fix(workflow): tolerate absent upstream jinja2 variables in LLM nodes (#38655)

[natemiller23]

Summary

Fixes #38655 — when a conditional branch feeds an LLM node whose Jinja2 template guards an optional upstream variable with {% if var is defined and var %}, Dify crashed before template evaluation with Variable <name> not found.

The crash originated in graphon's LLMNode._fetch_jinja_inputs, which raised VariableNotFoundError for every entry in prompt_config.jinja2_variables whose upstream selector was missing from the variable pool. graphon's own renderer (_render_jinja2_message) already treats a missing variable as an empty string, so the strict pre-check was the only thing failing.

Fix

Patch graphon's LLMNode._fetch_jinja_inputs at node-registration time (register_nodes in api/core/workflow/node_factory.py) so an absent upstream jinja2 variable is tolerated and skipped instead of raising. The user-authored is defined guard now decides whether the branch renders — exactly as the issue requests.

• graphon is an external pinned dependency (graphon==0.6.0), so we monkey-patch the live class once at registration time rather than fork the package.
• Prompt-template inputs, context and memory paths are unchanged (still required).
• The patch is idempotent (guarded by a marker attribute).

Testing notes

Unit tests added / updated:

• api/tests/unit_tests/core/workflow/test_node_factory.py::TestJinjaMissingVariableTolerance
  • test_absent_jinja_variable_is_tolerated: absent jinja2 variable no longer raises VariableNotFoundError; present variables still collected.
  • test_only_present_variables_collected: only present upstream variables are returned in the jinja input mapping.
• api/tests/unit_tests/core/workflow/nodes/llm/test_node.py
  • Updated test_fetch_jinja_inputs_raises_for_missing_variable -> test_fetch_jinja_inputs_tolerates_missing_variable to assert the new intended behavior (returns {} instead of raising).

Run with:

uv run --project api --python 3.12 --no-default-groups --group dev --group storage --group tools --group trace-all pytest api/tests/unit_tests/core/workflow/test_node_factory.py::TestJinjaMissingVariableTolerance api/tests/unit_tests/core/workflow/nodes/llm/test_node.py -k jinja -q

Result: all green (the regression suite for the LLM node jinja path passes).

OpenJobs

• openjobs.bot
• https://openjobs.bot/jobs/b13fc23b-8eff-477a-afa3-79bdcd77f665
• b13fc23b-8eff-477a-afa3-79bdcd77f665

Refs: #38655

[crazywoola]

Hi @natemiller23, thanks for opening this pull request.

Why this is being closed

Issue #38655 has multiple open pull requests. Under the duplicate-PR rule, pull requests from the issue author are ranked first, then creation time is used as the FIFO tie-breaker. PR #39153 is the retained pull request, so this one is being closed.

Next steps

Please coordinate on PR #39153 if you would like to contribute further. Thank you for understanding.