The image apps build FROM, with its health check (closes #45)

The Dockerfile's last stage is now the image of "Deployment" in SPEC.md:
Ubuntu 26.04 with ca-certificates, nix-bin and runit from a dated
snapshot whose InRelease files are checked by hash, nixpkgs from its
release file checked by SHA-256, runsvinit built at a fixed commit, and
smallwebwaf as a runit service. smallwebwaf answers
/_smallwebwaf/healthz, and `smallwebwaf healthcheck` is the image's
HEALTHCHECK. script/example-app builds an app on the image and checks
it end to end.

The Nix profile comes last on the PATH: first, busybox from nixpkgs
replaced runit's own runsvdir and sv. SPEC.md is corrected to match
what was built.

Model: opus-5-5

Dockerfile

@@ -33,16 +33,12 @@ RUN go test -count=1 -timeout 90s -race -cover ./... || \
     { echo "--- Rerunning with -v for details ---"; \
       go test -count=1 -timeout 90s -race -v ./...; exit 1; }
 
-# Build stage, and the last one: a plain `docker build .` names no
-# target and so builds this one. Nothing is wanted from the two phases
-# above; the copies are what make BuildKit build them first, so this
-# image cannot be produced unless lint and test passed. The image an
-# app's Dockerfile builds FROM comes with milestone 2
-# (https://git.eeqj.de/sneak/smallwebwaf/issues/12); until then this
-# stage builds the binary and can run it.
+# Build stage. Nothing is wanted from the two phases above; the copies
+# are what make BuildKit build them first, so the image, which needs this
+# stage, cannot be produced unless lint and test passed.
 #
 # golang 1.27.1-trixie, 2026-09-19
-FROM golang@sha256:3b77fc618ec235a1ab412de7737f120dd507c57e8d87de4cbb7994fb94275ed5
+FROM golang@sha256:3b77fc618ec235a1ab412de7737f120dd507c57e8d87de4cbb7994fb94275ed5 AS builder
 
 COPY --from=lint /src/go.sum /dev/null
 COPY --from=test /src/go.sum /dev/null
@@ -61,5 +57,90 @@ RUN CGO_ENABLED=0 go build -trimpath \
     -ldflags="-s -w -X main.Version=${VERSION}" \
     -o /usr/local/bin/smallwebwaf ./cmd/smallwebwaf
 
