Add script/update-policy to re-vendor policy files (closes #162)

One command fetches the vendored files from prompts at a commit, keeps a repo's own entries, commits and pushes. An entry above a file's closing comment that the canonical file lacks stops it before it writes. A run with nothing to commit still pushes, so a retry delivers a failed push. The canonical .dockerignore gains the closing comment the script needs. test/update-policy.sh tests the script in the Dockerfile test phase.

Model: opus-5-5

.dockerignore

@@ -76,3 +76,7 @@
 **/.idea
 **/.vscode
 **/*.sublime-*
+
+# This repository's own entries, such as its build outputs, go below
+# this comment, and a re-vendor keeps them. Anchor a binary built at the
+# root: `/myapp`, never `**/myapp`, which also matches `cmd/myapp/`.

Dockerfile

@@ -42,7 +42,7 @@ RUN script/bootstrap
 
 COPY . .
 
-RUN echo "No tests defined."
+RUN timeout 90 test/update-policy.sh
 
 # Development environment, and the last stage: a plain `docker build .`
 # names no target and so builds this one. Nothing is wanted from the two

Makefile

@@ -1,4 +1,4 @@
-.PHONY: bootstrap setup test lint fmt fmt-check check docker hooks
+.PHONY: bootstrap setup test lint fmt fmt-check check docker hooks update-policy
 
 # Makefile targets are thin shims; the implementations live in script/
 # per the scripts-to-rule-them-all pattern (see the Entrypoints section
@@ -30,3 +30,6 @@ docker:
 
 hooks:
 	@script/install-precommit
+
+update-policy:
+	@script/update-policy

README.md

@@ -118,8 +118,8 @@ alpine. We provide:
   `script/docker` for the image tag
 - `script/test` —
   `docker build --build-arg CACHEBUST="$(date +%s)-$$" --target test --output type=cacheonly .`,
-  building the `test` phase of the `Dockerfile` without writing an image (no
+  building the `test` phase of the `Dockerfile`, which runs
-  tests defined here)
+  `test/update-policy.sh`, without writing an image
 - `script/lint` —
   `docker build --build-arg CACHEBUST="$(date +%s)-$$" --target lint --output type=cacheonly .`,
   building the `lint` phase, which runs prettier over the markdown files,
@@ -141,6 +141,9 @@ alpine. We provide:
   `script/check`
 - `script/install-precommit` — installs the git pre-commit hook (our own
   extension); `make hooks` shims to it
+- `script/update-policy` — fetch the vendored policy files from this repo,
+  commit them and push (our own extension); it is for the repos that vendor them
+  and stops when run here
 
 `make hooks` installs the pre-commit hook that runs `script/precommit`.
 
@@ -172,8 +175,7 @@ share, and evolve prompts across projects.
 The repository is a collection of Markdown files organized in the `prompts/`
 subdirectory. Each file contains one or more related prompts or policy
 documents. There is no build step or runtime component; the prompts are consumed
-by copying them into other projects or referencing them directly. `examples/`
+by copying them into other projects or referencing them directly.
-holds the example `Dockerfile` files that `prompts/REPO_POLICIES.md` links to.
 
 ## Template Repos
 

TODO.md

@@ -21,13 +21,12 @@ fmt-check, and commit.
 
 # Completed Steps
 
-- 2026-11-09: Compacted `prompts/REPO_POLICIES.md` from 1,165 lines and 8,556
+- 2026-11-09: `script/update-policy` and `make update-policy` update a repo's
-  words to 982 lines and 7,281 words
+  vendored policy files with one command
-  (https://git.eeqj.de/sneak/prompts/issues/158). Every rule keeps its meaning.
+  (https://git.eeqj.de/sneak/prompts/issues/162). They fetch the files from
-  The rules now sit under headings, the pinning rule first. Each rule that was
+  `prompts`, keep the repo's own entries, commit and push. The canonical
-  stated in several places is stated once. The Makefile copies of the Go and
+  `.dockerignore` now ends with the comment that the repo's own entries go
-  Python test commands are gone, because the `Dockerfile` examples carry the
+  below. The template repositories get the script in their own issues.
-  same commands. Both checklists name the `.golangci.yml` section.
 - 2026-11-07: Go code is now formatted with `gofmt` and then `golines -m 77`
   (https://git.eeqj.de/sneak/prompts/issues/160). `prompts/REPO_POLICIES.md`
   says how `script/fmt`, `script/fmt-check` and `script/bootstrap` do it and

