Write the deployment guide and an example Caddy config (closes #89)

README.md gains a "Deployment" section: what the reverse proxy in front
of pixa must do (terminate TLS, pass Host, Origin and Referer on
unchanged, set X-Forwarded-For with trusted_proxies to match, wait at
least downstream_timeout, optionally refuse /metrics) and what pixa does
itself; that the state directory needs a persistent volume, what
cache_max_bytes counts and why to set it; the health check for a load
balancer; what SIGTERM does and the exit codes; and what running outside
Docker needs. configs/Caddyfile is the example, as Caddy needs no
settings beyond the host name and pixa's address.

Model: opus-5-5

TODO.md

@@ -38,6 +38,30 @@ P2: security: referer blacklist
   counts, the health check for a load balancer, what a stop does and its exit
   codes, and what running outside Docker needs; `configs/Caddyfile` is the
   example, checked with `caddy validate`.
+- 2026-10-04 the metrics basic auth, CORS preflight, request logging and
+  metrics recording have tests (closes #79): `MetricsAuth` on its own answers
+  401 with a challenge without credentials or with a wrong username or password
+  and lets the configured ones through; a preflight request gets `*` for any
+  origin when `access_control_allow_origin` is `*` and no
+  `Access-Control-Allow-Origin` from another origin than the configured one; a
+  `POST /` carrying the signing key leaves no trace of it in the request log
+  line, and the login handler's own log lines leave out the submitted key; the
+  metrics middleware on its own records a request it served, and the router
+  records nothing while no metrics username is set. Not tested: that the router
+  puts the basic auth in front of `/metrics` and records requests when a
+  metrics username is set. Only one test per package can set up `/metrics`, and
+  in `internal/server` that is `TestMaintenanceModeKeepsOtherRoutes`, which
+  needs the owner's approval to change; #180 holds it. Tests only; the basic
+  auth library already compares the password in constant time.
+- 2026-10-04 the image route's signature check and error answers are tested
+  (closes #76): new tests in `internal/handlers`, with no network, check the
+  status and JSON error body for a missing, wrong, unpadded, upper-case or
+  expired signature on a host not on the allowlist, or a valid one sent for
+  its parent domain, a sibling host, a subdomain or the host with another
+  domain appended (401), an unparseable path (400), `localhost` as the
+  upstream host (403) and an upstream error (502); that an allowlisted host is
+  served without a signature, another host only with a valid one; and the
+  answers of `/robots.txt` and the health check. No code changes.
 - 2026-10-04 request IDs returned and passed on, and `/v1/e/` revalidates
   (closes #84): pixa's own `RequestID` middleware, in place of chi's, gives each
   request an ID, its own `X-Request-ID` when that is at most 64 letters, digits,

internal/handlers/auth_login_log_internal_test.go

@@ -0,0 +1,59 @@
+package handlers
+
+import (
+	"bytes"
+	"log/slog"
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"strings"
+	"testing"
+
+	"sneak.berlin/go/pixa/internal/config"
+	"sneak.berlin/go/pixa/internal/session"
+)
+
+// TestLoginLogLeavesOutSubmittedKey verifies that the log lines for a
+// failed and for a successful login do not contain the submitted key.
+func TestLoginLogLeavesOutSubmittedKey(t *testing.T) {
+	t.Parallel()
+
+	const wrongKey = "wrong-signing-key-fedcba9876543210"
+
+	var buf bytes.Buffer
+
+	sessMgr, err := session.NewManager(testSigningKey)
+	if err != nil {
+		t.Fatalf("session.NewManager() error = %v", err)
+	}
+
+	h := &Handlers{
+		log:     slog.New(slog.NewJSONHandler(&buf, nil)),
+		config:  &config.Config{SigningKey: testSigningKey},
+		sessMgr: sessMgr,
+	}
+
+	submittedKeys := []string{wrongKey, testSigningKey}
+
+	for _, key := range submittedKeys {
+		form := url.Values{loginKeyField: {key}}
+		req := httptest.NewRequestWithContext(
+			t.Context(), http.MethodPost, "/",
+			strings.NewReader(form.Encode()))
+		req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+
+		h.handleLoginPost(httptest.NewRecorder(), req)
+	}
+
+	for _, msg := range []string{"failed login attempt", "successful login"} {
+		if !strings.Contains(buf.String(), msg) {
+			t.Fatalf("log missing %q; got %q", msg, buf.String())
+		}
+	}
+
+	for _, key := range submittedKeys {
+		if strings.Contains(buf.String(), key) {
+			t.Errorf("log contains submitted key %q; got %q", key, buf.String())
+		}
+	}
+}

