Fix job runner stdout missing newline

[jennan]

Kia ora,

While implementing a new job runner, I realised that the if the job runner submit method returns its stdout text without a newline, then a workflow with more than one task will fail. After further investigations with @hjoliver , we discovered that this is due to text handling in JobRunnerManager.jobs_submit, that doesn't properly add a newline to the text message if it is missing. This PR implements a fix.

You can reproduce the bug by removing the newline of the second output of the background job runner submit method (see https://github.com/cylc/cylc-flow/blob/master/cylc/flow/job_runner_handlers/background.py#L92) and try to run a workflow with at least 2 tasks.

I am not sure how to turn this into a test to reproduce the bug. Any guidance on this would be welcome 🙂.

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• No dependency changes
• Tests are included (or explain why tests are not needed).
• Changelog fragment included if this is a change that can affect users
• No cylc-doc PR needed
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[hjoliver]

Thanks @jennan !

@oliver-sanders (UK team) noted overnight that he ran into this some time ago, but wasn't sure it was a bug so added a note on the need for newline in the handler API documentation: https://github.com/cylc/cylc-flow/blob/master/cylc/flow/job_runner_handlers/documentation.py#L426-L428

However, now that you've tracked down the offending code, by inspection (and also by comparison with similar in jobs_kill(...), it is definitely a bug.

Most of the current job-runners don't provide an explicit submit method, they just use the command template as a shortcut. However, the background handler does, and it deliberately adds a newline to its return value. https://github.com/cylc/cylc-flow/blob/master/cylc/flow/job_runner_handlers/background.py#L92

@jennan - can you tweak this PR to:

• not add the newline in the background handler (above), as it's not necessary now
• and remove the newline advice in the docs (above)

[hjoliver]

Regarding testing: without this fix, if a job-runner submit method returns job-ID without a newline, it causes the cylc jobs-submit stdout to be misformatted (two lines concatenated) such that the scheduler can't parse it correctly - for one out of the multiple jobs submitted.

So I think it would be sufficient to refactor the job runner manger submit method to use a helper method that formats and returns the final string to be written to stdout, instead of writing incrementally to stdout as it does now. That helper method could then easily be unit tested with an inline doctest.

[hjoliver]

@jennan - no cylc-doc PR is needed for this. It could do with a change log entry though, just in case anyone else is working on job runners at the moment (unlikely but not impossible!). Just add a file changes.d/6081.fix.md with a short description (see other md files in there for example).