examples/go-lint-system-libraries.Dockerfile

@@ -1,8 +0,0 @@
-FROM golangci/golangci-lint@sha256:... AS lint
-RUN apt-get update \
-    && apt-get install -y --no-install-recommends libvips-dev \
-    && rm -rf /var/lib/apt/lists/*
-WORKDIR /src
-COPY go.mod go.sum ./
-RUN go mod download
-ARG CACHEBUST

examples/go.Dockerfile

@@ -1,85 +0,0 @@
-# In every stage ARG CACHEBUST comes right after the steps that install
-# system packages and download Go's modules, or right after FROM in a
-# stage with neither, and every step that lints, tests or compiles
-# comes after it. script/ passes a new value on every build, which
-# reruns every RUN step after it. Docker keeps the layers above it
-# until the base image, their own lines or go.mod and go.sum change.
-# The steps that compile mount Go's build cache, and golangci-lint run
-# also mounts golangci-lint's; both survive from build to build.
-
-# Lint phase
-# golangci/golangci-lint:v2.x.x, YYYY-MM-DD
-FROM golangci/golangci-lint@sha256:... AS lint
-WORKDIR /src
-COPY go.mod go.sum ./
-RUN go mod download
-ARG CACHEBUST
-COPY . .
-RUN --mount=type=cache,target=/root/.cache/go-build \
-    --mount=type=cache,target=/root/.cache/golangci-lint \
-    golangci-lint run --config .golangci.yml ./...
-
-# Test phase. -race needs cgo and so a C compiler, which the Debian Go
-# image ships and the alpine one does not. -count=1 because the mounted
-# cache also holds test results, which go test would otherwise replay.
-# A first run that timed out gets no rerun, because a rerun would only
-# wait for the same hang.
-# golang:1.x, YYYY-MM-DD
-FROM golang@sha256:... AS test
-WORKDIR /src
-COPY go.mod go.sum ./
-RUN go mod download
-ARG CACHEBUST
-COPY . .
-RUN --mount=type=cache,target=/root/.cache/go-build \
-    status=0; \
-    output=$(go test -count=1 -timeout 90s -race -cover ./... 2>&1) || status=$?; \
-    printf '%s\n' "$output"; \
-    test "$status" -eq 0 && exit 0; \
-    case "$output" in \
-        *"panic: test timed out"*|*"*** Test killed"*) exit 1 ;; \
-    esac; \
-    echo "--- Rerunning with -v for details ---"; \
-    go test -count=1 -timeout 90s -race -v ./...; \
-    exit 1
-
-# Build stage. Nothing is wanted from either phase above; the copies
-# are what make BuildKit build them first, so nothing after them can
-# run unless lint and test passed.
-# golang:1.x-alpine, YYYY-MM-DD
-FROM golang@sha256:... AS builder
-# git comes from the base image's own distribution package index, which
-# is trusted, so it needs no version and no hash.
-RUN apk add --no-cache git
-WORKDIR /src
-COPY go.mod go.sum ./
-RUN go mod download
-ARG CACHEBUST
-COPY --from=lint /src/go.sum /dev/null
-COPY --from=test /src/go.sum /dev/null
-# A tar-stream context keeps the sender's file owners, which git refuses.
-RUN git config --system --add safe.directory /src
-COPY . .
-
-# The VERSION build arg when one is given, otherwise
-# `git describe --tags --always` on the .git in the build context. With
-# .git present, a version that is still empty, dev or unknown fails the
-# build: git is missing or could not read the checkout.
-ARG VERSION
-RUN --mount=type=cache,target=/root/.cache/go-build \
-    VERSION="${VERSION:-$(git describe --tags --always)}"; \
-    if [ -e .git ]; then \
-        case "$VERSION" in ""|dev|unknown) \
-            echo "version is '$VERSION' although .git is present" >&2; \
-            exit 1 ;; \
-        esac; \
-    fi; \
-    CGO_ENABLED=0 go build -trimpath \
-        -ldflags="-s -w -X main.Version=${VERSION}" \
-        -o /app ./cmd/app/
-
-# Runtime stage, and the last one
-FROM alpine@sha256:...
-ARG CACHEBUST
-COPY --from=builder /app /usr/local/bin/app
-ENTRYPOINT ["app"]

