pad/.github/workflows/nix.yml

name: Nix

on:
  push:
    branches: [main]
  pull_request:
    branches: [main]

concurrency:
  # PR runs share a group per ref and supersede each other — a newer push to the
  # same PR makes the older run's answer worthless.
  #
  # PUSH runs get a group of their OWN, one per commit, and this is deliberate
  # (TASK-2954). A run on `main` may owe main a vendorHash heal, and an evicted
  # run never performs it. `cancel-in-progress: false` is not enough on its own —
  # GitHub holds only ONE pending run per group, so a third push evicts the
  # queued second, and the eviction looks exactly like a run that decided there
  # was nothing to do. A per-commit group cannot be evicted by anything.
  #
  # What an evicted heal costs is now DELAY, not the fix. Since BUG-2974 the
  # heal is gated on the committed hash rather than on what the push touched, so
  # the next push to main meets the same stale hash and heals it from its own
  # run. The per-commit group is what makes the heal land on the merge that
  # caused it instead of waiting for whatever lands next.
  #
  # What that trades for: two main runs can reach the push step at once, and the
  # loser gets a non-fast-forward rejection and goes red. That is the right
  # direction — a red job is read, and the next push to main heals anyway.
  group: nix-${{ github.event_name == 'push' && github.sha || github.ref }}
  cancel-in-progress: ${{ github.event_name == 'pull_request' }}

permissions:
  contents: read

# All third-party Actions are pinned to a 40-char commit SHA with a trailing
# '# vX.Y.Z' comment so a compromised maintainer or moved tag cannot silently
# execute attacker code in CI. Bump the SHA + comment together when updating.

jobs:
  nix:
    name: Nix build & check
    runs-on: ubuntu-latest
    # NO write permission here, deliberately — see `push-vendor-hash` below.
    # This job builds; it never pushes.
    outputs:
      vendor_hash_bumped: ${{ steps.vendorhash.outputs.bumped }}
    steps:
      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1

      - uses: DeterminateSystems/nix-installer-action@3138316df39ed29be04236d7ffc686fa525866aa # v23

      # THE RED-BASELINE FIX (TASK-2954). `vendorHash` pins the Go module set by
      # content hash; Dependabot updates go.mod/go.sum and has no idea this file
      # exists, so EVERY Go bump PR failed the steps below on a hash mismatch,
      # structurally and forever. A check that is always red is not a check —
      # a bump that genuinely breaks the build looked identical, at a glance, to
      # one that only moved the hash.
      #
      # This recomputes the hash in the WORKING TREE, so the steps below judge
      # the build with this ref's actual module set. Nothing is pushed from here
      # and no token is used: a Dependabot PR run gets a read-only GITHUB_TOKEN
      # (it runs as if from a fork), and the `permissions` key does NOT lift that
      # — only a repository-wide setting does, which would hand fork PRs write
      # tokens too. The push therefore happens on `push: main` instead, in the
      # job below, where the run is not Dependabot's.
      #
      # It runs on every run, not just Dependabot's: a human's own go.mod change
      # moves the hash the same way, and a check that is honest only for one
      # author is the shape this item exists to remove.
      #
      # It runs BEFORE `nix flake check`, which builds the package too and would
      # hit the same mismatch first. It is deliberately not a gate: if the build
      # fails for any reason other than a go-modules hash mismatch, the script
      # exits non-zero, this step still succeeds, and the real steps below report
      # the real failure. The gate stays exactly where it was.
      - name: Recompute vendorHash before judging the build
        id: vendorhash
        run: |
          set -o pipefail
          if nix build .#default --print-build-logs 2>&1 | tee /tmp/nix-build.log; then
            echo "Build is green; vendorHash is current."
            exit 0
          fi
          if ! new_hash="$(nix/bump-vendor-hash.sh /tmp/nix-build.log)"; then
            echo "Not a go-modules hash mismatch — leaving it to the build steps below."
            exit 0
          fi
          echo "vendorHash -> $new_hash"
          echo "bumped=true" >> "$GITHUB_OUTPUT"