+# runsvinit, the image's entrypoint, built at the last commit of its
+# archived repository. It has no go.mod, and `go build` of its directory
+# needs one; it uses only the standard library, so the one written here
+# names nothing else.
+#
+# golang 1.27.1-trixie, 2026-09-19
+FROM golang@sha256:3b77fc618ec235a1ab412de7737f120dd507c57e8d87de4cbb7994fb94275ed5 AS runsvinit
+
+RUN git clone --quiet https://github.com/peterbourgon/runsvinit /src
+WORKDIR /src
+RUN git checkout --quiet --detach b4b2c785308b1ce785b6155c7fe5f16879080193 \
+    && go mod init github.com/peterbourgon/runsvinit \
+    && CGO_ENABLED=0 go build -trimpath -ldflags="-s -w" \
+        -o /usr/local/bin/runsvinit .
+
+# The image an app's Dockerfile builds FROM, described under "Deployment"
+# in SPEC.md. It is the last stage, so a plain `docker build .` builds it.
+#
+# ubuntu 26.04, 2026-09-27
+FROM ubuntu@sha256:f144425ff09be612d6d9ad965196e9cdc23dae1f42110a8a11a3e9a8198759f7
+
+# ca-certificates, nix-bin and runit, from Ubuntu's archive as it was at
+# the snapshot moment, which is never earlier than the Ubuntu image above.
+# apt checks every package against the snapshot's InRelease files, and
+# this step checks those against the hashes named here, which are those
+# of the amd64 archive: other architectures use Ubuntu's ports archive.
+# apt also fetches the live archive's InRelease files, which change daily
+# and which the install does not use. The snapshot service is HTTPS only
+# and this image has no CA certificates yet, so this step uses the Go
+# image's.
+RUN --mount=type=bind,from=builder,source=/etc/ssl/certs/ca-certificates.crt,target=/tmp/go-image-ca.crt \
+    apt-get update --snapshot 20261001T000000Z \
+        -o Acquire::https::CaInfo=/tmp/go-image-ca.crt \
+    && printf '%s\n' \
+        '45f95ce276cdba3e41870516a130e03c58b8b7a79e9546b0efe9e526d255740c  snapshot.ubuntu.com_ubuntu_20261001T000000Z_dists_resolute_InRelease' \
+        '802e675dd9de4c7f3916434a95e7c1d8eec0e82886622d7805ab19a2c6fe0365  snapshot.ubuntu.com_ubuntu_20261001T000000Z_dists_resolute-updates_InRelease' \
+        '64b3353f0bd4970b4f7271962245bcea9ff24d4cc7bea16b433f8a60e42ca3dd  snapshot.ubuntu.com_ubuntu_20261001T000000Z_dists_resolute-backports_InRelease' \
+        '1d5041572116a8b23aabf79ac7439ad8af83d57ad3fb0f9aa0d4523ec10c5908  snapshot.ubuntu.com_ubuntu_20261001T000000Z_dists_resolute-security_InRelease' \
+        | (cd /var/lib/apt/lists && sha256sum --check --strict) \
+    && DEBIAN_FRONTEND=noninteractive apt-get install --yes --no-install-recommends \
+        --snapshot 20261001T000000Z \
+        -o Acquire::https::CaInfo=/tmp/go-image-ca.crt \
+        ca-certificates nix-bin runit \
+    && rm -rf /var/lib/apt/lists/*
+
+# Nix run by root expects a group of build users, which nix-bin does not
+# create; with the setting empty, root's builds run without them.
+RUN mkdir /etc/nix && echo 'build-users-group =' > /etc/nix/nix.conf
+
+# nixpkgs, from its release file, checked by SHA-256, and set up for root
+# as `nixpkgs`, so that an app's Dockerfile installs a package with
+# `nix-env -iA nixpkgs.<name>`. curl and xz come with nix-bin.
+RUN curl -fsSL -o /tmp/nixexprs.tar.xz \
+        https://releases.nixos.org/nixos/26.05/nixos-26.05.11045.774debe7a0d1/nixexprs.tar.xz \
+    && echo 'b2994104605601690023a5a6a3bb5a07b2bd1716b4e3b208cba1056dacd2ab08  /tmp/nixexprs.tar.xz' \
+        | sha256sum --check --strict \
+    && mkdir -p /root/.nix-defexpr/nixpkgs \
+    && tar -xJf /tmp/nixexprs.tar.xz -C /root/.nix-defexpr/nixpkgs --strip-components=1 \
+    && rm /tmp/nixexprs.tar.xz
+
+# What root installs with nix-env lands in root's profile. This path to
+# it works for every user, unlike /root/.nix-profile: only root can
+# enter /root. It comes last, so that no package shadows the image's
+# own tools: busybox, for one, brings an sv that looks for services
+# elsewhere.
+ENV PATH=${PATH}:/nix/var/nix/profiles/default/bin
+
+COPY --from=runsvinit /usr/local/bin/runsvinit /usr/local/bin/runsvinit
+COPY --from=builder /usr/local/bin/smallwebwaf /usr/local/bin/smallwebwaf
+
+RUN groupadd --system --gid 65532 smallwebwaf \
+    && useradd --system --uid 65532 --gid smallwebwaf --no-create-home \
+        --shell /usr/sbin/nologin smallwebwaf
+
+# runsvinit starts runit's runsvdir on /etc/service, where Ubuntu's sv
+# looks too.
+COPY --chmod=755 share/smallwebwaf.run /etc/service/smallwebwaf/run
+
 EXPOSE 8080
-ENTRYPOINT ["/usr/local/bin/smallwebwaf"]
+
+# traefik sends a container no requests until it is healthy, so the
+# check runs every second from the start until it first passes, for up
+# to a minute, and every 30 seconds after that.
+HEALTHCHECK --start-period=1m --start-interval=1s \
+    CMD ["/usr/local/bin/smallwebwaf", "healthcheck"]
+
+ENTRYPOINT ["/usr/local/bin/runsvinit"]

Makefile

@@ -1,8 +1,10 @@
-.PHONY: bootstrap setup test lint fmt fmt-check check docker hooks build run
+.PHONY: bootstrap setup test lint fmt fmt-check check docker hooks build run \
+	example-app
 
 # Makefile targets are thin shims; the implementations live in script/
 # per the scripts-to-rule-them-all pattern (see the Entrypoints section
-# of README.md). build and run are for working on the code by hand.
+# of README.md). build and run are for working on the code by hand;
+# example-app checks the image with an app built on it.
 
 bootstrap:
 	@script/bootstrap
@@ -36,3 +38,6 @@ build:
 
 run:
 	@script/run
+
+example-app:
+	@script/example-app

README.md

@@ -11,33 +11,36 @@ request with defaults chosen for a service on the open internet. It keeps its
 state in memory and in JSON files you can read and edit, and writes a detailed
 JSON log line for every request.
 
-Status: the first milestone is built
-(https://git.eeqj.de/sneak/smallwebwaf/issues/13), and the rate limits and
-country lists of the second (https://git.eeqj.de/sneak/smallwebwaf/issues/14).
-`smallwebwaf` passes each request to the app and the app's answer back,
-unchanged, within its timeouts and size limits, works out each client's address,
-refuses a client that sends too many requests or comes from a country you
-refuse, and writes a JSON log line for every request. The image an app builds on
-comes with the rest of milestone 2, and the rest of the design after that, in
-the order of the build order in [`SPEC.md`](SPEC.md). The survey of existing
-tools that led to the design is in [`EVALUATION.md`](EVALUATION.md).
+Status: the first two milestones are built
+(https://git.eeqj.de/sneak/smallwebwaf/issues/13 and
+https://git.eeqj.de/sneak/smallwebwaf/issues/14). `smallwebwaf` passes each
+request to the app and the app's answer back, unchanged, within its timeouts and
+size limits, works out each client's address, refuses a client that sends too
+many requests or comes from a country you refuse, and writes a JSON log line for
+every request. It comes as the image the app's own image is built on. The rest
+of the design comes after that, in the order of the build order in
+[`SPEC.md`](SPEC.md). The survey of existing tools that led to the design is in
+[`EVALUATION.md`](EVALUATION.md).
 
