Harden http.Server: slowloris timeouts and form body limit (closes #92)

The http.Server now sets ReadHeaderTimeout (10s), which bounds the slow header dribble that ReadTimeout alone does not, and IdleTimeout (120s), which bounds keep-alive reuse. Server construction moved into a small helper so a test can assert the timeouts without binding a listener.

POST / and POST /generate bodies are capped at 1 MiB and an oversized body returns 413.

What a reader would trip over: the CSRF library reads its token from the form and swallows a parse error, so a cap applied only inside it would surface as 403. The body limit therefore parses the form under the cap before the CSRF check; the parsed form is reused afterwards. A test covers an oversized body that carries a valid token.

Judgement call: WriteTimeout stays at 60s; it also bounds how long a large image may take to send over a slow link.

Model: opus-4-8 (implementation, review); fable-5-1 (landing message)

internal/server/http.go

@@ -9,24 +9,40 @@ import (
 
 // HTTP server configuration constants.
 const (
-	HTTPReadTimeout    = 30 * time.Second
-	HTTPWriteTimeout   = 60 * time.Second
+	HTTPReadTimeout = 30 * time.Second
+	// HTTPReadHeaderTimeout bounds the request-header read on its own,
+	// short, so a slowloris client dribbling headers is dropped well
+	// before it ties up a connection for the whole ReadTimeout window.
+	HTTPReadHeaderTimeout = 10 * time.Second
+	HTTPWriteTimeout      = 60 * time.Second
+	// HTTPIdleTimeout bounds how long an idle keep-alive connection is
+	// held open, so idle connections cannot accumulate without limit on a
+	// service targeting high concurrency.
+	HTTPIdleTimeout    = 120 * time.Second
 	HTTPMaxHeaderBytes = 8 << 10 // 8KB
 )
 
-func (s *Server) serveUntilShutdown() {
-	listenAddr := fmt.Sprintf(":%d", s.config.Port)
-	s.httpServer = &http.Server{
-		Addr:           listenAddr,
-		ReadTimeout:    HTTPReadTimeout,
-		WriteTimeout:   HTTPWriteTimeout,
-		MaxHeaderBytes: HTTPMaxHeaderBytes,
-		Handler:        s,
+// newHTTPServer builds the http.Server with the hardening timeouts and
+// limits applied. It is separate from serveUntilShutdown so the
+// configuration can be asserted in a test without binding a listener.
+func (s *Server) newHTTPServer() *http.Server {
+	return &http.Server{
+		Addr:              fmt.Sprintf(":%d", s.config.Port),
+		ReadTimeout:       HTTPReadTimeout,
+		ReadHeaderTimeout: HTTPReadHeaderTimeout,
+		WriteTimeout:      HTTPWriteTimeout,
+		IdleTimeout:       HTTPIdleTimeout,
+		MaxHeaderBytes:    HTTPMaxHeaderBytes,
+		Handler:           s,
 	}
+}
+
+func (s *Server) serveUntilShutdown() {
+	s.httpServer = s.newHTTPServer()
 
 	s.SetupRoutes()
 
-	s.log.Info("http begin listen", "listenaddr", listenAddr)
+	s.log.Info("http begin listen", "listenaddr", s.httpServer.Addr)
 
 	err := s.httpServer.ListenAndServe()
 	if err != nil && !errors.Is(err, http.ErrServerClosed) {

internal/server/http_internal_test.go

@@ -0,0 +1,65 @@
+package server
+
+import (
+	"testing"
+	"time"
+
+	"sneak.berlin/go/pixa/internal/config"
+)
+
+// TestNewHTTPServerTimeouts verifies that the constructed http.Server
+// carries every hardening timeout wired onto it, including the slowloris
+// defense (ReadHeaderTimeout) and the keep-alive bound (IdleTimeout). This
+// guards against a field being defined but never set on the server, so
+// each assertion compares the server field to its constant.
+func TestNewHTTPServerTimeouts(t *testing.T) {
+	t.Parallel()
+
+	s := &Server{config: &config.Config{Port: 8080}}
+
+	srv := s.newHTTPServer()
+
+	fields := []struct {
+		name string
+		got  time.Duration
+		want time.Duration
+	}{
+		{"ReadTimeout", srv.ReadTimeout, HTTPReadTimeout},
+		{"ReadHeaderTimeout", srv.ReadHeaderTimeout, HTTPReadHeaderTimeout},
+		{"WriteTimeout", srv.WriteTimeout, HTTPWriteTimeout},
+		{"IdleTimeout", srv.IdleTimeout, HTTPIdleTimeout},
+	}
+
+	for _, f := range fields {
+		if f.got != f.want {
+			t.Errorf("%s = %v, want %v", f.name, f.got, f.want)
+		}
+	}
+
+	if srv.MaxHeaderBytes != HTTPMaxHeaderBytes {
+		t.Errorf("MaxHeaderBytes = %d, want %d",
+			srv.MaxHeaderBytes, HTTPMaxHeaderBytes)
+	}
+
+	if srv.Handler != s {
+		t.Error("Handler is not the server")
+	}
+}
+
+// TestHardeningTimeoutValues pins the intent behind the two new timeouts
+// without hard-coding brittle exact durations: the header-read phase is
+// bounded strictly shorter than the whole-request read (the slowloris
+// dribble), and idle keep-alive connections are bounded rather than held
+// open forever.
+func TestHardeningTimeoutValues(t *testing.T) {
+	t.Parallel()
+
+	if HTTPReadHeaderTimeout <= 0 || HTTPReadHeaderTimeout > HTTPReadTimeout {
+		t.Errorf("ReadHeaderTimeout = %v, want positive and <= ReadTimeout %v",
+			HTTPReadHeaderTimeout, HTTPReadTimeout)
+	}
+
+	if HTTPIdleTimeout <= 0 {
+		t.Errorf("IdleTimeout = %v, want positive bound", HTTPIdleTimeout)
+	}
+}

internal/server/routes.go

@@ -8,6 +8,7 @@ import (
 	"github.com/go-chi/chi/v5/middleware"
 	"github.com/prometheus/client_golang/prometheus/promhttp"
 
+	"sneak.berlin/go/pixa/internal/handlers"
 	"sneak.berlin/go/pixa/internal/static"
 )
 
@@ -46,8 +47,10 @@ func (s *Server) SetupRoutes() {
 
 	// Login/generator UI. The form routes carry CSRF protection; the
 	// token cookie is independent of the session cookie, so it also
-	// covers the login POST, where no session exists yet.
+	// covers the login POST, where no session exists yet. LimitBody caps
+	// the POST body ahead of CSRF, which reads its token from that body.
 	s.router.Group(func(r chi.Router) {
+		r.Use(s.h.LimitBody(handlers.MaxFormBytes))
 		r.Use(s.h.CSRF())
 		r.Get("/", s.h.HandleRoot())
 		r.Post("/", s.h.HandleRoot())