Add a read-only Settings page for the loaded configuration (closes #402)

The page at /settings, behind the login and linked from the
navigation bar, lists every field of the configuration the server
started with: each by its environment variable name, with the
README's description and the value in effect. METRICS_PASSWORD and
SENTRY_DSN show only as set or not set; their values never reach the
template. The handlers now take the loaded Config from the dependency
graph.

Model: opus-5-5

README.md

@@ -145,6 +145,11 @@ TTY detection, and security headers are always applied.
 | `TRUSTED_PROXIES`       | CIDRs whose forwarded headers are trusted. A set value replaces the default. If any client can reach webhooker, or the proxy in front of it, from an RFC 1918 source address, set it to the proxy's address alone. See [Trusted proxies](#trusted-proxies) | `10.0.0.0/8,172.16.0.0/12,192.168.0.0/16` (RFC 1918) |
 | `ALLOWED_EGRESS_CIDRS`  | CIDRs that delivery targets may reach despite the SSRF blocklist. Read [Allowing egress to your own network](#allowing-egress-to-your-own-network) before setting it | `""` (none) |
 
+The Settings page of the web UI (`/settings`, behind the login) lists
+every one of these with the value the running server loaded. It is
+read-only, and it shows `METRICS_PASSWORD` and `SENTRY_DSN` only as
+set or not set, never their values.
+
 #### Allowing egress to your own network
 
 By default every delivery target must resolve to a public address. The
@@ -2713,6 +2718,7 @@ abuse limit later; they are tracked as future work.
 | ------ | ------------------------ | ----------- |
 | `GET`  | `/user/{username}`       | User profile page |
 | `POST` | `/user/{username}/password` | Change the user's password (5 per minute per bucket, then `429`; `503` if no verification slot frees up within 5s, or immediately if 16 requests are already queued for one) |
+| `GET`  | `/settings`              | Read-only list of the configuration the server is running with; `METRICS_PASSWORD` and `SENTRY_DSN` show only as set or not set |
 | `GET`  | `/sources`               | List user's webhooks |
 | `GET`  | `/sources/new`           | Create webhook form |
 | `POST` | `/sources/new`           | Create webhook submission |
@@ -2825,6 +2831,7 @@ webhooker/
 │   │   ├── healthcheck.go     # Health check handler
 │   │   ├── index.go           # Index page handler
 │   │   ├── profile.go         # User profile handler
+│   │   ├── settings.go        # Read-only Settings page handler
 │   │   ├── source_management.go  # Webhook CRUD handlers
 │   │   └── webhook.go         # Webhook receiver handler
 │   ├── healthcheck/

internal/handlers/handlers.go

@@ -13,6 +13,7 @@ import (
 	"sync/atomic"
 
 	"go.uber.org/fx"
+	"sneak.berlin/go/webhooker/internal/config"
 	"sneak.berlin/go/webhooker/internal/database"
 	"sneak.berlin/go/webhooker/internal/delivery"
 	"sneak.berlin/go/webhooker/internal/globals"
@@ -53,6 +54,7 @@ type HandlersParams struct {
 
 	Logger       *logger.Logger
 	Globals      *globals.Globals
+	Config       *config.Config
 	Database     *database.Database
 	WebhookDBMgr *database.WebhookDBManager
 	Healthcheck  *healthcheck.Healthcheck
@@ -129,6 +131,7 @@ func New(
 	s.templates = map[string]*template.Template{
 		"login.html":         parsePageTemplate("login.html"),
 		"profile.html":       parsePageTemplate("profile.html"),
+		"settings.html":      parsePageTemplate("settings.html"),
 		"sources_list.html":  parsePageTemplate("sources_list.html"),
 		"sources_new.html":   parsePageTemplate("sources_new.html"),
 		"source_detail.html": parsePageTemplate("source_detail.html"),

internal/handlers/handlers_test.go

@@ -83,16 +83,25 @@ func newTestApp(
 ) *fxtest.App {
 	t.Helper()
 