examples/python-test-phase.Dockerfile

@@ -1,24 +0,0 @@
-# Test phase. Both pytest runs share one 90-second timeout, so a hang
-# uses it once and gets no rerun.
-# debian:trixie-slim, YYYY-MM-DD
-FROM debian@sha256:... AS test
-# Python, pip and venv come from the distribution, which is trusted, so
-# they need no version and no hash. make and git are base tools that
-# script/bootstrap installs when they are missing. Installed here, they
-# stay in their own layer when script/ changes.
-RUN apt-get update \
-    && apt-get install -y --no-install-recommends ca-certificates make git \
-        python3 python3-venv \
-    && rm -rf /var/lib/apt/lists/*
-# script/bootstrap installs pipenv in a virtual environment at
-# $HOME/.pipenv, and the tests below run that pipenv.
-ENV PATH=/root/.pipenv/bin:$PATH
-WORKDIR /app
-COPY script/ script/
-COPY Pipfile Pipfile.lock ./
-RUN script/bootstrap
-ARG CACHEBUST
-COPY . .
-RUN timeout 90 sh -c 'pipenv run pytest || { \
-        echo "--- Rerunning with -v for details ---"; \
-        pipenv run pytest -v; exit 1; }'

prompts/EXISTING_REPO_CHECKLIST.md

@@ -71,8 +71,8 @@ with your task.
       also mounts golangci-lint's
       (`--mount=type=cache,target=/root/.cache/golangci-lint`), and no step
       mounts anything else; both `go test` runs carry `-count=1`; a first run
-      that timed out gets no rerun, as in `examples/go.Dockerfile` in the
+      that timed out gets no rerun, as in the canonical Go `Dockerfile` in
-      `prompts` repo
+      `REPO_POLICIES.md`
 - [ ] Python: a `test` phase that installs its packages from `Pipfile.lock` does
       it above `ARG CACHEBUST`, with `COPY script/ script/`,
       `COPY Pipfile Pipfile.lock ./` and `RUN script/bootstrap`. When
@@ -92,8 +92,8 @@ with your task.
     installs no Python, because a step above it already did. `Pipfile` names no
     `python_version`, because `--deploy` stops when the distribution's Python
     differs from it. The tests run after `ARG CACHEBUST`, and every other stage
-    installs its Python packages after it, as in
+    installs its Python packages after it, as in the Python `test` phase example
-    `examples/python-test-phase.Dockerfile` in the `prompts` repo
+    in `REPO_POLICIES.md`
 
 - [ ] The gate phases invoke their tools directly, never through `make lint` or
       `script/test` — those are themselves a `docker build` and would recurse
@@ -163,7 +163,7 @@ with your task.
     - [ ] Go: `go.mod`, `go.sum`, `.golangci.yml` (fetch from
           `https://git.eeqj.de/sneak/prompts/raw/branch/main/.golangci.yml` and,
           in the same commit, set the lint phase digest to the one named in the
-          `.golangci.yml` section of `REPO_POLICIES.md`)
+          `.golangci.yml` paragraph of `REPO_POLICIES.md`)
     - [ ] JS: `package.json`, `yarn.lock`, `.yarnrc.yml`, `.yarn/releases/`,
           `.gitattributes`, `.prettierrc`, `.prettierignore` (fetch the last two
           from `https://git.eeqj.de/sneak/prompts/raw/branch/main/.prettierrc`
@@ -205,12 +205,18 @@ with your task.
 - [ ] `Makefile` exists in root — reference
       `https://git.eeqj.de/sneak/prompts/raw/branch/main/Makefile`
 - [ ] Has targets: `test`, `lint`, `fmt`, `fmt-check`, `check`, `docker`,
-      `hooks`
+      `hooks`, `update-policy`
 - [ ] Target implementations live in `script/` (scripts-to-rule-them-all);
       Makefile targets are thin shims calling them — model scripts at
       `https://git.eeqj.de/sneak/prompts/raw/branch/main/script/<name>`
 - [ ] `script/precommit` exists and the pre-commit hook (installed by
       `script/install-precommit`, shimmed by `make hooks`) runs it
+- [ ] `script/update-policy` exists, identical to the model script, and
+      `make update-policy` shims to it. Before its first use, the repo's own
+      entries in `.gitignore`, `.editorconfig` and `.dockerignore` go below the
+      comment that ends each canonical file. An entry above that comment which
+      the canonical file lacks makes the script stop before it writes anything,
+      naming the file and the entry.
 - [ ] README has an **Entrypoints** section documenting the `script/`
       entrypoints and linking the standard
 - [ ] `script/lint` and `script/test` each run