 ## Getting started
 
-`smallwebwaf` is one Go binary. Until milestone 2 brings its image, build and
-run it from a clone, with Go installed:
+Build the `smallwebwaf` image from a clone:
 
 ```sh
 git clone https://git.eeqj.de/sneak/smallwebwaf.git
 cd smallwebwaf
-make build
-SWWAF_UPSTREAM_URL=http://127.0.0.1:3000 ./bin/smallwebwaf
+make docker
 ```
 
-It then listens on port 8080 and passes every request to the app at
-`SWWAF_UPSTREAM_URL`, here an app on port 3000; with no setting at all, to an
-app on `127.0.0.1:8081`. On `SIGTERM` or `SIGINT` it stops taking requests and
-gives those in progress five seconds to finish.
+`make docker` runs the tests and the linter, then builds the image, tagged
+`smallwebwaf`, for amd64. Push it to a registry your hosts pull from, and build
+each app's image on it, pinned by digest, as "How it works, in short" below
+shows. `make example-app` builds a small app on the image, the one in
+`deploy/example-app`, and checks that it works.
+
+To work on the code, `make build` builds the binary alone, with Go installed,
+and `make run` builds and runs it, listening on port 8080 in front of an app at
+`SWWAF_UPSTREAM_URL`, by default `http://127.0.0.1:8081`.
 
 ## What it does so far
 
@@ -79,6 +82,8 @@ gives those in progress five seconds to finish.
   lookup" below); with neither set, no visitor's address leaves the host. A
   client on a private, loopback or link-local address has no country, and
   neither list checks it.
+- Answers `GET /_smallwebwaf/healthz` itself with `200` and `ok`, before any
+  check and without asking the app, for the image's health check.
 - Writes a line in the request log for each request (see "Request log" below).
 
 ## Settings
@@ -155,8 +160,9 @@ refused ones included:
 - `action` is `forward` for a request passed to the app, `country_denied` for
   one refused for its client's country, `rate_limited` for one refused for a
   rate limit, `too_large` for a request or response over its size limit,
-  `timed_out` for one that ran out of time, and `upstream_error` when the app
-  could not be reached or its answer broke off.
+  `timed_out` for one that ran out of time, `upstream_error` when the app could
+  not be reached or its answer broke off, and `admin` for one `smallwebwaf`
+  answered at its own endpoint.
 - `limit_hit` is there for a request refused for a rate limit, and names the
   window whose limit it went over: `minute`, `hour` or `day`, the shortest if it
   went over several.
@@ -353,9 +359,9 @@ main "$@"
   side by side, each as its own user, and start either again a second after it
   exits. Leave out `ENTRYPOINT` and `USER` from the app's Dockerfile.
 - `nix-env -iA nixpkgs.<name>` installs a package from the nixpkgs in the image,
-  and the app finds it on its `PATH`. That nixpkgs is fixed at one commit, so
-  the same `smallwebwaf` image always gives the app the same packages; newer
-  ones come with a newer `smallwebwaf` image.
+  and the app finds it on its `PATH`, after Ubuntu's own commands. That nixpkgs
+  is fixed at one commit, so the same `smallwebwaf` image always gives the app
+  the same packages; newer ones come with a newer `smallwebwaf` image.
 - Deploy it as you deploy any app, with traefik's labels on this one container
   pointing at port 8080. upaas needs no change for this.
 - The app has to trust `127.0.0.1` and `::1` for forwarded headers, besides the
@@ -373,7 +379,10 @@ main "$@"
   the health check on `127.0.0.1`.
 - `smallwebwaf` keeps its state files in `/var/lib/smallwebwaf`. Mount a volume
   there to keep bans and client history when a deploy replaces the container;
-  without one, it still starts.
+  without one, it still starts. The state files come in milestone 3 or later;
+  until then it writes nothing to disk and needs no volume.
+- `docker stop` has runit stop both processes. `smallwebwaf` then stops taking
+  requests and gives those in progress five seconds to finish.
 
 A rule file is one rule per line: a name, what to match against, what to do, and
 a regex.
@@ -444,7 +453,8 @@ refusal comes with `SWWAF_ALLOW_NETS` in milestone 3 or later.
 
 - `cmd/smallwebwaf`: the binary, which only calls `internal/smallwebwaf`.
 - `internal/smallwebwaf`: the process: it reads the settings, listens, serves
-  requests until `SIGTERM` or `SIGINT`, and stops.
+  requests until `SIGTERM` or `SIGINT`, and stops. Run as
+  `smallwebwaf healthcheck`, it is the image's health check instead.
 - `internal/config`: reads the settings, the one place they are read.
 - `internal/proxy`: what happens to each request: it works out the client, runs
   the checks, passes the request to the app and the answer back with the
@@ -458,6 +468,11 @@ refusal comes with `SWWAF_ALLOW_NETS` in milestone 3 or later.
   it over a rate limit.
 - `internal/requestlog`: the lines on stdout: the request log line and the
   process's own messages.
+- `Dockerfile`: the lint and test phases, then the image, whose last stage
+  installs Ubuntu's packages, nixpkgs, `runsvinit` and `smallwebwaf`, with
+  `share/smallwebwaf.run` as runit's `run` script for `smallwebwaf`.
+- `deploy/example-app`: an app built on the image, which `script/example-app`
+  checks.
 
