fix(#13844): canonicalize job to avoid nil pointer deference

[tristanpemble]

I am new to this codebase, so unsure I have the best approach, but this fixes my issue in #13844. I traced the issue down to the nil pointer here, which is called from this path.

This code is called after the job registration succeeds on the server. I believe the root of the issue is that when submitting an API job via the CLI, it is canonicalized server-side, but not client-side. Registration succeeds on the server, and then this code path is called. It references paths that have not been canonicalized with their default values, and fails.

Another option would be to canonicalize the job before registration (maybe in the JobGetter when loading the job?), but being new to the codebase, I am wary of the unintended change in behavior that might create for endusers.

[hashicorp-cla]

All committers have signed the CLA.

[Amier3]

Hey @tristanpemble

Thanks for making this PR and congrats on the first contribution. We'll take a look at this soon 😄

[tgross]

Hi @tristanpemble! Thanks for this PR!

The fix you've got here will totally work for the case of the CLI, but it'll still be broken for other consumers of the HTTP API. The panic you've hit in #13844 is at api/jobs.go#L788. The p pointer is nil, and that causes the panic when we try to access one of its fields. But in this case, we have a safe fallback behavior later in the Next method. So a fix like the diff below should fix both the CLI panic you've hit and prevent anyone else from hitting the same bug if they don't call Canonicalize():

-if *p.SpecType == PeriodicSpecCron {
+if p != nil && *p.SpecType == PeriodicSpecCron {

If you can make this change I'd be happy to get this PR merged. You can also add a changelog entry file at .changelog/13845.txt with the following text:

```improvement
api: Fixed a bug where accessing the next periodic invocation for a job could panic if not set
```

[tgross]

Just FYI, there's no need to have the issue number in the comments. That's what git blame is for 😀

[tgross]

Hi! 👋 I want to get this PR landed in the upcoming Nomad 1.5.0, so I've pushed a commit with the requested fix and a changelog entry. Once that's green I'll get this merged.