prompts/NEW_REPO_CHECKLIST.md

@@ -62,6 +62,8 @@ Template files can be fetched from:
       `CLAUDE.md` or `.claude/`, and never separate memory files
 - [ ] `Dockerfile` and `.dockerignore` — fetch `.dockerignore` from
       `https://git.eeqj.de/sneak/prompts/raw/branch/main/.dockerignore`
+    - The repo's own entries go at the end of the file, where a re-vendor keeps
+      them.
     - Extend `.dockerignore` with the repo's own host-built artifacts, giving
       every depth-independent pattern a `**/` prefix — but write a repo-root
       binary anchored, `/myapp` and never `**/myapp`, which would also match
@@ -138,8 +140,8 @@ Template files can be fetched from:
       also mounts golangci-lint's
       (`--mount=type=cache,target=/root/.cache/golangci-lint`), and no step
       mounts anything else; both `go test` runs carry `-count=1`; a first run
-      that timed out gets no rerun, as in `examples/go.Dockerfile` in the
+      that timed out gets no rerun, as in the canonical Go `Dockerfile` in
-      `prompts` repo
+      `REPO_POLICIES.md`
     - Python: a `test` phase that installs its packages from `Pipfile.lock` does
       it above `ARG CACHEBUST`, with `COPY script/ script/`,
       `COPY Pipfile Pipfile.lock ./` and `RUN script/bootstrap`. When
@@ -160,8 +162,8 @@ Template files can be fetched from:
         because a step above it already did. `Pipfile` names no
         `python_version`, because `--deploy` stops when the distribution's
         Python differs from it. The tests run after `ARG CACHEBUST`, and every
-        other stage installs its Python packages after it, as in
+        other stage installs its Python packages after it, as in the Python
-        `examples/python-test-phase.Dockerfile` in the `prompts` repo
+        `test` phase example in `REPO_POLICIES.md`
 
     - Server: the final stage builds and runs the application
     - Non-server: the final stage brings up the dev environment
@@ -178,7 +180,7 @@ Template files can be fetched from:
     - [ ] Go: `go mod init sneak.berlin/go/<name>`, `.golangci.yml` (fetch from
           `https://git.eeqj.de/sneak/prompts/raw/branch/main/.golangci.yml` and,
           in the same commit, set the lint phase digest to the one named in the
-          `.golangci.yml` section of `REPO_POLICIES.md`)
+          `.golangci.yml` paragraph of `REPO_POLICIES.md`)
     - [ ] JS, or any repository that runs prettier through yarn: make the first
           `yarn.lock` with the steps below. `script/bootstrap` installs no yarn
           until `yarn.lock` exists, so run them with any distribution's node and
@@ -264,6 +266,9 @@ are thin shims calling them. Model scripts:
 - [ ] `script/install-precommit` — installs the pre-commit hook that runs
       `script/precommit`
 - [ ] `make hooks` — shims to `script/install-precommit`
+- [ ] `script/update-policy` / `make update-policy` — fetches the vendored
+      policy files from the `prompts` repo, commits them and pushes; identical
+      across repos
 - [ ] README **Entrypoints** section documents the scripts and links the
       standard
 

