[copilot-finds] Bug: First yielded value in orchestrator run() is not validated, causing silent hangs

[github-actions]

Problem

RuntimeOrchestrationContext.run() does not validate that the first value yielded by an orchestrator generator is a Task instance. The resume() method (which processes subsequent yields) already validates this with an instanceof Task check and throws a clear error:

// In resume() — line 184
if (!(value instanceof Task)) {
  throw new Error("The orchestrator generator yielded a non-Task object");
}

However, run() (which processes the first yield) silently accepts any value — null, undefined, plain objects, or primitives — and assigns it to _previousTask without validation. There was even a TODO comment acknowledging this gap: "// TODO: check if the task is null?".

File: packages/durabletask-js/src/worker/runtime-orchestration-context.ts, lines 109–132

Root Cause

The run() method was written without the same validation guard that resume() has. When the generator yields a non-Task value on its first yield:

1. _previousTask is set to the non-Task value (e.g., null, 42, {foo: "bar"})
2. The instanceof Task check on the isComplete guard fails silently — no error, just skips
3. Later, when resume() is called by a completion event, it checks _previousTask.isFailed and _previousTask.isComplete — both are undefined on non-Task values
4. The generator is never advanced, and the orchestration hangs indefinitely with no error

Impact

• Severity: Medium — causes orchestrations to hang silently with no error message
• Affected scenarios: Any orchestrator that accidentally yields a non-Task value on its first yield (e.g., a raw Promise, null, or forgetting to call a context method that returns a Task)
• Debugging difficulty: High — no error is thrown, no log is emitted; the orchestration simply stops progressing

[github-actions]

Branch copilot-finds/bug/validate-first-yield-in-orchestrator-run has been pushed with the fix and tests. A PR could not be created automatically because GitHub Actions is not permitted to create pull requests in this repo.

To create the PR manually:

gh pr create --head copilot-finds/bug/validate-first-yield-in-orchestrator-run --base main \
  --title "[copilot-finds] Bug: Validate first yielded value in orchestrator run() to prevent silent hangs" \
  --label copilot-finds

Changes summary

Source fix (runtime-orchestration-context.ts):

• Added instanceof Task validation on the first yielded value in run(), matching the existing validation in resume()
• Non-Task yields now throw: "The orchestrator generator yielded a non-Task object"
• Removed stale TODO comment and redundant instanceof guard

Tests (orchestration_executor.spec.ts):

• 4 new tests: yielding null, undefined, plain object, or primitive as first value → orchestration fails with clear error

All 911 unit tests pass. Lint passes.