      - name: Carry the recomputed package.nix to the push job
        if: steps.vendorhash.outputs.bumped == 'true'
        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
        with:
          name: recomputed-package-nix
          path: nix/package.nix
          retention-days: 1

      - name: nix flake check
        run: nix flake check --print-build-logs

      - name: nix build
        run: nix build .#default --print-build-logs

      - name: Smoke test binary
        run: |
          ./result/bin/pad --version
          ./result/bin/pad --help

      # The Go toolchain here exists only to install govulncheck for the
      # scan step below — the pad binary under test was built by Nix above.
      - uses: actions/setup-go@b7ad1dad31e06c5925ef5d2fc7ad053ef454303e # v7.0.0
        with:
          go-version: "1.26"

      - name: Scan Nix-built binary with govulncheck
        # CI's main govulncheck job scans a `go build` binary, which honours
        # go.mod's `toolchain go1.26.10` line — but nixpkgs pins
        # GOTOOLCHAIN=local, so the Nix artifact is built with whatever Go
        # patch the nixos-26.05 channel ships and can carry stdlib advisories
        # the main gate can no longer see (BUG-2567). This scans the artifact
        # that actually ships via Nix, against nix/accepted-advisories.txt:
        # known-accepted advisories stay green and visible, any NEW advisory
        # fails the job, and a warning fires when an accepted advisory clears
        # (the signal to prune the list and eventually close BUG-2567).
        #
        # govulncheck pinned to the same release as ci.yml — bump the two
        # together. GOTOOLCHAIN=auto on the install mirrors ci.yml's
        # rationale: setup-go exports GOTOOLCHAIN=local, and if govulncheck's
        # own go.mod ever requires a patch newer than setup-go's resolved
        # one, `auto` lets Go fetch it instead of failing the install.
        run: |
          GOTOOLCHAIN=auto go install golang.org/x/vuln/cmd/govulncheck@v1.2.0
          GOVULNCHECK="$(go env GOPATH)/bin/govulncheck" nix/vulnscan.sh result/bin/pad