+	return newTestAppWithConfig(
+		t, &config.Config{DataDir: t.TempDir()}, targets...,
+	)
+}
+
+// newTestAppWithConfig is newTestApp over a caller-supplied Config.
+func newTestAppWithConfig(
+	t *testing.T,
+	cfg *config.Config,
+	targets ...any,
+) *fxtest.App {
+	t.Helper()
+
 	return fxtest.New(
 		t,
 		fx.Provide(
 			globals.New,
 			logger.New,
-			func() *config.Config {
-				return &config.Config{
-					DataDir: t.TempDir(),
-				}
-			},
+			func() *config.Config { return cfg },
 			database.New,
 			database.NewWebhookDBManager,
 			healthcheck.New,

internal/handlers/settings.go

@@ -0,0 +1,130 @@
+package handlers
+
+import (
+	"net/http"
+	"net/netip"
+	"strconv"
+	"strings"
+
+	"sneak.berlin/go/webhooker/internal/config"
+)
+
+// notSet is what the Settings page shows for a value that is empty.
+const notSet = "not set"
+
+// settingRow is one line of the Settings page: an environment
+// variable, what it controls, and the value the server loaded for it.
+type settingRow struct {
+	Name        string
+	Description string
+	Value       string
+}
+
+// HandleSettings returns a handler for the read-only Settings page,
+// which lists the configuration the server started with.
+func (h *Handlers) HandleSettings() http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		h.renderTemplate(w, r, "settings.html", map[string]any{
+			"Settings": settingRows(h.params.Config),
+		})
+	}
+}
+
+// settingRows lists every field of cfg under the environment variable
+// it is read from, in the order of the README's configuration table.
+// METRICS_PASSWORD and SENTRY_DSN are credentials, so their values
+// never reach the page: only whether they are set.
+func settingRows(cfg *config.Config) []settingRow {
+	metricsUsername := cfg.MetricsUsername
+	if metricsUsername == "" {
+		metricsUsername = notSet
+	}
+
+	return []settingRow{
+		{"WEBHOOKER_ENVIRONMENT", "dev or prod", cfg.Environment},
+		{"PORT", "HTTP listen port", strconv.Itoa(cfg.Port)},
+		{
+			"BIND_ADDRESS",
+			"IP address the HTTP listener binds",
+			cfg.BindAddress,
+		},
+		{
+			"DATA_DIR",
+			"Directory for all SQLite databases",
+			cfg.DataDir,
+		},
+		{
+			"DEBUG",
+			"Enable debug logging",
+			strconv.FormatBool(cfg.Debug),
+		},
+		{
+			"MAINTENANCE_MODE",
+			"Report maintenanceMode: true in the healthcheck JSON. " +
+				"It does not change how any request is served",
+			strconv.FormatBool(cfg.MaintenanceMode),
+		},
+		{
+			"METRICS_USERNAME",
+			"Basic auth username for /metrics",
+			metricsUsername,
+		},
+		{
+			"METRICS_PASSWORD",
+			"Basic auth password for /metrics",
+			setOrNotSet(cfg.MetricsPassword),
+		},
+		{
+			"SENTRY_DSN",
+			"Error reporting DSN. Unset leaves error reporting off",
+			setOrNotSet(cfg.SentryDSN),
+		},
+		{
+			"RETENTION_SWEEP_INTERVAL",
+			"How often the retention reaper and archive sweeper run",
+			cfg.RetentionSweepInterval.String(),
+		},
+		{
+			"SESSION_IDLE_TIMEOUT",
+			"Idle session timeout. Zero or negative disables idle " +
+				"expiry",
+			cfg.SessionIdleTimeout.String(),
+		},
+		{
+			"RECEIVER_RATE_LIMIT",
+			"Receiver requests per minute per IP per entrypoint " +
+				"(10x that per IP across the route)",
+			strconv.Itoa(cfg.ReceiverRateLimit),
+		},
+		{
+			"TRUSTED_PROXIES",
+			"CIDRs whose forwarded headers are trusted",
+			cidrList(cfg.TrustedProxies),
+		},
+		{
+			"ALLOWED_EGRESS_CIDRS",
+			"CIDRs that delivery targets may reach despite the " +
+				"SSRF blocklist",
+			cidrList(cfg.AllowedEgressCIDRs),
+		},
+	}
+}
+
+// setOrNotSet is how the Settings page shows a credential: whether it
+// has a value, never the value itself.
+func setOrNotSet(value string) string {
+	if value == "" {
+		return notSet
+	}
+
+	return "set"
+}
+
+// cidrList renders a CIDR list setting for the Settings page.
+func cidrList(prefixes []netip.Prefix) string {
+	if len(prefixes) == 0 {
+		return "none"
+	}
+
+	return strings.Join(config.PrefixStrings(prefixes), ", ")
+}

