fix(runtime): resolve deadlock on converging conditional paths

[Domenic-MZS]

fix(runtime): resolve deadlock on converging conditional paths

Fixes #5501

📝 The Deadlock Problem

The issue described by @Sohorev is a classic "deadlock" in graph-based systems, specifically in Directed Acyclic Graphs (DAGs) that incorporate conditional branching.

When there was a condition (IF/Condition Node) that split the flow into Path A and Path B, and then both paths later converged into a single Node C, the following occurred:

• The condition chose Path A.
• The system marked Path B to be "skipped" (ignored).
• Node C (the converging node) was prepared and detected that it had two dependencies: one coming from A and one from B.
• Node C waited for inputs from both.
• The input from A arrived correctly.
• The input from B never arrived because Node B was ignored and never executed.
• Node C was stuck waiting eternally in waitingNodes.
• The execution queue (nodeExecutionQueue) emptied, and the process terminated silently because no active nodes remained, but Node C was never executed.
  ...

---

To avoid this, the processNodeOutputs function was modified, so that when the system determines which nodes should be ignored (ignoreNodeIds), it also "notifies" the nodes depending on them (the descendant nodes) that the specific input would not arrive, so that they would stop waiting for it.

This was achieved by simulating a null value to propagate within the ignored nodes to its dependents. This satisfied the dependency and allows the convergence node to execute using only the data from the active branch. (Ignore null value is already implemented so this works like a charm).

TL;DR

Previously, when conditional branches merged into a single node, execution would silently halt. The merging node would wait indefinitely for input from the "skipped" branch.

This fix propagates the ignored status to downstream dependencies by injecting null inputs. This satisfies the waiting condition (hasReceivedRequiredInputs) and allows the flow to proceed using data from the active branch only.

[gemini-code-assist]

Summary of Changes

Hello @Domenic-MZS, 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 resolves a critical deadlock issue in the runtime execution of graph-based systems, particularly in Directed Acyclic Graphs (DAGs) that utilize conditional branching. Previously, when a conditional path was skipped, any downstream node that converged from both the active and skipped paths would become stuck, waiting indefinitely for an input that would never arrive. The implemented solution modifies the processNodeOutputs function to actively notify dependent nodes of skipped paths by propagating null values, thereby allowing converging nodes to correctly satisfy their input requirements and continue execution using only the valid data from active branches.

Highlights

• Deadlock Resolution: Addresses a critical deadlock scenario in Directed Acyclic Graphs (DAGs) with conditional branching, where converging nodes would previously wait indefinitely for inputs from skipped paths.
• Skipped Path Propagation: Introduces a mechanism to actively propagate the 'skipped' status of an ignored node to its downstream dependencies by simulating a null value input.
• Dependency Satisfaction: Ensures that converging nodes correctly receive a null input from skipped branches, thereby satisfying their dependencies and allowing execution to proceed using only valid data from active branches.

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[gemini-code-assist]

Code Review

This pull request effectively resolves a deadlock issue in conditional graph paths by propagating a null signal down the skipped branches. The implementation is clear and directly addresses the problem described. I have one suggestion to improve the efficiency of finding downstream nodes, but overall this is a great fix.

[Domenic-MZS]

Does not work when:

A: If block
B: Human in the loop
C: Text Node

A-------B----|
|-------------C

Where
A->B
B->C
A->C

After resuming the execution from B, C is not executed...

[HenryHengZJ]

ah I see the reason now, thanks for pointing out the Human In the Loop part as well