ci: run only quick for changes no other gate reads

.github/workflows/ci.yml

@@ -28,15 +28,37 @@ jobs:
   # be added to one without the other. Plain text parsing rather than `just
   # --dump`: this job would otherwise spend three minutes entering the Nix
   # shell to produce a list.
+  #
+  # A change that touches nothing but documents and the other workflows needs
+  # only the `quick` gate, which includes the workflow lint; select-gates.sh
+  # decides, and falls back to every gate whenever the change cannot be
+  # compared with its base. The branch rules name `all gates`, which still
+  # runs, so a skipped gate never leaves a required check pending.
   gates:
     runs-on: ubuntu-latest
     outputs:
       gates: ${{ steps.read.outputs.gates }}
     steps:
       - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+        with:
+          fetch-depth: 0
       - id: read
+        env:
+          EVENT: ${{ github.event_name }}
+          PR_BASE: ${{ github.event.pull_request.base.sha }}
+          PUSH_BEFORE: ${{ github.event.before }}
         run: |
-          gates="$(sed -n 's/^check: *//p' justfile | jq -Rce 'split(" ") | map(select(length > 0)) | select(length > 0)')"
+          all="$(sed -n 's/^check: *//p' justfile | jq -Rce 'split(" ") | map(select(length > 0)) | select(length > 0)')"
+          case "$EVENT" in
+            pull_request) base="$PR_BASE" ;;
+            push) base="$PUSH_BEFORE" ;;
+            *) base="" ;;
+          esac
+          changed=""
+          if [ -n "$base" ]; then
+            changed="$(git diff --name-only "$base" HEAD)" || changed=""
+          fi
+          gates="$(printf '%s\n' "$changed" | scripts/select-gates.sh "$all")"
           echo "gates=$gates" >> "$GITHUB_OUTPUT"
 
   # One job per gate, so a red build names the gate that failed and a slow gate

justfile

@@ -96,6 +96,7 @@ scripts:
     python3 scripts/sleep-check.py --self-test
     python3 scripts/console.py --self-test
     python3 scripts/bench-console.py --self-test
+    bash scripts/select-gates-test.sh
 
 # Proves the library crates are genuinely no_std, against the target the
 # firmware actually runs on. rustc ships no prebuilt core for xtensa.

scripts/select-gates-test.sh

@@ -0,0 +1,46 @@
+#!/usr/bin/env bash
+# Behaviour of select-gates.sh: which CI gates a set of changed files needs.
+set -u
+cd "$(dirname "$0")" || exit 1
+
+FULL='["quick","lint","host-tests","device-checks","images"]'
+QUICK='["quick"]'
+failed=0
+
+# selected <expected> <description> <changed files...>
+selected() {
+    local expected=$1 description=$2 actual
+    shift 2
+    actual=$(printf '%s\n' "$@" | ./select-gates.sh "$FULL")
+    if [ "$actual" != "$expected" ]; then
+        echo "FAIL: $description: expected $expected, got $actual"
+        failed=1
+    fi
+}
+
+# Given / When / Then in each call: the files are the given, the call is the
+# when, the expected list is the then.
+selected "$QUICK" "a root markdown file needs only quick" README.md
+selected "$QUICK" "documents need only quick" docs/public-checklist.md
+selected "$QUICK" "another workflow needs only quick" .github/workflows/release-please.yml
+selected "$QUICK" "the dependabot config needs only quick" .github/dependabot.yml
+selected "$QUICK" "a licence file needs only quick" LICENSE-MIT
+selected "$QUICK" "several unrelated files need only quick" README.md CHANGELOG.md docs/a.md
+selected "$FULL" "the pipeline itself needs every gate" .github/workflows/ci.yml
+selected "$FULL" "source needs every gate" crates/teddiebox-core/src/lib.rs
+selected "$FULL" "a markdown file inside a crate needs every gate" crates/teddiebox-core/README.md
+selected "$FULL" "the justfile needs every gate" justfile
+selected "$FULL" "a lockfile needs every gate" firmware/Cargo.lock
+selected "$FULL" "one related file among unrelated ones needs every gate" README.md crates/trf7962a/src/lib.rs
+selected "$FULL" "no changed files needs every gate"
+
+# Given a gate list without quick, When only unrelated files changed,
+# Then nothing can be skipped safely.
+actual=$(printf 'README.md\n' | ./select-gates.sh '["lint","images"]')
+if [ "$actual" != '["lint","images"]' ]; then
+    echo "FAIL: a list without quick runs in full: got $actual"
+    failed=1
+fi
+
+[ "$failed" = 0 ] && echo "select-gates: all passed"
+exit "$failed"

scripts/select-gates.sh

@@ -0,0 +1,32 @@
+#!/usr/bin/env bash
+# Reads the changed file names on stdin and the JSON list of every CI gate as
+# the first argument, and prints the JSON list of gates the change needs.
+#
+# A change confined to files no compiler, test or linter but `quick` reads
+# needs only `quick`. Anything else, an empty change, or a list without
+# `quick` needs every gate: a wrong skip hides a break, a wrong run costs
+# minutes.
+set -eu
+
+all=$1
+needs_all=false
+seen=false
+
+while IFS= read -r file; do
+    [ -n "$file" ] || continue
+    seen=true
+    case "$file" in
+        .github/workflows/ci.yml) needs_all=true ;;
+        .github/workflows/* | .github/dependabot.yml | docs/*) ;;
+        */*) needs_all=true ;;
+        *.md | LICENSE-*) ;;
+        *) needs_all=true ;;
+    esac
+done
+
+quick=$(jq -c 'map(select(. == "quick"))' <<<"$all")
+if [ "$seen" = false ] || [ "$needs_all" = true ] || [ "$quick" = '[]' ]; then
+    jq -c . <<<"$all"
+else
+    echo "$quick"
+fi