 Besides the Go standard library, `github.com/hashicorp/golang-lru/v2` keeps the
 table of clients to 20,000 and the GeoJS answers to 100,000, dropping the least
@@ -472,7 +487,7 @@ the `Makefile` targets are thin shims that call them. The scripts are POSIX sh,
 so that they run in minimal containers.
 
 - `script/bootstrap`: installs what the other scripts need on the host: `make`,
-  `git`, Go for `gofmt`, node and yarn, and prettier.
+  `git`, `curl`, Go for `gofmt`, node and yarn, and prettier.
 - `script/setup`: readies a fresh clone: runs `script/bootstrap`, then
   `script/install-precommit`.
 - `script/projectname`: prints the project's name, `smallwebwaf`, which
@@ -492,13 +507,16 @@ so that they run in minimal containers.
   working on the code by hand; `make build` runs it.
 - `script/run`: builds `bin/smallwebwaf` with `script/build` and runs it;
   `make run` runs it.
+- `script/example-app`: builds the image and, on it, the example app in
+  `deploy/example-app`, runs it, and checks that the health check passes, that a
+  request reaches the app through `smallwebwaf`, and that `sv stop` and
+  `docker stop` stop it in order; then removes the container and both images. It
+  needs network access, for nixpkgs' binary cache, and `script/check` does not
+  run it; `make example-app` does.
 
 ## TODO
 
-- Milestone 2: the image an app builds on
-  (https://git.eeqj.de/sneak/smallwebwaf/issues/14); its rate limits and country
-  lists are built.
-- The rest of the design, in the order of the build order in
+- Milestone 3 and the rest of the design, in the order of the build order in
   [`SPEC.md`](SPEC.md).
 
 ## Documents

SPEC.md

@@ -1263,14 +1263,19 @@ image's digest does. `apt-get update` keeps the snapshot's `InRelease` files,
 which apt checks against the archive's signature, in `/var/lib/apt/lists/`; each
 lists the SHA-256 hash of the package lists it covers, and each package list the
 hash of every package in it. The Dockerfile also names the SHA-256 hash of each
-`InRelease` file apt uses, and the build checks them after `apt-get update` and
-before `apt-get install`, so every package apt installs is checked, through
-those files, against hashes the Dockerfile names. The snapshot service is
-reached over HTTPS, and the Ubuntu image has no CA certificates of its own, so
-this one install uses those of the Go image that `smallwebwaf` is built in,
-which is pinned by digest too: apt's `Acquire::https::CaInfo` option names that
-image's CA certificate file, `/etc/ssl/certs/ca-certificates.crt`, copied into
-the build.
+of the snapshot's `InRelease` files, and the build checks them after
+`apt-get update` and before `apt-get install`, so every package apt installs is
+checked, through those files, against hashes the Dockerfile names.
+`apt-get update` also fetches the live archive's `InRelease` files into the same
+directory; their hashes change whenever the archive does, and the install does
+not use them, so the check leaves them out. The hashes are those of the archive
+for amd64, and so the image is built for amd64: other architectures use Ubuntu's
+ports archive, whose `InRelease` files differ. The snapshot service is reached
+over HTTPS, and the Ubuntu image has no CA certificates of its own, so this one
+install uses those of the Go image that `smallwebwaf` is built in, which is
+pinned by digest too: apt's `Acquire::https::CaInfo` option names that image's
+CA certificate file, `/etc/ssl/certs/ca-certificates.crt`, mounted for that one
+step.
 
 Packages from nixpkgs: nixpkgs is fixed at one commit of its newest release
 branch, `nixos-26.05` today. For each commit of the branch that has passed its
@@ -1281,15 +1286,18 @@ bytes can change. The image's Dockerfile names the release and the SHA-256 hash
 of that file, which the release's page lists, and the build checks the hash
 before unpacking it. nixpkgs is set up for root under the name `nixpkgs`, so the
 app's Dockerfile installs a package with `nix-env -iA nixpkgs.<name>`, and
-whatever it installs is on the `PATH` of every service. Because nixpkgs stays at
-that commit, an app built on the same `smallwebwaf` image gets the same packages
-each time it is built. Unpacked, nixpkgs takes about 500 MiB of disk, more on
-some filesystems such as ZFS, and each package an app installs from it adds its
-own size, with everything it depends on. A newer commit of the branch, with its
-security fixes, comes with a newer `smallwebwaf` image, as do Ubuntu's own
-fixes; an app takes them by changing the digest in its `FROM` line. When nixpkgs
-makes its next release, every six months, the image moves to that release's
-branch.
+whatever it installs is on the `PATH` of every service: the image adds root's
+Nix profile, `/nix/var/nix/profiles/default/bin`, at the end of the `PATH`,
+after Ubuntu's own directories, so that no package hides the image's own
+commands. busybox, for one, brings its own `sv`, which looks for services
+elsewhere. Because nixpkgs stays at that commit, an app built on the same
+`smallwebwaf` image gets the same packages each time it is built. Unpacked,
+nixpkgs takes about 500 MiB of disk, more on some filesystems such as ZFS, and
+each package an app installs from it adds its own size, with everything it
+depends on. A newer commit of the branch, with its security fixes, comes with a
+newer `smallwebwaf` image, as do Ubuntu's own fixes; an app takes them by
+changing the digest in its `FROM` line. When nixpkgs makes its next release,
+every six months, the image moves to that release's branch.
 
