batches that never touch system.batchlog. Only logged multi-partition batches (mutate_atomic=true) are stored as blobs in system.batchlog and require size validation.

Changed:

* Moved verify_batch_size() call in execute_without_conditions() directly
  into the logged multi-partition branch for clearer intent
* The check is now placed exactly where mutate_atomic=true, making the
  code more intuitive

Result:
Batch Type  Single-Partition  Size Check
LOGGED      false             YES
LOGGED      true              NO
UNLOGGED    N/A               NO

Tests:

* test_unlogged_batch_size_not_checked: Unlogged batch >1024KB succeeds
* test_logged_multi_partition_batch_size_checked: Logged multi-partition batch >1024KB fails
* test_logged_single_partition_batch_size_not_checked: Logged single-partition batch >1024KB succeeds

Fixes #27605

Co-authored-by: mykaul <yaniv.kaul@scylladb.com>

cql3/statements/batch_statement.cc

@@ -304,8 +304,6 @@ future<coordinator_result<>> batch_statement::execute_without_conditions(
         }
     }));
 #endif
-    verify_batch_size(qp, mutations);
-
     bool mutate_atomic = true;
     if (_type != type::LOGGED) {
         _stats.batches_pure_unlogged += 1;
@@ -313,6 +311,7 @@ future<coordinator_result<>> batch_statement::execute_without_conditions(
     } else {
         if (mutations.size() > 1) {
             _stats.batches_pure_logged += 1;
+            verify_batch_size(qp, mutations);
         } else {
             _stats.batches_unlogged_from_logged += 1;
             mutate_atomic = false;

test/cqlpy/test_batch.py

@@ -74,3 +74,58 @@ def test_batch_with_error(cql, table1):
         # exceptions::exception_code::SERVER_ERROR, it gets converted to NoHostAvailable by the driver
         with pytest.raises(NoHostAvailable, match="Value too large"):
             cql.execute(generate_big_batch(table1, 100) + injection_key)
+
+
+def test_unlogged_batch_size_not_checked(cql, test_keyspace):
+    """Verifies that UNLOGGED batches are NOT subject to batch size limits.
+    
+    Unlogged batches are applied as independent mutations and don't go through
+    the system.batchlog table, so their collective size is irrelevant.
+    This test should succeed even with a batch larger than the fail threshold.
+    """
+    with new_test_table(cql, test_keyspace, "k int primary key, t text") as table:
+        # Create a batch larger than the fail threshold (1024 KB)
+        # This would fail for a logged batch, but should succeed for unlogged
+        statements = [f"INSERT INTO {table} (k, t) VALUES ({idx}, '{'x' * 743}')" for idx in range(1100)]
+        unlogged_batch = "BEGIN UNLOGGED BATCH\n" + "\n".join(statements) + "\n APPLY BATCH\n"
+        
+        # This should not raise an exception
+        cql.execute(unlogged_batch)
+        
+        # Verify the data was inserted
+        result = cql.execute(f"SELECT COUNT(*) FROM {table}")
+        assert result.one()[0] == 1100
+
+
+def test_logged_multi_partition_batch_size_checked(cql, test_keyspace):
+    """Verifies that LOGGED batches targeting multiple partitions ARE subject to batch size limits.
+    
+    Logged multi-partition batches go through system.batchlog and must be size-checked.
+    """
+    with new_test_table(cql, test_keyspace, "k int primary key, t text") as table:
+        # Create a batch larger than the fail threshold (1024 KB) with multiple partitions
+        statements = [f"INSERT INTO {table} (k, t) VALUES ({idx}, '{'x' * 743}')" for idx in range(1100)]
+        logged_batch = "BEGIN BATCH\n" + "\n".join(statements) + "\n APPLY BATCH\n"
+        
+        # This should raise "Batch too large" exception
+        with pytest.raises(InvalidRequest, match="Batch too large"):
+            cql.execute(logged_batch)
+
+
+def test_logged_single_partition_batch_size_not_checked(cql, test_keyspace):
+    """Verifies that LOGGED batches targeting a single partition are NOT subject to batch size limits.
+    
+    Logged single-partition batches don't go through system.batchlog,
+    so their collective size is not relevant.
+    """
+    with new_test_table(cql, test_keyspace, "k int, c int, t text, primary key (k, c)") as table:
+        # Create a batch larger than the fail threshold (1024 KB) but all targeting the same partition
+        statements = [f"INSERT INTO {table} (k, c, t) VALUES (1, {idx}, '{'x' * 743}')" for idx in range(1100)]
+        logged_batch = "BEGIN BATCH\n" + "\n".join(statements) + "\n APPLY BATCH\n"
+        
+        # This should not raise an exception since it's a single-partition batch
+        cql.execute(logged_batch)
+        
+        # Verify the data was inserted
+        result = cql.execute(f"SELECT COUNT(*) FROM {table} WHERE k = 1")
+        assert result.one()[0] == 1100