internal/handlers/image_errors_internal_test.go

@@ -0,0 +1,267 @@
+package handlers
+
+import (
+	"encoding/json"
+	"fmt"
+	"image/color"
+	"log/slog"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+	"testing/fstest"
+	"time"
+
+	"github.com/go-chi/chi/v5"
+	"sneak.berlin/go/pixa/internal/httpfetcher"
+	"sneak.berlin/go/pixa/internal/imgcache"
+	"sneak.berlin/go/pixa/internal/signature"
+)
+
+// allowlistedHost is the only host on the allowlist of the image route
+// newImageRoute builds.
+const allowlistedHost = "allowed.example.com"
+
+// newImageRoute returns the image route of a Handlers whose service fetches
+// with fetcher and checks signatures with testSigningKey.
+func newImageRoute(t *testing.T, fetcher httpfetcher.Fetcher) http.Handler {
+	t.Helper()
+
+	cache, err := imgcache.NewCache(setupTestDB(t), imgcache.CacheConfig{
+		StateDir:    t.TempDir(),
+		CacheTTL:    time.Hour,
+		NegativeTTL: 5 * time.Minute,
+	})
+	if err != nil {
+		t.Fatalf("failed to create cache: %v", err)
+	}
+
+	svc, err := imgcache.NewService(&imgcache.ServiceConfig{
+		Cache:      cache,
+		Fetcher:    fetcher,
+		SigningKey: testSigningKey,
+		Allowlist:  []string{allowlistedHost},
+	})
+	if err != nil {
+		t.Fatalf("failed to create service: %v", err)
+	}
+
+	h := &Handlers{imgSvc: svc, log: slog.New(slog.DiscardHandler)}
+
+	r := chi.NewRouter()
+	r.Get("/v1/image/*", h.HandleImage())
+
+	return r
+}
+
+// newPhotoFetcher returns a mock fetcher that serves a JPEG at photoPath on
+// each of hosts, and answers any other URL with an upstream error.
+func newPhotoFetcher(t *testing.T, hosts ...string) *httpfetcher.MockFetcher {
+	t.Helper()
+
+	photo := &fstest.MapFile{
+		Data: generateTestJPEG(t, 100, 100, color.RGBA{255, 0, 0, 255}),
+	}
+
+	files := fstest.MapFS{}
+	for _, host := range hosts {
+		files[host+photoPath] = photo
+	}
+
+	return httpfetcher.NewMock(files)
+}
+
+// photoURL returns the image route URL of photoPath on host, as a 50x50 JPEG.
+func photoURL(host string) string {
+	return "/v1/image/" + host + photoPath + "/50x50.jpeg"
+}
+
+// photoURLWithSig returns photoURL(host) with sig and expires as its sig and
+// exp.
+func photoURLWithSig(host, sig string, expires time.Time) string {
+	return fmt.Sprintf("%s?sig=%s&exp=%d", photoURL(host), sig, expires.Unix())
+}
+
+// photoSignature returns the signature of photoURL(host) at the default
+// quality and fit, made with key and expiring at expires.
+func photoSignature(key, host string, expires time.Time) string {
+	return signature.New(key).Sign(&signature.Request{
+		SourceHost: host,
+		SourcePath: photoPath,
+		Width:      50,
+		Height:     50,
+		Format:     string(imgcache.FormatJPEG),
+		Quality:    85,
+		FitMode:    string(imgcache.FitCover),
+		Expires:    expires,
+	})
+}
+
+// sendGet sends a GET for target to route and returns the response.
+func sendGet(
+	t *testing.T, route http.Handler, target string,
+) *httptest.ResponseRecorder {
+	t.Helper()
+
+	req := httptest.NewRequestWithContext(t.Context(), http.MethodGet, target, nil)
+	rec := httptest.NewRecorder()
+
+	route.ServeHTTP(rec, req)
+	t.Logf("GET %s: %d", target, rec.Code)
+
+	return rec
+}
+
+// checkErrorBody checks that rec has status wantStatus and the JSON error body
+// the image route sends: wantError, wantStatus and the time in RFC 3339.
+func checkErrorBody(
+	t *testing.T, rec *httptest.ResponseRecorder, wantStatus int, wantError string,
+) {
+	t.Helper()
+
+	if rec.Code != wantStatus {
+		t.Errorf("status = %d, want %d", rec.Code, wantStatus)
+	}
+
+	if ct := rec.Header().Get("Content-Type"); ct != "application/json" {
+		t.Errorf("Content-Type = %q, want application/json", ct)
+	}
+
+	var body struct {
+		Error     string `json:"error"`
+		Status    int    `json:"status"`
+		Timestamp string `json:"timestamp"`
+	}
+
+	err := json.NewDecoder(rec.Body).Decode(&body)
+	if err != nil {
+		t.Fatalf("decoding response body: %v", err)
+	}
+
+	if body.Error != wantError || body.Status != wantStatus {
+		t.Errorf("body error and status = %q %d, want %q %d",
+			body.Error, body.Status, wantError, wantStatus)
+	}
+
+	_, err = time.Parse(time.RFC3339, body.Timestamp)
+	if err != nil {
+		t.Errorf("body timestamp: %v", err)
+	}
+}
+