script/update-policy

@@ -0,0 +1,200 @@
+#!/bin/sh
+# script/update-policy: replace this repo's vendored policy files with
+# the canonical ones from the prompts repo, commit them and push.
+# Identical in all repos. Our own extension to
+# scripts-to-rule-them-all.
+#
+# Usage: script/update-policy [full commit hash of prompts]
+# Without a hash it uses the head of the prompts main branch.
+#
+# The script writes nothing until every file has been fetched and
+# merged. A repo's own entries are kept:
+#   - .gitignore, .editorconfig, .dockerignore: the lines below the
+#     canonical file's last line. An entry above that line which the
+#     canonical file lacks stops the script.
+#   - .golangci.yml: the entries the repo added to the deny list of the
+#     test-support rule
+set -eu
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+PROMPTS_URL="https://git.eeqj.de/sneak/prompts"
+
+# Paths in the prompts repo. A file this repo does not carry is skipped.
+VENDORED_FILES="
+.dockerignore
+.editorconfig
+.gitea/workflows/check.yml
+.gitignore
+.golangci.yml
+.prettierrc
+prompts/API_POLICIES.md
+prompts/CODE_STYLEGUIDE.md
+prompts/CODE_STYLEGUIDE_GO.md
+prompts/CODE_STYLEGUIDE_JS.md
+prompts/CODE_STYLEGUIDE_PYTHON.md
+prompts/EXISTING_REPO_CHECKLIST.md
+prompts/GO_HTTP_SERVER_CONVENTIONS.md
+prompts/LLM_PROSE_TELLS.md
+prompts/NEW_REPO_CHECKLIST.md
+prompts/REPO_POLICIES.md
+"
+
+die() {
+    echo "update-policy: $*" >&2
+    exit 1
+}
+
+# local_path <path in prompts>: print where this repo keeps the file, or
+# nothing if git does not track it here. A prompts/NAME.md lands at
+# docs/NAME.md or NAME.md.
+local_path() {
+    name="${1#prompts/}"
+    if [ "$name" != "$1" ] && [ -n "$(git ls-files -- "docs/$name")" ]; then
+        echo "docs/$name"
+    elif [ -n "$(git ls-files -- "$name")" ]; then
+        echo "$name"
+    fi
+}
+
+# keep_lines_below_last <fetched> <local>: append to the fetched file
+# whatever follows its last line in the local file. Stop if a line above
+# that one is missing from the fetched file, unless it is blank or a
+# comment: replacing the file would delete it.
+keep_lines_below_last() {
+    last="$(tail -n 1 "$1")"
+    number="$(grep -n -F -x -- "$last" "$2" | head -n 1 | cut -d: -f1)"
+    if [ -z "$last" ] || [ -z "$number" ]; then
+        die "$2 lacks this line; add it and put the repo's own entries below it: $last"
+    fi
+    own="$(awk -v end="$number" '
+        NR == FNR { fetched[$0]; next }
+        FNR < end && !/^#|^[ \t]*$/ && !($0 in fetched)
+    ' "$1" "$2")"
+    if [ -n "$own" ]; then
+        die "$2 has entries above its last canonical line that the canonical file lacks; move each below that line to keep it, or delete it:
+$own"
+    fi
+    tail -n "+$((number + 1))" "$2" >> "$1"
+}
+
+# keep_deny_entries <fetched> <local>: add to the deny list of the
+# test-support rule in the fetched file the entries only the local file
+# has. An entry is a "- pkg:" line, the lines under it and any comment
+# above it.
+keep_deny_entries() {
+    awk '
+        function indent(line) { return match(line, /[^ ]/) - 1 }
+        function print_local_entries(i) {
+            for (i = 1; i <= count; i++)
+                if (!(pkg[i] in fetched_pkg))
+                    printf "%s", entry[i]
+        }
+
+        FNR == 1 { in_rule = 0; in_list = 0 }
+
+        in_list && /[^ ]/ && indent($0) <= list_indent {
+            in_list = 0
+            if (FILENAME == ARGV[2]) print_local_entries()
+        }
+
+        in_list {
+            is_entry = ($1 == "-" && $2 == "pkg:")
+            if (FILENAME == ARGV[2]) {
+                if (is_entry) fetched_pkg[$3]
+                print
+            } else if (is_entry) {
+                count++
+                pkg[count] = $3
+                entry[count] = comment $0 "\n"
+                comment = ""
+            } else if (/^ *#/) {
+                comment = comment $0 "\n"
+            } else if (count) {
+                entry[count] = entry[count] $0 "\n"
+            }
+            next
+        }
+
+        /^ *test-support:/ { in_rule = 1 }
+        in_rule && /^ *deny:/ {
+            in_rule = 0
+            in_list = 1
+            list_indent = indent($0)
+            if (FILENAME == ARGV[2]) found = 1
+        }
+        FILENAME == ARGV[2] { print }
+
+        END {
+            if (in_list) print_local_entries()
+            exit !found
+        }
+    ' "$2" "$1" > "$1.merged" ||
+        die "the fetched $2 has no deny list in its test-support rule"
+    mv "$1.merged" "$1"
+}
+
+main() {
+    cd "$ROOT"
+
+    [ "$#" -le 1 ] || die "usage: script/update-policy [full commit hash]"
+
+    project="$(script/projectname)"
+    if [ "$project" = "prompts" ]; then
+        die "this repo is the source of the policy files"
+    fi
+
+    commit="${1:-}"
+    if [ -z "$commit" ]; then
+        head_line="$(git ls-remote "$PROMPTS_URL.git" refs/heads/main)"
+        commit="${head_line%%[[:space:]]*}"
+    fi
+    printf '%s\n' "$commit" | grep -Eqx '[0-9a-f]{40}' ||
+        die "not a full commit hash: '$commit'"
+
+    tmp="$(mktemp -d)"
+    trap 'rm -rf "$tmp"' EXIT
+
+    carried=""
+    for file in $VENDORED_FILES; do
+        path="$(local_path "$file")"
+        [ -n "$path" ] || continue
+        if [ -n "$(git status --porcelain -- "$path")" ]; then
+            die "$path has uncommitted changes"
+        fi
+        mkdir -p "$tmp/$(dirname "$path")"
+        curl --fail --silent --show-error --max-time 60 \
+            --output "$tmp/$path" \
+            "$PROMPTS_URL/raw/commit/$commit/$file" ||
+            die "cannot fetch $file at $commit"
+        carried="$carried $path"
+    done
+    [ -n "$carried" ] || die "this repo carries none of the vendored files"
+
+    for path in $carried; do
+        case "$path" in
+            .gitignore | .editorconfig | .dockerignore)
+                keep_lines_below_last "$tmp/$path" "$path"
+                ;;
+            .golangci.yml)
+                keep_deny_entries "$tmp/$path" "$path"
+                ;;
+        esac
+    done
+
+    for path in $carried; do
+        cp "$tmp/$path" "$path"
+        git add -- "$path"
+    done
+
+    if git diff --cached --quiet -- $carried; then
+        echo "update-policy: already at prompts $commit, nothing to commit"
+    else
+        git commit -m "Update policy files from prompts $commit" -- $carried
+    fi
+
+    # Also pushes when there was nothing to commit, so a run after a
+    # failed push delivers the commit that push left behind.
+    git push origin HEAD || die "push failed; fix that and run this again"
+}
+
+main "$@"

