[RFC 0089] Collect non-source package meta attribute (NixOS#89)

Co-authored-by: asymmetric <lorenzo@mailbox.org>

rfcs/0089-collect-non-source-package-meta.md

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+---
+feature: collect-non-source-package-meta
+start-date: 2021-03-14
+author: Robert Scott
+co-authors: (find a buddy later to help out with the RFC)
+shepherd-team: @asymmetric, @aforemny, @alyssais
+shepherd-leader: @asymmetric
+related-issues: (will contain links to implementation PRs)
+---
+
+# Summary
+[summary]: #summary
+
+Collect and maintain a new `meta` attribute in packages allowing users to easily
+identify and manage their preference for binary (more broadly "non-source")
+packages.
+
+# Motivation
+[motivation]: #motivation
+
+Different users have different expectations from a software distribution. We
+acknowledge that much with the collection of license information and the
+existence of the `allowUnfree` nixpkgs option, much as Debian maintains a
+separate `-nonfree` repository.
+
+Similarly, there are a number of different reasons users may have to disfavour
+those packages not built-from-source:
+
+- Transparency: an ever-growing concern with more focus than ever on
+  supply-chain attacks.
+- Malleability: being able to conveniently override packages with patches or an
+  altered build process is a key advantage of Nix, and for nixpkgs maintainers
+  it's not generally possible to backport security fixes to binary packages.
+
+For some users, these concerns are enough to deter them from using Nix entirely.
+
+# Detailed design
+[design]: #detailed-design
+
+Add a new `meta` attribute to non-source-built packages, `sourceProvenance`.
+The value of this attribute being a list of at least one entry from a
+collection of possibilities maintained in `lib.sourceTypes`. These possibilities
+should have entries to represent at least "built from source", "binary native
+code", "binary bytecode" and "binary firmware".
+
+Packages built from source can be left as-is with the assumption of a missing
+attribute being the equivalent of `[ lib.sourceTypes.fromSource ]`.
+
+Multiple values present in a package's `sourceProvenance` would be used to
+mean that the package contains parts that fall under each of these categories.
+However, a "source type" not appearing in a package's `sourceProvenance` would
+_not_ necessarily mean that the package _doesn't_ contain parts which fall
+under that category - it could simply mean that a package hasn't been fully
+assessed yet. See "Future work" for discussion of adding the ability to make
+such a "comprehensive" declaration.
+
+Add a mechanism to allow `.nixpkgs/config.nix` to specify
+`allowNonSource = false` to prevent use of these packages in a similar manner
+to `allowUnfree`. An `allowNonSourcePredicate` parameter would allow the
+distinction to be customized, but the default predicate should take into account
+the possible hierarchical nature of `lib.sourceTypes` entries.
+
+# Alternatives
+[alternatives]: #alternatives
+
+The original proposal used a simple boolean attribute to declare whether the
+package contains any binary parts, mostly in an attempt to avoid having
+to go down the route of devising and debating an ontology of source types. This
+was deemed by the shepherding meeting to not embrace extensibility enough.
+
+Another suggestion involved using a single value from a collection of "source
+types" to describe the source provenance in an effort to avoid complexity, but
+this appeared to have the disadvantages of requiring an ontology to be agreed
+upon and still yet not providing sufficient flexibility to cover many cases.
+The missing ability to express multiple provenances might even encourage
+maintainers to proliferate source types that represent combinations of others.
+
+There already exists a rather informally-applied convention of adding a `-bin`
+suffix to the package names of "binary packages". This is non-ideal because:
+
+- It doesn't allow a user to filter the use of these packages in a better way
+  than simply not requesting a package with a `-bin` suffix. Binary-package
+  _dependencies_ of non-`-bin` packages will still be installed regardless.
+- It falls into the terminology trap over the term "binary", and if we expanded
+  the definition of what a "binary" package is, *very many* packages in nixpkgs
+  would have to be renamed, causing not only visual clutter but possible
+  breakage and churn.
+
+If we _don't_ do anything about this, then I think we continue to signal to
+users who have such concerns over the source of their software that
+nixpkgs/NixOS isn't for them. Far from being a concern just for obscure
+extremists, most Debian users would probably balk at our appetite for binary
+packages.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+- It could spur us to disappear into endless navel-gazing conversations over
+  the `lib.sourceTypes` ontology.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+Exact attribute names and contents of `lib.sourceTypes` are open for debate.
+
+# Future work
+[future]: #future-work
+
+The author is willing to spend a significant amount of time finding and marking
+non-source packages in nixpkgs.
+
+Addition of a simple accompanying boolean flag could allow the meaning of the
+`sourceProvenance` field to be changed to imply the declaration is
+"comprehensive" and that "source types" missing from the declaration are not
+present. This is something that could be added once a maintainer has thoroughly
+inspected a package, but should not place extra burden on someone wanting to
+simply flag up that they have spotted some binary element in the package.