+// TestHandleImage_ErrorAnswers checks the status and the JSON error body the
+// image route answers each request below with. The JPEG at photoPath exists on
+// signedHost and on each host below that differs from it, so a request refused
+// with 401 would otherwise be served.
+func TestHandleImage_ErrorAnswers(t *testing.T) {
+	t.Parallel()
+
+	// A signature for signedHost must not verify for any of these.
+	parentHost := "example.com"
+	siblingHost := "other.example.com"
+	subdomainHost := "img." + signedHost
+	appendedHost := signedHost + ".example.net"
+
+	photos := newPhotoFetcher(t,
+		signedHost, parentHost, siblingHost, subdomainHost, appendedHost)
+	// The real fetcher refuses localhost before any lookup or connection.
+	realFetcher := httpfetcher.New(httpfetcher.DefaultConfig())
+
+	exp := time.Now().Add(time.Hour)
+	expired := time.Now().Add(-time.Hour)
+	sig := photoSignature(testSigningKey, signedHost, exp)
+	otherKeySig := photoSignature("another-signing-key", signedHost, exp)
+	expiredSig := photoSignature(testSigningKey, signedHost, expired)
+	localhostSig := photoSignature(testSigningKey, "localhost", exp)
+
+	// The error every request refused for its signature gets.
+	const unauthorized = "unauthorized"
+
+	tests := []struct {
+		name       string
+		fetcher    httpfetcher.Fetcher
+		target     string
+		wantStatus int
+		wantError  string
+	}{
+		{"no sig or exp", photos, photoURL(signedHost),
+			http.StatusUnauthorized, unauthorized},
+		{"exp but no sig", photos,
+			fmt.Sprintf("%s?exp=%d", photoURL(signedHost), exp.Unix()),
+			http.StatusUnauthorized, unauthorized},
+		{"sig made with another key", photos,
+			photoURLWithSig(signedHost, otherKeySig, exp),
+			http.StatusUnauthorized, unauthorized},
+		{"sig without its = padding", photos,
+			photoURLWithSig(signedHost, strings.TrimRight(sig, "="), exp),
+			http.StatusUnauthorized, unauthorized},
+		{"sig in upper case", photos,
+			photoURLWithSig(signedHost, strings.ToUpper(sig), exp),
+			http.StatusUnauthorized, unauthorized},
+		{"expired sig", photos, photoURLWithSig(signedHost, expiredSig, expired),
+			http.StatusUnauthorized, unauthorized},
+		{"sig sent for the parent domain", photos,
+			photoURLWithSig(parentHost, sig, exp),
+			http.StatusUnauthorized, unauthorized},
+		{"sig sent for a sibling host", photos,
+			photoURLWithSig(siblingHost, sig, exp),
+			http.StatusUnauthorized, unauthorized},
+		{"sig sent for a subdomain", photos,
+			photoURLWithSig(subdomainHost, sig, exp),
+			http.StatusUnauthorized, unauthorized},
+		{"sig sent with another domain appended", photos,
+			photoURLWithSig(appendedHost, sig, exp),
+			http.StatusUnauthorized, unauthorized},
+		{"unparseable path", photos,
+			"/v1/image/" + allowlistedHost + photoPath + "/big.jpeg",
+			http.StatusBadRequest, "invalid image URL: invalid size format"},
+		{"blocked upstream address", realFetcher,
+			photoURLWithSig("localhost", localhostSig, exp),
+			http.StatusForbidden, "forbidden"},
+		{"upstream error", photos,
+			"/v1/image/" + allowlistedHost + "/images/missing.jpg/50x50.jpeg",
+			http.StatusBadGateway, "upstream error"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			rec := sendGet(t, newImageRoute(t, tt.fetcher), tt.target)
+			checkErrorBody(t, rec, tt.wantStatus, tt.wantError)
+		})
+	}
+}
+
+// TestHandleImage_AllowlistOrSignature checks that the image route serves an
+// image without a signature for a host on the allowlist only, and for another
+// host only with a valid signature.
+func TestHandleImage_AllowlistOrSignature(t *testing.T) {
+	t.Parallel()
+
+	photos := newPhotoFetcher(t, allowlistedHost, signedHost)
+	exp := time.Now().Add(time.Hour)
+	sig := photoSignature(testSigningKey, signedHost, exp)
+
+	tests := []struct {
+		name       string
+		target     string
+		wantStatus int
+	}{
+		{"allowlisted host, no sig", photoURL(allowlistedHost), http.StatusOK},
+		{"other host, no sig", photoURL(signedHost), http.StatusUnauthorized},
+		{"other host, valid sig", photoURLWithSig(signedHost, sig, exp),
+			http.StatusOK},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			rec := sendGet(t, newImageRoute(t, photos), tt.target)
+			if rec.Code != tt.wantStatus {
+				t.Errorf("status = %d, want %d", rec.Code, tt.wantStatus)
+			}
+		})
+	}
+}