internal/handlers/settings_test.go

@@ -0,0 +1,139 @@
+package handlers_test
+
+import (
+	"context"
+	"html"
+	"net/http"
+	"net/http/httptest"
+	"net/netip"
+	"regexp"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"sneak.berlin/go/webhooker/internal/config"
+	"sneak.berlin/go/webhooker/internal/handlers"
+	"sneak.berlin/go/webhooker/internal/session"
+)
+
+// settingsShown renders the Settings page over cfg as a logged-in user
+// and returns the value it shows for each variable name, plus the
+// whole page.
+func settingsShown(
+	t *testing.T, cfg *config.Config,
+) (map[string]string, string) {
+	t.Helper()
+
+	var h *handlers.Handlers
+
+	var sess *session.Session
+
+	app := newTestAppWithConfig(t, cfg, &h, &sess)
+	app.RequireStart()
+
+	t.Cleanup(app.RequireStop)
+
+	req := httptest.NewRequestWithContext(
+		context.Background(), http.MethodGet, "/settings", nil,
+	)
+	for _, c := range authenticatedCookies(t, sess, "id", "admin") {
+		req.AddCookie(c)
+	}
+
+	w := httptest.NewRecorder()
+	h.HandleSettings().ServeHTTP(w, req)
+	require.Equal(t, http.StatusOK, w.Code)
+
+	body := w.Body.String()
+
+	row := regexp.MustCompile(
+		`<code[^>]*>([A-Z_]+)</code>\s*<code[^>]*>([^<]*)</code>`,
+	)
+
+	shown := map[string]string{}
+	for _, match := range row.FindAllStringSubmatch(body, -1) {
+		shown[match[1]] = html.UnescapeString(match[2])
+	}
+
+	return shown, body
+}
+
+func TestSettingsPageShowsLoadedConfiguration(t *testing.T) {
+	t.Parallel()
+
+	const metricsPassword = "metrics-password-1f9a"
+
+	// No two rows show the same value: DEBUG and MAINTENANCE_MODE, and
+	// METRICS_PASSWORD and SENTRY_DSN, get opposite values, so each row
+	// is checked against its own field.
+	cfg := &config.Config{
+		DataDir:                t.TempDir(),
+		Debug:                  true,
+		MaintenanceMode:        false,
+		Environment:            config.EnvironmentDev,
+		MetricsUsername:        "scraper",
+		MetricsPassword:        metricsPassword,
+		Port:                   9123,
+		SentryDSN:              "",
+		BindAddress:            "192.0.2.10",
+		RetentionSweepInterval: 17 * time.Minute,
+		SessionIdleTimeout:     3 * time.Hour,
+		ReceiverRateLimit:      77,
+		TrustedProxies: []netip.Prefix{
+			netip.MustParsePrefix("10.1.0.0/16"),
+		},
+		AllowedEgressCIDRs: []netip.Prefix{
+			netip.MustParsePrefix("192.168.5.0/24"),
+			netip.MustParsePrefix("fd00::/8"),
+		},
+	}
+
+	shown, body := settingsShown(t, cfg)
+
+	assert.Equal(t, map[string]string{
+		"WEBHOOKER_ENVIRONMENT":    "dev",
+		"PORT":                     "9123",
+		"BIND_ADDRESS":             "192.0.2.10",
+		"DATA_DIR":                 cfg.DataDir,
+		"DEBUG":                    "true",
+		"MAINTENANCE_MODE":         "false",
+		"METRICS_USERNAME":         "scraper",
+		"METRICS_PASSWORD":         "set",
+		"SENTRY_DSN":               "not set",
+		"RETENTION_SWEEP_INTERVAL": "17m0s",
+		"SESSION_IDLE_TIMEOUT":     "3h0m0s",
+		"RECEIVER_RATE_LIMIT":      "77",
+		"TRUSTED_PROXIES":          "10.1.0.0/16",
+		"ALLOWED_EGRESS_CIDRS":     "192.168.5.0/24, fd00::/8",
+	}, shown)
+
+	assert.NotContains(t, body, metricsPassword)
+	assert.Contains(
+		t, body, `href="/settings"`,
+		"the navigation bar links to the page",
+	)
+}
+
+func TestSettingsPageShowsUnsetValues(t *testing.T) {
+	t.Parallel()
+
+	const (
+		sentryKey = "dsnkey7c2e"
+		sentryDSN = "https://" + sentryKey + "@errors.example.com/42"
+	)
+
+	// SENTRY_DSN is set here and empty in the test above, the opposite
+	// of METRICS_PASSWORD, so each secret is seen both set and not set.
+	shown, body := settingsShown(t, &config.Config{
+		DataDir:   t.TempDir(),
+		SentryDSN: sentryDSN,
+	})
+
+	assert.Equal(t, "not set", shown["METRICS_USERNAME"])
+	assert.Equal(t, "not set", shown["METRICS_PASSWORD"])
+	assert.Equal(t, "set", shown["SENTRY_DSN"])
+	assert.NotContains(t, body, sentryKey)
+	assert.Equal(t, "none", shown["TRUSTED_PROXIES"])
+	assert.Equal(t, "none", shown["ALLOWED_EGRESS_CIDRS"])
+}

