feat: Persist Variables for Enhanced Debugging Workflow (#20699)

This pull request introduces a feature aimed at improving the debugging experience during workflow editing. With the addition of variable persistence, the system will automatically retain the output variables from previously executed nodes. These persisted variables can then be reused when debugging subsequent nodes, eliminating the need for repetitive manual input.

By streamlining this aspect of the workflow, the feature minimizes user errors and significantly reduces debugging effort, offering a smoother and more efficient experience.

Key highlights of this change:

- Automatic persistence of output variables for executed nodes.
- Reuse of persisted variables to simplify input steps for nodes requiring them (e.g., `code`, `template`, `variable_assigner`).
- Enhanced debugging experience with reduced friction.

Closes #19735.

api/core/workflow/nodes/iteration/iteration_node.py

@@ -11,6 +11,7 @@ from flask import Flask, current_app
 
 from configs import dify_config
 from core.variables import ArrayVariable, IntegerVariable, NoneVariable
+from core.variables.segments import ArrayAnySegment, ArraySegment
 from core.workflow.entities.node_entities import (
     NodeRunResult,
 )
@@ -37,6 +38,7 @@ from core.workflow.nodes.base import BaseNode
 from core.workflow.nodes.enums import NodeType
 from core.workflow.nodes.event import NodeEvent, RunCompletedEvent
 from core.workflow.nodes.iteration.entities import ErrorHandleMode, IterationNodeData
+from factories.variable_factory import build_segment
 from libs.flask_utils import preserve_flask_contexts
 
 from .exc import (
@@ -72,6 +74,10 @@ class IterationNode(BaseNode[IterationNodeData]):
             },
         }
 
+    @classmethod
+    def version(cls) -> str:
+        return "1"
+
     def _run(self) -> Generator[NodeEvent | InNodeEvent, None, None]:
         """
         Run the node.
@@ -85,10 +91,17 @@ class IterationNode(BaseNode[IterationNodeData]):
             raise InvalidIteratorValueError(f"invalid iterator value: {variable}, please provide a list.")
 
         if isinstance(variable, NoneVariable) or len(variable.value) == 0:
+            # Try our best to preserve the type informat.
+            if isinstance(variable, ArraySegment):
+                output = variable.model_copy(update={"value": []})
+            else:
+                output = ArrayAnySegment(value=[])
             yield RunCompletedEvent(
                 run_result=NodeRunResult(
                     status=WorkflowNodeExecutionStatus.SUCCEEDED,
-                    outputs={"output": []},
+                    # TODO(QuantumGhost): is it possible to compute the type of `output`
+                    # from graph definition?
+                    outputs={"output": output},
                 )
             )
             return
@@ -231,6 +244,7 @@ class IterationNode(BaseNode[IterationNodeData]):
             # Flatten the list of lists
             if isinstance(outputs, list) and all(isinstance(output, list) for output in outputs):
                 outputs = [item for sublist in outputs for item in sublist]
+            output_segment = build_segment(outputs)
 
             yield IterationRunSucceededEvent(
                 iteration_id=self.id,
@@ -247,7 +261,7 @@ class IterationNode(BaseNode[IterationNodeData]):
             yield RunCompletedEvent(
                 run_result=NodeRunResult(
                     status=WorkflowNodeExecutionStatus.SUCCEEDED,
-                    outputs={"output": outputs},
+                    outputs={"output": output_segment},
                     metadata={
                         WorkflowNodeExecutionMetadataKey.ITERATION_DURATION_MAP: iter_run_map,
                         WorkflowNodeExecutionMetadataKey.TOTAL_TOKENS: graph_engine.graph_runtime_state.total_tokens,

api/core/workflow/nodes/iteration/iteration_start_node.py

@@ -13,6 +13,10 @@ class IterationStartNode(BaseNode[IterationStartNodeData]):
     _node_data_cls = IterationStartNodeData
     _node_type = NodeType.ITERATION_START
 
+    @classmethod
+    def version(cls) -> str:
+        return "1"
+
     def _run(self) -> NodeRunResult:
         """
         Run the node.