internal/handlers/robots_healthcheck_internal_test.go

@@ -0,0 +1,90 @@
+package handlers
+
+import (
+	"encoding/json"
+	"log/slog"
+	"net/http"
+	"testing"
+
+	"go.uber.org/fx/fxtest"
+	"sneak.berlin/go/pixa/internal/config"
+	"sneak.berlin/go/pixa/internal/globals"
+	"sneak.berlin/go/pixa/internal/healthcheck"
+	"sneak.berlin/go/pixa/internal/logger"
+)
+
+// TestHandleRobotsTxt checks that /robots.txt asks every crawler to stay off
+// the whole site.
+func TestHandleRobotsTxt(t *testing.T) {
+	t.Parallel()
+
+	h := &Handlers{log: slog.New(slog.DiscardHandler)}
+	rec := sendGet(t, h.HandleRobotsTxt(), "/robots.txt")
+
+	if rec.Code != http.StatusOK {
+		t.Errorf("status = %d, want %d", rec.Code, http.StatusOK)
+	}
+
+	if ct := rec.Header().Get("Content-Type"); ct != "text/plain" {
+		t.Errorf("Content-Type = %q, want text/plain", ct)
+	}
+
+	want := "User-agent: *\nDisallow: /\n"
+	if rec.Body.String() != want {
+		t.Errorf("body = %q, want %q", rec.Body.String(), want)
+	}
+}
+
+// TestHandleHealthCheck checks that the health check answers 200 with status
+// ok, the app's name and version, now, uptime_seconds, uptime_human and
+// maintenance_mode, which is true here: the health check stays 200 while
+// maintenance mode is on.
+func TestHandleHealthCheck(t *testing.T) {
+	t.Parallel()
+
+	lc := fxtest.NewLifecycle(t)
+
+	log, err := logger.New(lc, logger.Params{Globals: &globals.Globals{}})
+	if err != nil {
+		t.Fatalf("logger.New() error = %v", err)
+	}
+
+	hc, err := healthcheck.New(lc, healthcheck.Params{
+		Globals: &globals.Globals{Appname: "pixad", Version: "v1.2.3"},
+		Config:  &config.Config{MaintenanceMode: true},
+		Logger:  log,
+	})
+	if err != nil {
+		t.Fatalf("healthcheck.New() error = %v", err)
+	}
+
+	h := &Handlers{hc: hc, log: slog.New(slog.DiscardHandler)}
+	rec := sendGet(t, h.HandleHealthCheck(), "/.well-known/healthcheck.json")
+
+	if rec.Code != http.StatusOK {
+		t.Errorf("status = %d, want %d", rec.Code, http.StatusOK)
+	}
+
+	if ct := rec.Header().Get("Content-Type"); ct != "application/json" {
+		t.Errorf("Content-Type = %q, want application/json", ct)
+	}
+
+	var body map[string]any
+
+	err = json.NewDecoder(rec.Body).Decode(&body)
+	if err != nil {
+		t.Fatalf("decoding response body: %v", err)
+	}
+
+	if body["status"] != "ok" || body["appname"] != "pixad" ||
+		body["version"] != "v1.2.3" || body["maintenance_mode"] != true {
+		t.Errorf("body = %v, want status ok, appname pixad, version v1.2.3 "+
+			"and maintenance_mode true", body)
+	}
+
+	for _, key := range []string{"now", "uptime_seconds", "uptime_human"} {
+		if _, ok := body[key]; !ok {
+			t.Errorf("body = %v, has no %s", body, key)
+		}
+	}
+}

internal/middleware/middleware_internal_test.go

@@ -1,11 +1,16 @@
 package middleware
 
 import (
+	"bytes"
 	"log/slog"
 	"net/http"
 	"net/http/httptest"
+	"net/url"
+	"strings"
 	"testing"
 