test/update-policy.sh

@@ -0,0 +1,389 @@
+#!/bin/sh
+# test/update-policy.sh: test script/update-policy on throwaway repos.
+# Each scenario gets one temp directory holding a repo, its local bare
+# origin, the canonical files and a stub curl that serves them, so
+# nothing touches the network.
+set -eu
+
+UPDATE_POLICY="$(cd "$(dirname "$0")/.." && pwd -P)/script/update-policy"
+
+# The one commit the stub curl serves; every other commit gets a 404.
+export HASH=1111111111111111111111111111111111111111
+
+dir=""
+trap 'rm -rf "$dir"' EXIT
+
+fail() {
+    echo "FAIL: $scenario: $*" >&2
+    cat "$dir/output" >&2
+    exit 1
+}
+
+write_stub_curl() {
+    mkdir "$dir/bin"
+    cat > "$dir/bin/curl" << 'EOF'
+#!/bin/sh
+previous=""
+for argument in "$@"; do
+    [ "$previous" = "--output" ] && output="$argument"
+    previous="$argument"
+done
+url="$previous"
+base="https://git.eeqj.de/sneak/prompts/raw/commit/$HASH/"
+path="${url#"$base"}"
+source="$(dirname "$0")/../canonical/$path"
+if [ "$path" = "$url" ] || [ ! -f "$source" ]; then
+    exit 22
+fi
+cp "$source" "$output"
+EOF
+    chmod +x "$dir/bin/curl"
+}
+
+write_canonical_files() {
+    mkdir -p "$dir/canonical/prompts"
+    cat > "$dir/canonical/.gitignore" << 'EOF'
+*.log
+*.tmp
+
+# Own entries go below this comment.
+EOF
+    cat > "$dir/canonical/.dockerignore" << 'EOF'
+**/*.log
+**/*.tmp
+
+# Own entries go below this comment.
+EOF
+    cat > "$dir/canonical/.editorconfig" << 'EOF'
+root = true
+[*]
+indent_size = 4
+
+# Own sections go below this comment.
+EOF
+    cat > "$dir/canonical/.golangci.yml" << 'EOF'
+linters:
+  settings:
+    depguard:
+      rules:
+        test-support:
+          deny:
+            - pkg: net/http/httptest
+              desc: canonical
+            - pkg: net/http/cgi
+              desc: added canonically
+        other:
+          deny:
+            - pkg: example.com/other
+EOF
+    echo '{ "tabWidth": 4 }' > "$dir/canonical/.prettierrc"
+    echo "new policy" > "$dir/canonical/prompts/REPO_POLICIES.md"
+    echo "new api policy" > "$dir/canonical/prompts/API_POLICIES.md"
+}
+
+# write_outdated_files: this repo's files, one canonical version behind,
+# with the repo's own entries in the places the script keeps them. The
+# comment above the closing comment in .gitignore is replaced, not kept.
+write_outdated_files() {
+    cat > "$dir/repo/.gitignore" << 'EOF'
+*.log
+
+# An outdated comment.
+# Own entries go below this comment.
+/myapp
+EOF
+    cat > "$dir/repo/.dockerignore" << 'EOF'
+**/*.log
+
+# Own entries go below this comment.
+/myapp
+EOF
+    cat > "$dir/repo/.editorconfig" << 'EOF'
+root = true
+[*]
+
+# Own sections go below this comment.
+[*.py]
+indent_size = 2
+EOF
+    cat > "$dir/repo/.golangci.yml" << 'EOF'
+linters:
+  settings:
+    depguard:
+      rules:
+        test-support:
+          deny:
+            - pkg: net/http/httptest
+              desc: canonical
+            # Our own test helpers.
+            - pkg: example.com/app/internal/apptest
+              desc: own entry
+        other:
+          deny:
+            - pkg: example.com/other
+EOF
+    echo '{ "tabWidth": 2 }' > "$dir/repo/.prettierrc"
+    echo "old policy" > "$dir/repo/REPO_POLICIES.md"
+}
+
+# start_scenario <name>: make a new temp directory holding a repo with
+# the outdated files, not yet committed, and an empty origin.
+start_scenario() {
+    scenario="$1"
+    rm -rf "$dir"
+    dir="$(mktemp -d)"
+    : > "$dir/output"
+    write_stub_curl
+    write_canonical_files
+
+    git init -q --bare -b main "$dir/origin.git"