 The two processes:
 
@@ -1315,12 +1323,15 @@ The two processes:
 - The container's root filesystem stays writable: runit writes each service's
   status into its directory under `/etc/service`.
 
-The health check: the image's `HEALTHCHECK` passes while `smallwebwaf` answers
-`GET /_smallwebwaf/healthz` on `127.0.0.1`, at the port in `SWWAF_LISTEN_ADDR`,
-and the app accepts connections at the address in `SWWAF_UPSTREAM_URL`, and
-fails when either does not. The container therefore shows as healthy only while
-both processes are up. An app with a health check of its own can replace the
-image's `HEALTHCHECK` with one that checks both.
+The health check: the image's `HEALTHCHECK` runs `smallwebwaf healthcheck`,
+which passes while `smallwebwaf` answers `GET /_smallwebwaf/healthz` on
+`127.0.0.1`, at the port in `SWWAF_LISTEN_ADDR`, and the app accepts connections
+at the address in `SWWAF_UPSTREAM_URL`, and fails when either does not. The
+container therefore shows as healthy only while both processes are up. traefik
+sends a container no requests until it shows as healthy, so the check runs every
+second from the container's start until it first passes, for up to a minute, and
+every 30 seconds after that. An app with a health check of its own can replace
+the image's `HEALTHCHECK` with one that checks both.
 
 Ports: `smallwebwaf` listens on port 8080 on every address and on no other port;
 its health check, metrics and ban management are all on that listener, under
@@ -1609,7 +1620,9 @@ holds any token file.
     - The container image described under "Deployment", with runit and the
       container's health check. The health check calls `/_smallwebwaf/healthz`,
       so milestone 2 answers that path, although the other admin endpoints come
-      later.
+      later. The image's `/var/lib/smallwebwaf`, which the `run` script gives to
+      the `smallwebwaf` user, and `/etc/smallwebwaf/rules.d` come with the state
+      files and the rule files.
 - Milestone 3 and later: the rest of the design, in this order:
     - static lists, the bans that broken request limits lead to, the ban ledger
       and the JSON state files with edits taken in while running, exemptions,

deploy/example-app/Dockerfile

@@ -0,0 +1,20 @@
+# An app built on the smallwebwaf image, as under "Deployment" in
+# SPEC.md, which script/example-app builds and checks. The app is
+# busybox's web server, from the nixpkgs in the image, serving one page;
+# a real app copies in its own binary instead.
+#
+# A real app names the smallwebwaf image by digest. This one takes the
+# image script/example-app has just built, or else the one `make docker`
+# builds.
+ARG SMALLWEBWAF_IMAGE=smallwebwaf
+FROM ${SMALLWEBWAF_IMAGE}
+
+# Packages the app needs, from the nixpkgs in the image.
+RUN nix-env -iA nixpkgs.busybox
+
+# The app's page, and a user of its own to run it.
+RUN mkdir /var/www && echo 'hello from the example app' > /var/www/index.html
+RUN useradd --system --no-create-home --shell /usr/sbin/nologin app
+
+# The app's runit service.
+COPY --chmod=755 app.run /etc/service/app/run

deploy/example-app/app.run

@@ -0,0 +1,9 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+main() {
+    sleep 1
+    exec chpst -u app:app busybox httpd -f -p 127.0.0.1:8081 -h /var/www
+}
+
+main "$@"

internal/proxy/health_test.go

@@ -0,0 +1,47 @@
+package proxy_test
+
+import (
+	"net/http"
+	"sync/atomic"
+	"testing"
+
+	"sneak.berlin/go/smallwebwaf/internal/proxy"
+	"sneak.berlin/go/smallwebwaf/internal/requestlog"
+)
+
+func TestHealthEndpointIsAnsweredBeforeAnyCheck(t *testing.T) {
+	t.Parallel()
+
+	var calls atomic.Int32
+
+	app := startApp(t, func(http.ResponseWriter, *http.Request) {
+		calls.Add(1)
+	})
+	// With a limit of one request a minute, any request counted before
+	// the last one would have it refused.
+	addr, out := startProxy(t, app.URL, map[string]string{rateLimitPerMinute: "1"})
+
+	const healthChecks = 3
+
+	for range healthChecks {
+		got := get(t, addr, proxy.HealthPath)
+		wantStatus(t, got, http.StatusOK)
+
+		if string(got.body) != "ok\n" {
+			t.Errorf("health endpoint answered %q, want ok", got.body)
+		}
+	}
+
+	wantStatus(t, get(t, addr, "/"), http.StatusOK)
+
+	lines := out.requestLines(t, healthChecks+1)
+	for _, line := range lines[:healthChecks] {
+		wantLine(t, line, http.StatusOK, requestlog.ActionAdmin)
+	}
+
+	wantLine(t, lines[healthChecks], http.StatusOK, requestlog.ActionForward)
+
+	if calls.Load() != 1 {
+		t.Errorf("the app was called %d times, want once", calls.Load())
+	}
+}