+	"github.com/prometheus/client_golang/prometheus/promhttp"
+
 	"sneak.berlin/go/pixa/internal/config"
 )
 
@@ -56,6 +61,203 @@ func TestCORSAnswersWithConfiguredOrigin(t *testing.T) {
 	}
 }
 
+// TestCORSAnswersPreflightWithConfiguredOrigin checks that the CORS
+// middleware answers a preflight request, which the CORS library handles
+// apart from other requests, the same way: "*" lets any origin read
+// responses and a single origin lets only that origin read them.
+func TestCORSAnswersPreflightWithConfiguredOrigin(t *testing.T) {
+	t.Parallel()
+
+	const appOrigin = "https://app.example.com"
+
+	cases := []struct {
+		configured    string
+		requestOrigin string
+		want          string
+	}{
+		{"*", "https://any.example.com", "*"},
+		{appOrigin, appOrigin, appOrigin},
+		{appOrigin, "https://other.example.com", ""},
+	}
+
+	testHandler := http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		w.WriteHeader(http.StatusOK)
+	})
+
+	for _, tc := range cases {
+		mw := &Middleware{
+			log:    slog.Default(),
+			config: &config.Config{AccessControlAllowOrigin: tc.configured},
+		}
+
+		handler := mw.CORS()(testHandler)
+
+		// An OPTIONS request naming the method it asks about is the
+		// preflight a browser sends before some cross-origin requests.
+		req := httptest.NewRequestWithContext(
+			t.Context(), http.MethodOptions, "/v1/image/example.com/a.jpg/1x1.png", nil)
+		req.Header.Set("Origin", tc.requestOrigin)
+		req.Header.Set("Access-Control-Request-Method", http.MethodGet)
+
+		rec := httptest.NewRecorder()
+
+		handler.ServeHTTP(rec, req)
+
+		got := rec.Header().Get("Access-Control-Allow-Origin")
+		if got != tc.want {
+			t.Errorf("configured %q, preflight from %q: "+
+				"Access-Control-Allow-Origin = %q, want %q",
+				tc.configured, tc.requestOrigin, got, tc.want)
+		}
+	}
+}
+
+// TestMetricsAuthRequiresConfiguredCredentials checks that MetricsAuth on
+// its own answers 401 with a challenge to a request without credentials or
+// with a wrong username or password, and lets a request with the configured
+// username and password through. That the router puts it in front of
+// /metrics is not tested.
+func TestMetricsAuthRequiresConfiguredCredentials(t *testing.T) {
+	t.Parallel()
+
+	const (
+		username  = "metricsuser"
+		password  = "metricspass"
+		challenge = `Basic realm="metrics"`
+	)
+
+	// An empty username stands for a request sent without credentials.
+	cases := []struct {
+		name        string
+		username    string
+		password    string
+		wantReached bool
+	}{
+		{"no credentials", "", "", false},
+		{"wrong username", "someone", password, false},
+		{"wrong password", username, "wrongpass", false},
+		{"configured credentials", username, password, true},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			mw := &Middleware{
+				log: slog.Default(),
+				config: &config.Config{
+					MetricsUsername: username,
+					MetricsPassword: password,
+				},
+			}
+
+			reached := false
+			handler := mw.MetricsAuth()(http.HandlerFunc(
+				func(http.ResponseWriter, *http.Request) {
+					reached = true
+				}))
+
+			req := httptest.NewRequestWithContext(
+				t.Context(), http.MethodGet, "/metrics", nil)
+
+			if tc.username != "" {
+				req.SetBasicAuth(tc.username, tc.password)
+			}
+
+			rec := httptest.NewRecorder()
+
+			handler.ServeHTTP(rec, req)
+
+			if reached != tc.wantReached {
+				t.Fatalf("request reached /metrics = %v, want %v",
+					reached, tc.wantReached)
+			}
+
+			if tc.wantReached {
+				return
+			}
+
+			if rec.Code != http.StatusUnauthorized {
+				t.Errorf("status = %d, want %d",
+					rec.Code, http.StatusUnauthorized)
+			}
+
+			if got := rec.Header().Get("WWW-Authenticate"); got != challenge {
