ci: check for removed slices

We would not like for slices to be deleted randomly. The CI will check
for any deleted (or renamed) slices and trigger an error if there are
any.

Author: Rafid Bin Mostofa

.github/scripts/removed-slices/removed-slices

@@ -0,0 +1,56 @@
+#!/bin/bash
+
+# This script takes in a git ref and checks if any slices present in the former
+# ref (commit, branch etc) were removed. Must be run in the chisel-releases
+# root directory. It exits with an error (1) if removals found.
+#
+# Note: there may be errors if the current git status is dirty, since this
+#       script does "git checkout" to the old ref and back.
+#
+# Deps: yq (https://github.com/mikefarah/yq).
+
+set -eu
+
+if (( $# != 1 )); then
+  echo -e "Usage:\n\t$(basename "$0") <old-git-ref>"
+  exit 1
+fi
+
+export LC_COLLATE="C"
+oldref="$1"
+curref="$(git rev-parse --short HEAD)"
+
+# Revert to the current git ref on exit.
+revert() {
+  git checkout -q "$curref"
+}
+trap revert EXIT
+
+# Lists all the slices in the 'slices/' directory.
+list() {
+  local pkg f
+  echo "Checking slices in $(git rev-parse --short HEAD)..." >&2
+  # Iterate over the files in a sorted order to produce deterministic order.
+  for f in $(find "slices/" -name "*.yaml" | sort); do
+    pkg="$(yq '.package' "$f")"
+    # Get the slice names from the file, prefix with package name and an
+    # underscore and finally, sort.
+    yq '.slices | keys | .[]' "$f"    \
+      | sed "s/^/$pkg\_/"             \
+      | sort
+  done
+}
+
+# List of slices in current ref, in a file.
+cur="$(list)"
+
+# List of slices in old ref, in a file.
+git checkout -q "$oldref"
+old="$(list)"
+
+# Check which slices are removed.
+# The following 'diff' syntax only reports the deletions from the 'old' file.
+echo "The following slices from $oldref have been removed:"
+diff --changed-group-format='%<' --unchanged-group-format='' \
+  <(echo "$old") <(echo "$cur")                              \
+  && echo "None"

.github/workflows/removed-slices.yaml

@@ -0,0 +1,43 @@
+name: "Removed slices"
+
+on:
+  push:
+  pull_request:
+  workflow_call:
+
+jobs:
+  check:
+    name: "Check for removed slices"
+    runs-on: "ubuntu-latest"
+    env:
+      files_main: "main-files"
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          # We do need to fetch all history for all branches.
+          fetch-depth: 0
+
+      - name: "Set reference to main branch"
+        id: set-main-ref
+        run: |
+          if [[ "${{ github.base_ref }}" == "main" ]]; then
+            # For PRs to main, use the updated files.
+            # Leaving checkout_main_ref unset will checkout the PR head_ref.
+            echo "ref_main=" >> $GITHUB_OUTPUT
+          else
+            echo "ref_main=main" >> $GITHUB_OUTPUT
+          fi
+
+      - name: "Checkout main branch"
+        uses: actions/checkout@v4
+        with:
+          ref: ${{ steps.set-main-ref.outputs.ref_main }}
+          path: ${{ env.files_main }}
+
+      - name: "Install yq"
+        uses: mikefarah/yq@v4.45.4
+
+      - name: "Check for removed slices"
+        run: |
+          ln -s "${{ env.files_main }}/.github/scripts/removed-slices/removed-slices" .
+          ./removed-slices "${{ github.base_ref || github.event.before }}"