ci: check that the workflow references resolve

actionlint reads a local action's inputs but never checks its path
exists: uses: ./.github/actions/typo lints clean and fails only when the
job runs. Covers composite action paths, make targets including the ones
the examples matrix builds from $SANDERLING, and the scripts a run: step
invokes plus their executable bit.

Fails when it parses fewer references out of a file than that file
mentions, because a checker that matches nothing reports a safety it
never looked for.

.github/scripts/workflow-refs.sh

@@ -0,0 +1,104 @@
+#!/usr/bin/env bash
+# Checks that everything the workflows name actually exists: composite actions,
+# make targets, and the scripts a run: block invokes.
+#
+# This is the class actionlint does not cover. `uses: ./.github/actions/typo`
+# lints clean and fails only when the job runs, and these workflows are
+# dispatch-only or push-triggered, so that first run is after merge.
+set -euo pipefail
+
+root="$(cd "$(dirname "${BASH_SOURCE[0]}")/../.." && pwd)"
+
+ROOT="$root" python3 - <<'PY'
+import glob
+import json
+import os
+import re
+import sys
+
+root = os.environ["ROOT"]
+problems = []
+checked = 0
+
+
+def report(ok, label, detail=""):
+    global checked
+    checked += 1
+    if not ok:
+        problems.append("%s%s" % (label, detail))
+    print("  %-4s %s%s" % ("ok" if ok else "MISS", label, detail))
+
+
+def workflow_files():
+    return (sorted(glob.glob(os.path.join(root, ".github/workflows/*.yml")))
+            + sorted(glob.glob(os.path.join(root, ".github/actions/*/action.yml"))))
+
+
+def rel(path):
+    return os.path.relpath(path, root)
+
+
+# --- composite actions -------------------------------------------------------
+print("local action references:")
+local_refs = 0
+for path in workflow_files():
+    # Comments are not references. They mention paths as examples, and a version
+    # comment trails the `uses:` line of every pinned action.
+    body = re.sub(r"#[^\n]*", "", open(path).read())
+    found = re.findall(r"^\s*-?\s*uses:\s*(\./\S+)\s*$", body, re.M)
+    local_refs += len(found)
+    for ref in found:
+        target = os.path.join(root, ref[2:], "action.yml")
+        report(os.path.isfile(target), ref, "  (from %s)" % rel(path))
+    # A checker that silently matches nothing reports a safety it never looked
+    # for. If the file names a local action in a form the pattern above does not
+    # read, that is a broken checker, not a clean file.
+    mentions = len(re.findall(r"\./\.github/actions/", body))
+    if mentions > len(found):
+        sys.exit("workflow-refs: %s mentions ./.github/actions/ %d time(s) but this "
+                 "check only parsed %d `uses:` reference(s) out of it, so it is not "
+                 "reading the file it claims to read" % (rel(path), mentions, len(found)))
+
+if local_refs == 0:
+    sys.exit("workflow-refs: found no `uses: ./...` at all, so this check is not "
+             "reading the workflows it claims to read")
+
+# --- make targets ------------------------------------------------------------
+print("\nmake targets named by a run: step:")
+makefile = open(os.path.join(root, "Makefile")).read()
+targets = set(re.findall(r"^([A-Za-z0-9_.-]+):", makefile, re.M))
+wanted = set()
+for path in sorted(glob.glob(os.path.join(root, ".github/workflows/*.yml"))):
+    body = open(path).read()
+    # Comments are not run, and prose in them says things like "make the run
+    # always open the same way", which is not a target.
+    commands = re.sub(r"#[^\n]*", "", body)
+    for name in re.findall(r"\bmake\s+([a-z][a-z0-9-]*)\b", commands):
+        wanted.add(name)
+    # `make "sanderling-$SANDERLING"` is resolved from the matrix that feeds it
+    if "sanderling-$SANDERLING" in body:
+        table = re.search(r"examples='(\[.*?\])'", body, re.S)
+        if table is None:
+            sys.exit("workflow-refs: %s builds a make target from $SANDERLING but its "
+                     "examples table could not be read" % rel(path))
+        for entry in json.loads(table.group(1)):
+            wanted.add("sanderling-%s" % entry["sanderling"])
+for name in sorted(wanted):
+    report(name in targets, "make %s" % name)
+
+# --- scripts a run: step invokes ---------------------------------------------
+print("\nscripts a run: step invokes:")
+scripts = set()
+for path in workflow_files():
+    scripts |= set(re.findall(r"\.github/scripts/[A-Za-z0-9_.-]+\.sh", open(path).read()))
+for name in sorted(scripts):
+    full = os.path.join(root, name)
+    report(os.path.isfile(full), name)
+    if os.path.isfile(full):
+        report(os.access(full, os.X_OK), "%s is executable" % name)
+
+print("\n%d references checked" % checked)
+if problems:
+    sys.exit("workflow-refs: unresolved: %s" % ", ".join(problems))
+print("workflow-refs: ok")
+PY