internal/server/routes.go

@@ -141,6 +141,7 @@ func (s *Server) setupRoutes() {
 
 	s.setupPageRoutes()
 	s.setupUserRoutes()
+	s.setupSettingsRoutes()
 	s.setupSourceRoutes()
 	s.setupWebhookRoutes()
 }
@@ -182,6 +183,21 @@ func (s *Server) setupUserRoutes() {
 	})
 }
 
+// setupSettingsRoutes serves the Settings page. It is GET only:
+// configuration comes from the environment and nothing here changes
+// it.
+func (s *Server) setupSettingsRoutes() {
+	s.router.Route("/settings", func(r chi.Router) {
+		// MaxBodySize precedes CSRF and RequireAuth deliberately;
+		// see maxFormBodySize for why, and for what it costs.
+		r.Use(s.mw.MaxBodySize(maxFormBodySize))
+		r.Use(s.mw.CSRF())
+		r.Use(s.mw.NoCache())
+		r.Use(s.mw.RequireAuth())
+		r.Get("/", s.h.HandleSettings())
+	})
+}
+
 func (s *Server) setupSourceRoutes() {
 	s.router.Route("/sources", func(r chi.Router) {
 		// MaxBodySize precedes CSRF and RequireAuth deliberately;

internal/server/settings_test.go

@@ -0,0 +1,22 @@
+package server_test
+
+import (
+	"net/http"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestSettingsPageIsBehindLogin(t *testing.T) {
+	t.Parallel()
+
+	env := newTestEnv(t)
+
+	w := env.get("/settings", nil)
+	assert.Equal(t, http.StatusSeeOther, w.Code)
+	assert.Equal(t, "/pages/login", w.Header().Get("Location"))
+
+	w = env.get("/settings", env.authCookies(t, "id", "admin"))
+	assert.Equal(t, http.StatusOK, w.Code)
+	assert.Contains(t, w.Body.String(), "WEBHOOKER_ENVIRONMENT")
+}

templates/navbar.html

@@ -17,6 +17,7 @@
         <div class="hidden md:flex items-center gap-4">
             {{if .User}}
             <a href="/sources" class="btn-text">Webhooks</a>
+            <a href="/settings" class="btn-text">Settings</a>
             <a href="/user/{{.User.Username}}" class="btn-text">
                 <svg class="w-5 h-5 mr-1" fill="currentColor" viewBox="0 0 16 16">
                     <path d="M11 6a3 3 0 1 1-6 0 3 3 0 0 1 6 0z"/>
@@ -39,6 +40,7 @@
         <div class="flex flex-col gap-2">
             {{if .User}}
             <a href="/sources" class="btn-text w-full text-left">Webhooks</a>
+            <a href="/settings" class="btn-text w-full text-left">Settings</a>
             <a href="/user/{{.User.Username}}" class="btn-text w-full text-left">Profile</a>
             <form method="POST" action="/pages/logout">
                 <input type="hidden" name="csrf_token" value="{{.CSRFToken}}">

templates/settings.html

@@ -0,0 +1,24 @@
+{{template "base" .}}
+
+{{define "title"}}Settings - Webhooker{{end}}
+
+{{define "content"}}
+<div class="max-w-6xl mx-auto px-6 py-8">
+    <h1 class="text-2xl font-medium text-gray-900">Settings</h1>
+    <p class="text-sm text-gray-500 mt-1 mb-6">The configuration this server started with. It is set in the server's environment and cannot be changed here.</p>
+
+    <div class="card">
+        <div class="divide-y divide-gray-100">
+            {{range .Settings}}
+            <div class="p-4">
+                <div class="flex justify-between items-start gap-4">
+                    <code class="text-sm font-medium text-gray-900">{{.Name}}</code>
+                    <code class="text-sm text-gray-900 break-all">{{.Value}}</code>
+                </div>
+                <p class="text-sm text-gray-500 mt-1">{{.Description}}</p>
+            </div>
+            {{end}}
+        </div>
+    </div>
+</div>
+{{end}}