fix(api): defer streaming response until referenced variables are updated

[QuantumGhost]

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Summary

Fix #30831.

Root cause: VariableAssigner v1 does not override blocks_variable_output, so the response streaming coordinator treats it as non-blocking and may start streaming Answer nodes before the conversation variable update is applied.

Screenshots

Before	After

Checklist

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran make lint and make type-check (backend) and cd web && npx lint-staged (frontend) to appease the lint gods

[gemini-code-assist]

Summary of Changes

Hello @QuantumGhost, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses an issue where streaming API responses might prematurely send data before conversation variables, modified by a Variable Assigner node, have been fully updated. The core change introduces a mechanism within the VariableAssignerNode to signal when it is actively modifying a variable that is part of the streaming output. This allows the API to defer the streaming of affected variables until their final values are available, thereby ensuring that users receive accurate and up-to-date information in real-time streaming scenarios.

Highlights

• API Streaming Logic: Added a blocks_variable_output method to the VariableAssignerNode to identify if it modifies specific conversation variables. This new method is crucial for enabling the API to defer streaming responses until these variables are updated, ensuring data consistency.
• New Workflow Test Fixture: Introduced a new workflow test fixture (test_streaming_conversation_variables_v1_overwrite.yml) that simulates a scenario where a conversation variable is overwritten by a Variable Assigner node, providing a concrete test case for the fix.
• Unit Test for Deferred Streaming: Implemented a new unit test (test_streaming_conversation_variables_v1_overwrite_waits_for_assignment) to confirm that streaming responses correctly wait for the Variable Assigner node to update referenced conversation variables before emitting the final value, validating the intended behavior.

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[gemini-code-assist]

Code Review

This pull request effectively addresses an issue where a streaming response could be sent before a referenced conversation variable was updated by a VariableAssignerNode. The fix is well-implemented by introducing a blocks_variable_output method in the v1 VariableAssignerNode, allowing the workflow engine to correctly defer the stream. The addition of a dedicated test case using a new workflow fixture is excellent, as it specifically verifies that the streamed variable chunks contain the updated value, confirming the fix is working as intended. The code is clean, and the test is comprehensive. Great work.

[Nov1c444]

LGTM