internal/proxy/proxy.go

@@ -13,6 +13,7 @@ import (
 	"sneak.berlin/go/smallwebwaf/internal/config"
 	"sneak.berlin/go/smallwebwaf/internal/lookup"
 	"sneak.berlin/go/smallwebwaf/internal/ratelimit"
+	"sneak.berlin/go/smallwebwaf/internal/requestlog"
 )
 
 // The request line and headers a client may send, and how long a
@@ -34,6 +35,10 @@ const (
 	appIdleConnTimeout = 90 * time.Second
 )
 
+// HealthPath is smallwebwaf's health endpoint, which the container's
+// health check asks.
+const HealthPath = "/_smallwebwaf/healthz"
+
 // Params are what New needs.
 type Params struct {
 	Config *config.Config
@@ -111,6 +116,15 @@ func (h *handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	rq := h.newRequest(w, r)
 	defer rq.finish()
 
+	// The health endpoint is answered at once, before any check, so that
+	// a health checker is never refused. It does not ask the app.
+	if r.Method == http.MethodGet && r.URL.Path == HealthPath {
+		rq.line.Action = requestlog.ActionAdmin
+		_, _ = io.WriteString(rq.out, "ok\n")
+
+		return
+	}
+
 	refused := rq.check(r.Context())
 	if refused != nil {
 		rq.answer(*refused)

internal/requestlog/requestlog.go

@@ -28,6 +28,9 @@ const (
 	ActionRateLimited = "rate_limited"
 	// ActionCountryDenied is a request refused for its client's country.
 	ActionCountryDenied = "country_denied"
+	// ActionAdmin is a request smallwebwaf answered at one of its own
+	// endpoints, under /_smallwebwaf/.
+	ActionAdmin = "admin"
 )
 
 // timeLayout is RFC 3339 with milliseconds.

internal/smallwebwaf/healthcheck.go

@@ -0,0 +1,90 @@
+package smallwebwaf
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"io"
+	"net"
+	"net/http"
+	"net/url"
+	"time"
+
+	"sneak.berlin/go/smallwebwaf/internal/config"
+	"sneak.berlin/go/smallwebwaf/internal/proxy"
+)
+
+// healthCheckTimeout bounds the whole health check.
+const healthCheckTimeout = 5 * time.Second
+
+var errHealthEndpoint = errors.New("smallwebwaf's health endpoint answered")
+
+// HealthCheck is the container's health check. It returns 0 while
+// smallwebwaf answers its health endpoint on 127.0.0.1, at the port in
+// SWWAF_LISTEN_ADDR, and the app accepts connections at the address in
+// SWWAF_UPSTREAM_URL. Otherwise it writes why to stderr and returns 1.
+func HealthCheck(
+	ctx context.Context, lookupEnv func(string) (string, bool), stderr io.Writer,
+) int {
+	err := healthCheck(ctx, lookupEnv)
+	if err != nil {
+		_, _ = fmt.Fprintln(stderr, "unhealthy:", err)
+
+		return 1
+	}
+
+	return 0
+}
+
+func healthCheck(ctx context.Context, lookupEnv func(string) (string, bool)) error {
+	ctx, cancel := context.WithTimeout(ctx, healthCheckTimeout)
+	defer cancel()
+
+	cfg, err := config.FromEnvironment(lookupEnv)
+	if err != nil {
+		return fmt.Errorf("invalid setting: %w", err)
+	}
+
+	// The settings have checked that the address has a port.
+	_, port, _ := net.SplitHostPort(cfg.ListenAddr)
+	health := "http://" + net.JoinHostPort("127.0.0.1", port) + proxy.HealthPath
+
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, health, http.NoBody)
+	if err != nil {
+		return fmt.Errorf("make the request: %w", err)
+	}
+
+	res, err := http.DefaultClient.Do(req)
+	if err != nil {
+		return fmt.Errorf("ask smallwebwaf: %w", err)
+	}
+
+	_ = res.Body.Close()
+
+	if res.StatusCode != http.StatusOK {
+		return fmt.Errorf("%w %s", errHealthEndpoint, res.Status)
+	}
+
+	conn, err := (&net.Dialer{}).DialContext(ctx, "tcp", appAddress(cfg.UpstreamURL))
+	if err != nil {
+		return fmt.Errorf("connect to the app: %w", err)
+	}
+
+	_ = conn.Close()
+
+	return nil
+}
+
+// appAddress is the host and port of the app's URL, the port being the
+// scheme's own when the URL names none.
+func appAddress(app *url.URL) string {
+	port := app.Port()
+	if port == "" {
+		port = "80"
+		if app.Scheme == "https" {
+			port = "443"
+		}
+	}
+
+	return net.JoinHostPort(app.Hostname(), port)
+}