+				t.Errorf("WWW-Authenticate = %q, want %q", got, challenge)
+			}
+		})
+	}
+}
+
+// TestMetricsRecordsServedRequest checks that the metrics middleware
+// records a request it served, so /metrics reports it. It is the only test
+// in this package that sets up the metrics middleware, which registers with
+// the process-wide Prometheus registry and can do so only once.
+func TestMetricsRecordsServedRequest(t *testing.T) {
+	t.Parallel()
+
+	// The line /metrics shows once one GET /test has been served.
+	const want = `http_request_duration_seconds_count{` +
+		`code="200",handler="/test",method="GET",service=""} 1`
+
+	mw := &Middleware{log: slog.Default(), config: &config.Config{}}
+
+	handler := mw.Metrics()(http.HandlerFunc(
+		func(w http.ResponseWriter, _ *http.Request) {
+			w.WriteHeader(http.StatusOK)
+		}))
+
+	handler.ServeHTTP(httptest.NewRecorder(), httptest.NewRequestWithContext(
+		t.Context(), http.MethodGet, "/test", nil))
+
+	rec := httptest.NewRecorder()
+	promhttp.Handler().ServeHTTP(rec, httptest.NewRequestWithContext(
+		t.Context(), http.MethodGet, "/metrics", nil))
+
+	if !strings.Contains(rec.Body.String(), want) {
+		t.Errorf("/metrics does not report the GET /test served; "+
+			"want the line %q in:\n%s", want, rec.Body.String())
+	}
+}
+
+// TestLoggingLeavesOutSubmittedSigningKey checks that a login, a POST /
+// whose form carries the signing key, leaves no trace of the key in the
+// request's log line.
+func TestLoggingLeavesOutSubmittedSigningKey(t *testing.T) {
+	t.Parallel()
+
+	const signingKey = "test-signing-key-0123456789abcdef"
+
+	var buf bytes.Buffer
+
+	mw := newTestMiddleware(t, &buf)
+
+	// The handler reads the key from the form, as the login handler does.
+	handler := mw.Logging()(http.HandlerFunc(
+		func(w http.ResponseWriter, r *http.Request) {
+			if got := r.FormValue("key"); got != signingKey {
+				t.Errorf("key in form = %q, want %q", got, signingKey)
+			}
+
+			w.WriteHeader(http.StatusSeeOther)
+		}))
+
+	form := url.Values{"key": {signingKey}}
+	req := httptest.NewRequestWithContext(t.Context(), http.MethodPost, "/",
+		strings.NewReader(form.Encode()))
+	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+
+	handler.ServeHTTP(httptest.NewRecorder(), req)
+
+	if !strings.Contains(buf.String(), `"method":"POST"`) {
+		t.Fatalf("no log line for the request; got %q", buf.String())
+	}
+
+	if strings.Contains(buf.String(), signingKey) {
+		t.Errorf("log output contains the signing key; got %q", buf.String())
+	}
+}
+
 func TestSecurityHeaders(t *testing.T) {
 	t.Parallel()
 

internal/server/metrics_internal_test.go

@@ -0,0 +1,32 @@
+package server
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"github.com/prometheus/client_golang/prometheus/promhttp"
+)
+
+// TestNoMetricsRecordedWithoutMetricsUsername checks that with no metrics
+// username set the router records nothing about the requests it serves.
+// /metrics is not served then, so the process-wide Prometheus registry is
+// read directly. TestMaintenanceModeKeepsOtherRoutes records into the same
+// registry, but never a GET /robots.txt.
+func TestNoMetricsRecordedWithoutMetricsUsername(t *testing.T) {
+	t.Parallel()
+
+	s := newTestServer(t)
+	s.ServeHTTP(httptest.NewRecorder(), httptest.NewRequestWithContext(
+		t.Context(), http.MethodGet, "/robots.txt", nil))
+
+	rec := httptest.NewRecorder()
+	promhttp.Handler().ServeHTTP(rec, httptest.NewRequestWithContext(
+		t.Context(), http.MethodGet, "/metrics", nil))
+
+	if strings.Contains(rec.Body.String(), `handler="/robots.txt"`) {
+		t.Errorf("with no metrics username GET /robots.txt was recorded:\n%s",
+			rec.Body.String())
+	}
+}