job platforms: Forward lookup

[datamel]

Added forward lookup method

• checks if specified platform (will come from suite.rc file) is a valid platfrom (listed in global.rc file).
• checks in reverse order to ensure user platform selection overrides global setting.
• If no platform is selected by user, defaults to 'localhost'.
• If platform not found in list, returns a Platform error.

Tests added to ensure working as required.
These changes relate to Platform support #3453

Requirements 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).
• Appropriate tests are included (unit and/or functional).
• No change log entry required (invisible to users).
• No documentation update required as yet.

[oliver-sanders]

Looks good.

[hjoliver]

Looks good, but:

• one item name change suggested
• one question: sorry if I missed this in the proposal, but have we lost the ability to specify a job host in the suite config file if it has not already been defined in the global config file?

[wxtim]

one question: sorry if I missed this in the proposal, but have we lost the ability to specify a job host in the suite config file if it has not already been defined in the global config file?

Yes. But because the platform can be a regex (and therefore allow the creation of a lot of similar platforms) it will seem to the user like they can just add hosts, as long as the host fit the patterns. Additionally, any user who is really desperate can define their own platforms in their user config.

[hjoliver]

Here is an overview of what got changed by this pull request:

Complexity increasing per file
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- cylc/flow/platform_lookup.py  4
         

See the complete overview on Codacy