internal/smallwebwaf/healthcheck_internal_test.go

@@ -0,0 +1,27 @@
+package smallwebwaf
+
+import (
+	"net/url"
+	"testing"
+)
+
+func TestAppAddress(t *testing.T) {
+	t.Parallel()
+
+	for app, want := range map[string]string{
+		"http://127.0.0.1:8081": "127.0.0.1:8081",
+		"http://app":            "app:80",
+		"https://app/":          "app:443",
+		"https://[::1]":         "[::1]:443",
+	} {
+		parsed, err := url.Parse(app)
+		if err != nil {
+			t.Fatalf("parse %q: %v", app, err)
+		}
+
+		got := appAddress(parsed)
+		if got != want {
+			t.Errorf("appAddress(%q) is %q, want %q", app, got, want)
+		}
+	}
+}

internal/smallwebwaf/healthcheck_test.go

@@ -0,0 +1,79 @@
+package smallwebwaf_test
+
+import (
+	"bytes"
+	"context"
+	"net"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+	"time"
+
+	"sneak.berlin/go/smallwebwaf/internal/smallwebwaf"
+)
+
+func TestHealthCheck(t *testing.T) {
+	t.Parallel()
+
+	app := httptest.NewServer(http.NotFoundHandler())
+	defer app.Close()
+
+	ctx, stop := context.WithCancel(t.Context())
+	defer stop()
+
+	out := &output{}
+	exited := make(chan int, 1)
+
+	go func() {
+		exited <- run(ctx, map[string]string{
+			listenAddr:  localhost + ":0",
+			upstreamURL: app.URL,
+		}, out)
+	}()
+
+	addr, _ := out.line(t, "msg", "starting")["address"].(string)
+	_, port, _ := net.SplitHostPort(addr)
+	// The container's settings: an address to listen on with an empty
+	// host part, which the health check asks at 127.0.0.1.
+	env := map[string]string{listenAddr: ":" + port, upstreamURL: app.URL}
+
+	wantHealthCheck(t, env, 0, "")
+
+	app.Close()
+	wantHealthCheck(t, env, 1, "unhealthy: connect to the app: ")
+
+	stop()
+
+	select {
+	case <-exited:
+	case <-time.After(waitLimit):
+		t.Fatal("still running after being told to stop")
+	}
+
+	wantHealthCheck(t, env, 1, "unhealthy: ask smallwebwaf: ")
+	wantHealthCheck(t, map[string]string{listenAddr: "8080"}, 1,
+		"unhealthy: invalid setting: SWWAF_LISTEN_ADDR: ")
+}
+
+// wantHealthCheck runs the health check with the settings in env, and
+// checks its exit status and the start of what it writes to stderr,
+// which is nothing when message is empty.
+func wantHealthCheck(t *testing.T, env map[string]string, status int, message string) {
+	t.Helper()
+
+	var stderr bytes.Buffer
+
+	got := smallwebwaf.HealthCheck(t.Context(), func(name string) (string, bool) {
+		value, ok := env[name]
+
+		return value, ok
+	}, &stderr)
+
+	wrote := stderr.String()
+	if got != status || !strings.HasPrefix(wrote, message) ||
+		(message == "" && wrote != "") {
+		t.Errorf("health check returned %d and wrote %q, want %d and %q",
+			got, wrote, status, message)
+	}
+}

internal/smallwebwaf/smallwebwaf.go

@@ -37,8 +37,13 @@ type Params struct {
 }
 
 // Main runs smallwebwaf until SIGTERM or SIGINT, and returns the
-// process's exit status.
+// process's exit status. Run as `smallwebwaf healthcheck`, it is the
+// container's health check instead.
 func Main(version string) int {
+	if len(os.Args) == 2 && os.Args[1] == "healthcheck" {
+		return HealthCheck(context.Background(), os.LookupEnv, os.Stderr)
+	}
+
 	ctx, stop := signal.NotifyContext(context.Background(),
 		syscall.SIGTERM, os.Interrupt)
 	defer stop()

internal/smallwebwaf/smallwebwaf_test.go

@@ -24,8 +24,9 @@ const (
 	// testVersion is the version the tests give smallwebwaf.
 	testVersion = "test"
 	// localhost is where the tests listen.
-	localhost  = "127.0.0.1"
-	listenAddr = "SWWAF_LISTEN_ADDR"
+	localhost   = "127.0.0.1"
+	listenAddr  = "SWWAF_LISTEN_ADDR"
+	upstreamURL = "SWWAF_UPSTREAM_URL"
 )
 
 // output collects what smallwebwaf writes on stdout.
@@ -143,8 +144,8 @@ func TestServesUntilToldToStop(t *testing.T) {
 
 	go func() {
 		exited <- run(ctx, map[string]string{
-			listenAddr:           localhost + ":0",
-			"SWWAF_UPSTREAM_URL": app.URL,
+			listenAddr:  localhost + ":0",
+			upstreamURL: app.URL,
 		}, out)
 	}()
 