+    git init -q -b main "$dir/repo"
+    git -C "$dir/repo" config user.name "Test"
+    git -C "$dir/repo" config user.email "test@example.com"
+    git -C "$dir/repo" remote add origin "$dir/origin.git"
+
+    mkdir "$dir/repo/script"
+    cp "$UPDATE_POLICY" "$dir/repo/script/update-policy"
+    printf '#!/bin/sh\necho example\n' > "$dir/repo/script/projectname"
+    chmod +x "$dir/repo/script/projectname"
+    write_outdated_files
+}
+
+# commit_and_push_fixture <paths...>: commit those paths, remove the
+# rest of the outdated files, and record the commit as $before.
+commit_and_push_fixture() {
+    (
+        cd "$dir/repo"
+        git add script "$@"
+        git commit -q -m "Fixture"
+        git push -q origin HEAD
+        git clean -q -f -x
+    )
+    before="$(git -C "$dir/repo" rev-parse HEAD)"
+}
+
+run_update_policy() {
+    status=0
+    (cd "$dir/repo" && PATH="$dir/bin:$PATH" script/update-policy "$@") \
+        > "$dir/output" 2>&1 || status=$?
+}
+
+reject_pushes() {
+    printf '#!/bin/sh\nexit 1\n' > "$dir/origin.git/hooks/pre-receive"
+    chmod +x "$dir/origin.git/hooks/pre-receive"
+}
+
+accept_pushes() {
+    rm "$dir/origin.git/hooks/pre-receive"
+}
+
+head_of_repo() {
+    git -C "$dir/repo" rev-parse HEAD
+}
+
+head_of_origin() {
+    git -C "$dir/origin.git" rev-parse refs/heads/main
+}
+
+expect_success() {
+    [ "$status" -eq 0 ] || fail "exited $status"
+}
+
+expect_failure() {
+    [ "$status" -ne 0 ] || fail "exited 0"
+}
+
+expect_output() {
+    grep -q -F -e "$1" "$dir/output" || fail "output lacks: $1"
+}
+
+expect_output_without() {
+    if grep -q -F -e "$1" "$dir/output"; then
+        fail "output has: $1"
+    fi
+}
+
+# expect_file <path>: the file must equal standard input.
+expect_file() {
+    cat > "$dir/expected"
+    diff "$dir/expected" "$dir/repo/$1" >&2 || fail "$1 is wrong"
+}
+
+expect_absent() {
+    [ ! -e "$dir/repo/$1" ] || fail "$1 exists"
+}
+
+expect_nothing_written() {
+    [ -z "$(git -C "$dir/repo" status --porcelain)" ] ||
+        fail "the working tree changed"
+    [ "$(head_of_repo)" = "$before" ] || fail "the repo has a new commit"
+    [ "$(head_of_origin)" = "$before" ] || fail "origin has a new commit"
+}
+
+test_own_entries_kept() {
+    start_scenario "own entries kept in all four files"
+    commit_and_push_fixture .gitignore .dockerignore .editorconfig \
+        .golangci.yml .prettierrc REPO_POLICIES.md
+    run_update_policy "$HASH"
+    expect_success
+
+    expect_file .gitignore << 'EOF'
+*.log
+*.tmp
+
+# Own entries go below this comment.
+/myapp
+EOF
+    expect_file .dockerignore << 'EOF'
+**/*.log
+**/*.tmp
+
+# Own entries go below this comment.
+/myapp
+EOF
+    expect_file .editorconfig << 'EOF'
+root = true
+[*]
+indent_size = 4
+
+# Own sections go below this comment.
+[*.py]
+indent_size = 2
+EOF
+    expect_file .golangci.yml << 'EOF'
+linters:
+  settings:
+    depguard:
+      rules:
+        test-support:
+          deny:
+            - pkg: net/http/httptest
+              desc: canonical
+            - pkg: net/http/cgi
+              desc: added canonically
+            # Our own test helpers.
+            - pkg: example.com/app/internal/apptest
+              desc: own entry
+        other:
+          deny:
+            - pkg: example.com/other
+EOF
+    expect_file .prettierrc << 'EOF'
+{ "tabWidth": 4 }
+EOF
+    expect_file REPO_POLICIES.md << 'EOF'
+new policy
+EOF
+
+    [ "$(head_of_origin)" = "$(head_of_repo)" ] ||
+        fail "the commit was not pushed"
+    [ "$(head_of_repo)" != "$before" ] || fail "no commit was made"
