feat: enforce packager defaults

[drewstinnett]

Not completely addressing #100, but giving it a first go!

This PR adds in some deb specific defaults, specifically around:

• Although the docs are a little murky (some say this field must be set, others say it is just recommended), there are some 3rd party tools that will barf if priority isn't set, so it feels like a good idea to set this to a safe default on all debs
• Maintainer is a 'mandatory' field...leaving it unset still results in a usable deb package, however installing the deb package prints a nasty warning about not having a maintainer

Would love any feedback/suggestions around making this better.

One thing I'm a little torn on is setting the Maintainer field to a bogus value. Even though this field is marked as mandatory in some of the deb documentation, but I would hate to break existing setups by making nfpm for real require it, instead of just setting a bogus value.

When (or if 🤣) you're cool with the implementation details, I'll be happy to extend this to rpm/apk too. Thanks!!

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[erikgeiser]

My preferred way would be this: Set a bogus value but print out a "deprecation" warning. Don't document the bogus value but document that leaving is empty is "deprecated". After a while (or with v3 if necessary) we remove the bogus value and force the maintainer to be set.

Maybe another word would fit this purpose better than "deprecation", though.

What do you you think?

[drewstinnett]

@erikgeiser , that would work from my end! I will update the PR

[erikgeiser]

A safe value for the priority would be optional I guess. According to the docs: "This is the default priority for the majority of the archive. Unless a package should be installed by default on standard Debian systems, it should have a priority of optional. Packages with a priority of optional may conflict with each other."

Oh by the way, maybe the bogus value should be Unset Maintainer <unset@localhost>?

[erikgeiser]

That kind of makes it sound like the maintainer field will be removed. What about DEPRECATION WARNING: Leaving the 'maintainer' field unset will not be allowed in a future version. Same in the docs.

[drewstinnett]

Very good call, adjusting now. Ty for the thoughtful feedback on this!

[caarlos0]

LGTM!

thanks!

[codecov]

Codecov Report

Merging #372 (ccdd639) into master (df14bee) will increase coverage by 0.16%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #372      +/-   ##
==========================================
+ Coverage   65.03%   65.19%   +0.16%     
==========================================
  Files          14       14              
  Lines        1284     1290       +6     
==========================================
+ Hits          835      841       +6     
  Misses        313      313              
  Partials      136      136              

Impacted Files	Coverage Δ
deb/deb.go	67.17% <100.00%> (+0.60%)	⬆️

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