@@ -177,7 +178,7 @@ func wantStartingLine(t *testing.T, line map[string]any, appURL string) {
 	settings, _ := line["settings"].(map[string]any)
 	want := map[string]any{
 		listenAddr:                            localhost + ":0",
-		"SWWAF_UPSTREAM_URL":                  appURL,
+		upstreamURL:                           appURL,
 		"SWWAF_TRUSTED_PROXIES":               "10.0.0.0/8,172.16.0.0/12,192.168.0.0/16",
 		"SWWAF_CLIENT_REQUEST_TIMEOUT":        "60s",
 		"SWWAF_CLIENT_RESPONSE_TIMEOUT":       "30m",

script/bootstrap

@@ -131,6 +131,7 @@ main() {
 
     if missing make; then pkg_install gnumake make make make; fi
     if missing git; then pkg_install git git git git; fi
+    if missing curl; then pkg_install curl curl curl curl; fi
     if missing gofmt; then pkg_install go golang go go; fi
 
     ensure_node

script/example-app

@@ -0,0 +1,88 @@
+#!/bin/sh
+# script/example-app: build the image, and on it the example app in
+# deploy/example-app, then run the app's container and check that the
+# health check passes, that a request is served through smallwebwaf,
+# that `sv stop` stops smallwebwaf in order, and that `docker stop`
+# stops the container without having to kill it. The container and both
+# images are removed however the script ends. Building the app needs
+# network access, for nixpkgs' binary cache. script/check does not run
+# this.
+set -eu
+
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
+ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
+
+# Named after this run, so that runs in other clones on the same host
+# never touch each other's.
+NAME="$("$SCRIPT_DIR/projectname")-example-$$"
+IMAGE="$NAME-base"
+APP_IMAGE="$NAME-app"
+CONTAINER="$NAME"
+
+cleanup() {
+    docker rm --force "$CONTAINER" >/dev/null 2>&1 || true
+    docker rmi --force "$APP_IMAGE" "$IMAGE" >/dev/null 2>&1 || true
+}
+
+fail() {
+    echo "example-app: $*; the container's output:" >&2
+    docker logs "$CONTAINER" >&2 || true
+    exit 1
+}
+
+# wait_for <what fails> <command>...: run the command every second until
+# it succeeds, for at most a minute.
+wait_for() {
+    failure="$1"
+    shift
+    tries=0
+    until "$@"; do
+        tries=$((tries + 1))
+        [ "$tries" -lt 60 ] || fail "$failure"
+        sleep 1
+    done
+}
+
+healthy() {
+    status="$(docker inspect --format '{{.State.Health.Status}}' "$CONTAINER")"
+    [ "$status" = healthy ]
+}
+
+# logged <text>: the container's output holds text.
+logged() {
+    docker logs "$CONTAINER" 2>&1 | grep -qF "$1"
+}
+
+main() {
+    cd "$ROOT"
+    trap cleanup EXIT
+    trap 'exit 1' HUP INT TERM
+
+    docker build --no-cache -t "$IMAGE" .
+    docker build --no-cache --build-arg SMALLWEBWAF_IMAGE="$IMAGE" \
+        -t "$APP_IMAGE" deploy/example-app
+
+    docker run --detach --name "$CONTAINER" --publish 127.0.0.1::8080 \
+        "$APP_IMAGE" >/dev/null
+    wait_for "the health check did not pass" healthy
+    echo "example-app: the health check passes"
+
+    address="$(docker port "$CONTAINER" 8080/tcp)"
+    page="$(curl --fail --silent --show-error --max-time 10 "http://$address/")" ||
+        fail "no answer on port 8080"
+    [ "$page" = "hello from the example app" ] || fail "port 8080 answered $page"
+    wait_for "smallwebwaf logged no request it forwarded" logged '"action":"forward"'
+    echo "example-app: smallwebwaf passes a request to the app and its answer back"
+
+    docker exec "$CONTAINER" sv stop smallwebwaf >/dev/null ||
+        fail "sv stop smallwebwaf failed"
+    wait_for "smallwebwaf did not stop in order" logged '"msg":"stopped"'
+    echo "example-app: sv stop stops smallwebwaf in order"
+
+    docker stop "$CONTAINER" >/dev/null
+    status="$(docker inspect --format '{{.State.ExitCode}}' "$CONTAINER")"
+    [ "$status" = 0 ] || fail "docker stop left exit status $status"
+    echo "example-app: docker stop stops the container in order"
+}
+
+main "$@"

share/smallwebwaf.run

@@ -0,0 +1,12 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+# runit's run script for smallwebwaf, run again whenever smallwebwaf
+# exits; the wait spaces out the restarts. exec, so that the signal
+# `sv stop` sends reaches smallwebwaf itself.
+main() {
+    sleep 1
+    exec chpst -u smallwebwaf:smallwebwaf /usr/local/bin/smallwebwaf
+}
+
+main "$@"