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ADR 0019: A platform says whether its base can fetch

Status: accepted (2026-10-18)

Context​

A BinaryArtifact is downloaded at build time and verified against its declared sha256, and RpmFile is the same shape for an RPM taken from a URL; the Containerfile backend fetches both with curl. That makes curl an assumption about a base, and it was wrong once: the Ubuntu base ships none, so the build died with /bin/sh: 1: curl: not found minutes in, naming no resource. Invariant 9 asks the opposite, and this is a platform fact because which bases carry a fetcher varies by vendor, and the package name is the platform's, not Core's. Core knows neither curl nor ca-certificates (ADR 0004, the shape of iconCache.package).

Decision​

  1. A platform declares how its base gets a verified artifact onto the image:

    downloads:
      baseFetches: false   # this base ships no fetcher
      package: curl        # and the image installs this one

    baseFetches: true is the common case and names no package; baseFetches: false requires package.

  2. Composition fails on a plan that downloads from a platform that says nothing, naming the platform and the resource that asked for the download; a plan that downloads nothing composes anywhere.

  3. The composition installs the fetcher when the base cannot provide it: the package joins the plan's installed packages, carrying the provenance of the download that needed it, so the plan answers "why is this installed" without the reader consulting a platform resource.

  4. The backend keeps naming its own tool. The backend fetches with curl and its flags; what the platform declares is that a base can satisfy it. A command line in a resource would be a per-platform copy of backend knowledge, and the copies would drift.

  5. A fetch retries transient failures and bounds the attempts that never answer: a connection error, a 5xx and a timeout are retried within a bound; a 404 or a checksum that disagrees fails at once.

Consequences​

  • Ubuntu declares baseFetches: false with package: curl, measured on its base; the EL and Fedora platforms declare baseFetches: true, measured on theirs; RHEL's base cannot be pulled here (no subscription), so its declaration rests on the EL bases that can.
  • A new platform cannot be silent about this: composing a workstation that downloads onto one fails at composition, with the platform named, instead of failing on a missing command minutes into a build.
  • The plan's package list becomes the record of what the image installs to be able to fetch, and deskos check already reports installed packages as outside its scope.
  • curl remains Core's own choice, and the platform states the base can satisfy it, or the package that makes that true.
  • This covers the one capability a build cannot proceed without, and leaves the rest to be added the same way.