Fix custom job runs-on array and object forms silently dropped

[Copilot]

• Fix runs-on handling in buildCustomJobs() to support array and object forms
• Add unit tests for string, array, and object runs-on forms
• Add error path test case using unmarshalable value (channel), asserting error mentions job name
• Run make fmt, make lint, make build to validate

Original prompt

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This section details on the original issue you should resolve

<issue_title>[plan] Fix custom jobs runs-on array and object forms being silently dropped</issue_title>
<issue_description>## Objective

Fix buildCustomJobs() in pkg/workflow/compiler_jobs.go to correctly handle runs-on values that are arrays or objects, not just strings.

Context

From discussion #19383 (HIGH-2): The current code does a string type assertion on runs-on:

if runsOnStr, ok := runsOn.(string); ok {
    job.RunsOn = "runs-on: " + runsOnStr
}
// array/object: falls through silently, job.RunsOn stays ""

When runs-on is an array (e.g., [self-hosted, linux, large]), the assertion fails, job.RunsOn stays empty, and the compiled YAML is invalid — missing the required runs-on field. The top-level runs-on correctly uses extractTopLevelYAMLSection which handles all forms.

Approach

In pkg/workflow/compiler_jobs.go, in buildCustomJobs() around line 465, update the runs-on extraction:

1. For the string case: keep existing behavior ("runs-on: " + runsOnStr)
2. For array and object cases: use goccy/go-yaml to marshal the value back to a YAML snippet, then prepend "runs-on: "
3. Add a validation error if runs-on is present but cannot be marshaled

Example target output for array form:

runs-on:
  - self-hosted
  - linux
  - large

Files to Modify

• pkg/workflow/compiler_jobs.go — update runs-on handling in buildCustomJobs()

Acceptance Criteria

• runs-on: ubuntu-latest (string) continues to work correctly
• runs-on: [self-hosted, linux, large] (array) compiles to valid YAML
• runs-on: {group: my-runners} (object) compiles to valid YAML
• Invalid runs-on values produce a validation error (not silent empty output)
• Unit tests added for all three runs-on forms
• Run make agent-finish before committing

Generated by Plan Command for issue #discussion #19383 · ◷

• expires on Mar 5, 2026, 10:49 AM UTC

Comments on the Issue (you are @copilot in this section)

• Fixes [plan] Fix custom jobs runs-on array and object forms being silently dropped #19400

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[Copilot]

Pull request overview

Fixes buildCustomJobs() so custom job runs-on is no longer silently dropped when specified in YAML array/object form, preventing invalid compiled workflows.

Changes:

• Extend custom job runs-on extraction to support string, array, and object YAML forms (marshaling non-string forms to YAML and emitting an error on marshal failure).
• Add a unit test covering string/array/object runs-on forms for custom jobs.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
pkg/workflow/compiler_jobs.go	Updates custom job runs-on handling to marshal array/object forms into an indented YAML snippet instead of dropping the field.
pkg/workflow/compiler_jobs_test.go	Adds TestBuildCustomJobsRunsOnForms to verify string/array/object runs-on forms are preserved in job.RunsOn.

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[Copilot]

The new runs-on handling returns an error when YAML marshaling fails, but this test suite never exercises that path (and shouldErr is unused). Add a test case with an unmarshalable runs-on value (e.g., a function or channel) and assert that buildCustomJobs returns an error mentioning the job name; or remove shouldErr if the error-path test is intentionally out of scope.

[pelikhan]

@copilot Apply comments

[Copilot]

@copilot Apply comments

Done in 0ceede6. Added a fourth test case "unmarshalable value returns error" using make(chan int) (channels can't be marshaled by go-yaml), which confirms buildCustomJobs returns an error that includes the job name.