fix Mellon custom block required input handling (#13888)

* fix Mellon custom block required input handling

* test: assert Mellon required inputs at spec level

Author: wunianze666-netizen

src/diffusers/modular_pipelines/mellon_node_utils.py

@@ -1058,6 +1058,7 @@ def from_custom_block(
         inputs = []
         model_inputs = []
         outputs = []
+        required_inputs = []
 
         # Process block inputs
         for input_param in block.inputs:
@@ -1066,7 +1067,8 @@ def from_custom_block(
             if input_param.name in input_types:
                 input_param = copy.copy(input_param)
                 input_param.metadata = {"mellon": input_types[input_param.name]}
-            print(f" processing input: {input_param.name}, metadata: {input_param.metadata}")
+            if input_param.required:
+                required_inputs.append(input_param.name)
             inputs.append(input_param_to_mellon_param(input_param))
 
         # Process block outputs
@@ -1090,7 +1092,7 @@ def from_custom_block(
             "inputs": inputs,
             "model_inputs": model_inputs,
             "outputs": outputs,
-            "required_inputs": [],
+            "required_inputs": required_inputs,
             "required_model_inputs": [],
             "block_name": "custom",
         }

tests/modular_pipelines/test_modular_pipelines_custom_blocks.py

@@ -230,6 +230,17 @@ def test_custom_block_supported_components(self):
         assert len(pipe.components) == 1
         assert pipe.component_names[0] == "transformer"
 
+    def test_custom_block_mellon_config_preserves_required_inputs(self):
+        from diffusers.modular_pipelines.mellon_node_utils import MellonPipelineConfig
+
+        custom_block = DummyCustomBlockSimple()
+
+        mellon_config = MellonPipelineConfig.from_custom_block(custom_block)
+        custom_node = mellon_config.node_params["custom"]
+
+        assert mellon_config.node_specs["custom"]["required_inputs"] == ["prompt"]
+        assert custom_node["params"]["prompt"]["label"].endswith(" *")
+
     def test_trust_remote_code_not_propagated_to_external_repo(self):
         """When a modular pipeline repo references a component from an external repo that has custom
         code (auto_map in config), calling load_components(trust_remote_code=True) should NOT