Add comment and test for adding not null column in Iceberg

plugin/trino-iceberg/src/main/java/io/trino/plugin/iceberg/IcebergMetadata.java

@@ -1358,7 +1358,10 @@ private static Term toIcebergTerm(Schema schema, PartitionField partitionField)
     @Override
     public void addColumn(ConnectorSession session, ConnectorTableHandle tableHandle, ColumnMetadata column)
     {
-        // TODO https://github.com/trinodb/trino/issues/13587 Allow NOT NULL constraint when the table is empty
+        // Spark doesn't support adding a NOT NULL column to Iceberg tables
+        // Also, Spark throws an exception when reading the table if we add such columns and execute a rollback procedure
+        // because they keep returning the latest table definition even after the rollback https://github.com/apache/iceberg/issues/5591
+        // Even when a table is empty, this connector doesn't support adding not null columns to avoid the above Spark failure
         if (!column.isNullable()) {
             throw new TrinoException(NOT_SUPPORTED, "This connector does not support adding not null columns");
         }

testing/trino-product-tests/src/main/java/io/trino/tests/product/iceberg/TestIcebergSparkCompatibility.java

@@ -1998,6 +1998,25 @@ public void testUpdateOnPartitionColumn()
         onSpark().executeQuery("DROP TABLE " + sparkTableName);
     }
 
+    @Test(groups = {ICEBERG, PROFILE_SPECIFIC_TESTS})
+    public void testAddNotNullColumn()
+    {
+        String baseTableName = "test_add_not_null_column_" + randomTableSuffix();
+        String trinoTableName = trinoTableName(baseTableName);
+        String sparkTableName = sparkTableName(baseTableName);
+
+        onTrino().executeQuery("CREATE TABLE " + trinoTableName + " AS SELECT 1 col");
+        assertThat(onTrino().executeQuery("SELECT * FROM " + trinoTableName)).containsOnly(row(1));
+
+        assertQueryFailure(() -> onTrino().executeQuery("ALTER TABLE " + trinoTableName + " ADD COLUMN new_col INT NOT NULL"))
+                .hasMessageMatching(".*This connector does not support adding not null columns");
+        assertQueryFailure(() -> onSpark().executeQuery("ALTER TABLE " + sparkTableName + " ADD COLUMN new_col INT NOT NULL"))
+                .hasMessageMatching("(?s).*Unsupported table change: Incompatible change: cannot add required column.*");
+
+        assertThat(onTrino().executeQuery("SELECT * FROM " + trinoTableName)).containsOnly(row(1));
+        onTrino().executeQuery("DROP TABLE " + trinoTableName);
+    }
+
     @Test(groups = {ICEBERG, PROFILE_SPECIFIC_TESTS})
     public void testHandlingPartitionSchemaEvolutionInPartitionMetadata()
     {