+    git -C "$dir/repo" log -1 --format=%s | grep -q -F -e "$HASH" ||
+        fail "the commit message does not name the hash"
+}
+
+test_files_not_carried_are_skipped() {
+    start_scenario "files the repo does not carry skipped"
+    mkdir "$dir/repo/docs"
+    mv "$dir/repo/REPO_POLICIES.md" "$dir/repo/docs/REPO_POLICIES.md"
+    commit_and_push_fixture docs/REPO_POLICIES.md
+    run_update_policy "$HASH"
+    expect_success
+
+    expect_file docs/REPO_POLICIES.md << 'EOF'
+new policy
+EOF
+    expect_absent REPO_POLICIES.md
+    expect_absent API_POLICIES.md
+    expect_absent docs/API_POLICIES.md
+    expect_absent .gitignore
+    expect_absent .dockerignore
+    expect_absent .editorconfig
+    expect_absent .golangci.yml
+    expect_absent .prettierrc
+    [ "$(head_of_origin)" = "$(head_of_repo)" ] ||
+        fail "the commit was not pushed"
+}
+
+test_nothing_changed() {
+    start_scenario "nothing changed"
+    cp "$dir/canonical/.prettierrc" "$dir/repo/.prettierrc"
+    commit_and_push_fixture .prettierrc
+    run_update_policy "$HASH"
+    expect_success
+    expect_output "nothing to commit"
+    expect_nothing_written
+}
+
+test_bad_commit_hash() {
+    for hash in not-a-hash 0000000000000000000000000000000000000000; do
+        start_scenario "bad commit hash $hash stops with nothing written"
+        commit_and_push_fixture .gitignore .prettierrc REPO_POLICIES.md
+        run_update_policy "$hash"
+        expect_failure
+        expect_nothing_written
+    done
+}
+
+test_push_failure() {
+    start_scenario "push failure exits non-zero"
+    commit_and_push_fixture .prettierrc
+    reject_pushes
+    run_update_policy "$HASH"
+    expect_failure
+    expect_output "push failed"
+    [ "$(head_of_origin)" = "$before" ] || fail "origin has a new commit"
+}
+
+# Each of these files, with the repo's own entries above the closing
+# comment. The comment added with them must not be named.
+test_own_entry_above_closing_comment() {
+    for file in .gitignore .dockerignore .editorconfig; do
+        start_scenario "own entries above the closing comment of $file stop"
+        {
+            printf '/myapp\n/.tool/\n\n# A comment of our own.\n'
+            cat "$dir/repo/$file"
+        } > "$dir/merged"
+        mv "$dir/merged" "$dir/repo/$file"
+        commit_and_push_fixture .gitignore .dockerignore .editorconfig \
+            .prettierrc REPO_POLICIES.md
+        run_update_policy "$HASH"
+        expect_failure
+        expect_output "$file has entries"
+        expect_output "/myapp"
+        expect_output "/.tool/"
+        expect_output_without "A comment of our own."
+        expect_nothing_written
+    done
+}
+
+test_retry_after_failed_push() {
+    start_scenario "a retry after a failed push delivers the commit"
+    commit_and_push_fixture .prettierrc
+    reject_pushes
+    run_update_policy "$HASH"
+    expect_failure
+    committed="$(head_of_repo)"
+    [ "$committed" != "$before" ] || fail "no commit was made"
+    [ "$(head_of_origin)" = "$before" ] || fail "origin has a new commit"
+
+    accept_pushes
+    run_update_policy "$HASH"
+    expect_success
+    [ "$(head_of_repo)" = "$committed" ] || fail "the retry made a new commit"
+    [ "$(head_of_origin)" = "$committed" ] ||
+        fail "the retry did not deliver the commit"
+}
+
+main() {
+    test_own_entries_kept
+    test_files_not_carried_are_skipped
+    test_nothing_changed
+    test_bad_commit_hash
+    test_push_failure
+    test_own_entry_above_closing_comment
+    test_retry_after_failed_push
+    echo "update-policy tests passed"
+}
+
+main "$@"