[bug] `towncrier create` allows fragment creation with invalid filenames

[webknjaz]

So in sanitizers/chronographer-github-app#44, @praiskup reported $sbj as follows:

I opened this, because we hit a real-life issue with towncrier:

$ towncrier create foo.bar.baz.config
Created news fragment at /home/praiskup/rh/projects/mock/mock/releng/release-notes-next/foo.bar.baz.config

At least we were allowed to create such file, I assumed that it "is a correct snippet" then, and I'd expect that the action accepts it as well. But is this a bug of towncrier create?

This created a false sense of implied correctness, but I'm sure that towncrier build will get confused trying to identify this file and assign identifier+category.

https://towncrier.readthedocs.io/en/stable/tutorial.html#creating-news-fragments and https://towncrier.readthedocs.io/en/stable/cli.html#towncrier-create only roughly mentions what the shape of the filename is expected to look like.
From my earlier experience and reading the source code in the past, I know that what works is <number-ref>.<category>, <number-ref>.<category>.<discardable-suffix>, <number-ref>.<category>.<number>, <number-ref>.<category>.<number>.<discardable-suffix>, +<discardable-ref>.<category>, +<discardable-ref>.<category>.<discardable-suffix>, +<discardable-ref>.<category>.<number>, +<discardable-ref>.<category>.<number>.<discardable-suffix>.

But it doesn't look clear to a casual reader that neither <number-ref>, nor <discardable-ref> is allowed to contain a period.
So I imagine this could be turned into two issues:

• Make the create and build subcommands coherent, interpreting the file names consistently (with proper input validation).
• Ensure the expectations are spelled out in the docs.

[adiroiban]

Thanks for the report.
I am happy to review and approve a PR that improves the validation for the file names.

But it doesn't look clear to a casual reader that neither , nor is allowed to contain a period.

I consider towncrier a dev oriented tool, and I was expecting that for users of towncrier when they see <number-ref>.<category>.<discardable-suffix> it's clear that the dot/fullstop character is not allowed anywhere else.

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Personally, I don't use towncrier create... for me it's easier to just create a new file using my text editor

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I think that it would be nice to work on some sort of "canonical" release notes management process... we have #705 and you have already commented... so we need to make a decision and implement and document it.

so that the available fragments will be "shared knowledge"