  # THE ONLY JOB WITH A WRITE TOKEN, and it exists as a separate job precisely so
  # that the token is never minted in a context an untrusted change could reach.
  #
  # An `if:` on a STEP is not a security boundary — the job would still hold a
  # write-capable GITHUB_TOKEN, and `actions/checkout` persists it in the
  # workspace for every later command, so any change to this file inside a PR
  # could use it. An `if:` on a JOB is different in kind: the job does not start,
  # and no write token is issued for that run at all.
  #
  # WHY ON `push: main` AND NOT ON THE PR (ruled on TASK-2954's trail). A
  # Dependabot PR run gets a read-only token whatever `permissions` says. The
  # merge, though, is authored by a human, so the `push` run that follows is an
  # ordinary run with an ordinary token. The PR's own check was already honest,
  # because the job above recomputes in the working tree; `main` is what needs
  # the committed fix, and it gets it from the run of the merge itself — or,
  # when that run loses the push race, from the run of the next push to main.
  # The two rejected alternatives were a PAT in the Dependabot secret store (a
  # standing credential for a once-a-week fix) and enabling write tokens for
  # pull-request workflows (which would give them to FORK PRs on a public repo —
  # the surface CONVE-2438 exists to keep closed).
  #
  # THE LOOP TERMINATES BECAUSE THIS PUSH TRIGGERS NOTHING. A push made with
  # the default `GITHUB_TOKEN` creates no workflow runs, so the commit this job
  # pushes gets no run at all. Observed on the first heal this fix produced
  # (BUG-2974, day 62): `gh run list --branch main` spans cbfc073e ->
  # 31d11e76 -> 0b16be49 and skips the heal commit 249a8f87 entirely. Read the
  # BRANCH listing, not `gh run list --commit` — that query also returned
  # nothing for 0b16be49, which demonstrably has runs, so it cannot tell
  # "no runs" from "no answer". The loop is closed by construction rather than
  # by being green.
  #
  # THE CORRECTED-TREE TERMINATOR IS THE BACKSTOP, AND IT HAS NEVER RUN. If
  # this push did trigger a run, the build job above would find the hash
  # correct, print "Build is green; vendorHash is current.", never set
  # `bumped`, and this job would not start — the FIX's own effect on the build,
  # holding regardless of what any push touched. That is an argument, not a
  # measurement: nothing has ever exercised it. It becomes load-bearing the
  # moment this push uses a PAT or a GitHub App token, which is the obvious
  # edit for anyone who wants CI to validate the healed tree. Making that
  # change inherits an untested terminator; exercise it before relying on it.
  #
  # A CONSEQUENCE OF THE SAME MECHANISM, not a defect: the healed tip carries
  # no CI of its own. Its tree is the merge commit plus the one hash line that
  # merge's own Nix run computed and built against, and the next push to main
  # builds on it normally.
  #
  # THE RANGE-VS-STATE DISTINCTION IS LOAD-BEARING, AND IT WAS GOT WRONG ONCE
  # (BUG-2974). This
  # comment used to name the loop guard as a gate asking whether the push had
  # touched `go.mod`/`go.sum`, and that gate existed as a step below. It is the
  # right question for loop prevention and the WRONG one for recovery: after a
  # lost push race the tree that needs healing was written by an EARLIER push,
  # so every later merge that did not itself move the module set refused to fix
  # it, exited GREEN, and left main carrying a hash a clean nix build rejects.
  # The gate is gone. The state comparison below is the only gate, and being a
  # question about state it is equally true for a hash this push broke and one
  # an earlier push broke and could not land.
  #
  # If another merge lands while this job runs, the push is REJECTED as a
  # non-fast-forward and this job goes RED rather than overwriting anything —
  # and the heal is genuinely not lost now, because the next push to main meets
  # the same stale hash, recomputes it, and heals it from its own run.
  #
  # `needs: nix` without `if: always()` is the second half: this runs only when
  # the build job SUCCEEDED, so a module set whose corrected tree still fails
  # `nix flake check`, the smoke test or the vulnerability scan is never
  # committed to main.
  push-vendor-hash:
    name: Heal main's vendorHash
    needs: nix
    if: >-
      github.event_name == 'push' &&
      github.ref == 'refs/heads/main' &&
      needs.nix.outputs.vendor_hash_bumped == 'true'
    runs-on: ubuntu-latest
    permissions:
      contents: write
    steps:
      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
        with:
          # Nothing here reads history any more — BUG-2974 removed the gate that
          # did. The depth stays full rather than dropping to the default shallow
          # clone: this is the clone the heal commit is pushed from, full depth is
          # what every run of this job has ever used, and a shallow push is not a
          # variable worth introducing on a job that runs only when a Go bump
          # actually moves the module set.
          fetch-depth: 0

      - uses: actions/download-artifact@3e5f45b2cfb9172054b4087a40e8e0b5a5461e7c # v8.0.1
        with:
          name: recomputed-package-nix
          path: nix/

      # THE GATE, and the only one, lives in the script: does what main COMMITTED
      # differ from what the build job RECOMPUTED? The artifact exists only
      # because the build job found a mismatch, but comparing there keeps the job
      # honest about the tree it is actually looking at rather than about what
      # another job reported. It is a file rather than a `run:` block so that
      # nix/heal-vendor-hash_test.sh can drive it against a real git remote —
      # BUG-2974 shipped green for a day because nothing could exercise it.
      - name: Commit and push the recomputed hash
        run: nix/heal-vendor-hash.sh
        env:
          SHA: ${{ github.sha }}