Check the image has the commands before CI finds out it does not

Two failures in a row were the same shape: a command the workflow calls
was not in the image. git-lfs, then file(1). Each cost a full run to
learn, and the android job is expensive to be wrong in -- the step that
fails is at the end, so every attempt paid twenty-eight minutes of
cross-compile first to reach the line that could not work.

Both were visible in ten seconds from here. `docker run <image> command -v
file` is the whole diagnosis; it just never occurred to anybody to ask
before pushing.

So the question gets asked automatically. This reads the `run:` blocks out
of the workflows, pulls the commands worth doubting -- the ones a minimal
Debian plausibly lacks, not `cd` -- and checks each against the image its
job declares. It does not run the workflow and is not a replacement for
one. It answers exactly the question that was expensive to answer.

`git lfs` is handled specially and the comment says why: it is a
subcommand, so the first word of the line is `git`, which is always there.
Taking first words alone would have missed the original bug -- and did,
in the first version of this script.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docker/ci-preflight.sh

@@ -0,0 +1,97 @@
+#!/usr/bin/env bash
+# Check that every command the workflows invoke exists in the image that will
+# run it.
+#
+# This exists because two CI failures in a row were the same shape: the image
+# was missing a command, and finding out took a 28-minute cross-compile each
+# time because the step that would fail ran last. git-lfs and file(1) were both
+# absent for as long as the build panicked before ever reaching them.
+#
+# Nothing here runs the workflow. It answers one question -- is the toolchain
+# the steps assume actually installed -- in about ten seconds.
+#
+#   ./docker/ci-preflight.sh              # every job
+#   ./docker/ci-preflight.sh android      # one job
+set -uo pipefail
+
+HERE="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+REPO="$(cd "${HERE}/.." && pwd)"
+WANT="${1:-}"
+FAILED=0
+
+# Commands a `run:` block invokes that are worth asserting: the ones a minimal
+# image plausibly lacks. Shell builtins and coreutils are not interesting; a
+# missing `cd` is not the failure mode anybody has.
+readonly INTERESTING='^(git|git-lfs|file|zip|unzip|keytool|curl|cargo|rustup|apt-get|find|sed|awk|base64|shred|adb|python3|node|jq)$'
+
+jobs_and_images() {
+    python3 - "$REPO" <<'PY'
+import sys, pathlib, yaml
+root = pathlib.Path(sys.argv[1])
+for wf in sorted((root / ".gitea/workflows").glob("*.yml")):
+    doc = yaml.safe_load(wf.read_text()) or {}
+    for job, spec in (doc.get("jobs") or {}).items():
+        image = ((spec.get("container") or {}).get("image"))
+        if not image:
+            continue
+        cmds = set()
+        for step in (spec.get("steps") or []):
+            run = step.get("run")
+            if not run:
+                continue
+            for line in run.splitlines():
+                line = line.strip()
+                if not line or line.startswith("#"):
+                    continue
+                # first word of the line, and of anything after a pipe
+                for part in line.split("|"):
+                    word = part.strip().split(" ")[0].strip()
+                    if word.isidentifier() or "-" in word:
+                        cmds.add(word)
+                # `git lfs` is a subcommand, so the first word is `git` and the
+                # thing that can actually be absent never appears. This is the
+                # exact bug that shipped an image with no git-lfs in it.
+                if "git lfs" in line:
+                    cmds.add("git-lfs")
+        print(f"{job}\t{image}\t{' '.join(sorted(cmds))}")
+PY
+}
+
+while IFS=$'\t' read -r job image cmds; do
+    [[ -n "${WANT}" && "${job}" != "${WANT}" ]] && continue
+    checked=()
+    for c in ${cmds}; do
+        [[ "${c}" =~ ${INTERESTING} ]] && checked+=("${c}")
+    done
+    # `git lfs` is a git subcommand, not a binary on PATH — ask git about it.
+    printf '\n== %s (%s)\n' "${job}" "${image}"
+    if [[ ${#checked[@]} -eq 0 ]]; then
+        echo "   nothing to check"
+        continue
+    fi
+    docker image inspect "${image}" >/dev/null 2>&1 || {
+        echo "   image not present locally — docker pull ${image}"
+        FAILED=1
+        continue
+    }
+    out=$(docker run --rm --entrypoint bash "${image}" -c '
+        for c in '"${checked[*]}"'; do
+            if [ "$c" = git-lfs ]; then
+                git lfs version >/dev/null 2>&1 && echo "ok       git lfs" || echo "MISSING  git lfs"
+            elif command -v "$c" >/dev/null 2>&1; then
+                echo "ok       $c"
+            else
+                echo "MISSING  $c"
+            fi
+        done' 2>&1)
+    echo "${out}" | sed 's/^/   /'
+    grep -q MISSING <<<"${out}" && FAILED=1
+done < <(jobs_and_images)
+
+echo
+if [[ ${FAILED} -eq 0 ]]; then
+    echo "preflight: every command the workflows invoke is present"
+else
+    echo "preflight: something the workflows invoke is not in the image — fix the Dockerfile before pushing"
+fi
+exit ${FAILED}