route-proxy: the challenge path falls through for real

autocert's HTTPHandler answers 404 itself for a token it does not hold
and never consults its fallback on the challenge path — the
predecessor's exact fault, rediscovered live when Mailu's renewal died
behind this proxy on cutover day. tokenOrRoute probes each authority
against a buffered writer and hands a token none of them holds to plain
routing, so a consumer's own ACME client answers its own challenge
through an ordinary path-scoped route. Four tests pin it, including the
cache-key shape a restart-surviving token actually has.

examples/route-proxy/challenge_test.go

@@ -0,0 +1,86 @@
+package main
+
+// The challenge path falls through for real. autocert's own HTTPHandler answers 404 itself for a
+// token it does not hold and never consults its fallback on the challenge path — the
+// predecessor's fault, the edge owning /.well-known/acme-challenge outright, rediscovered live
+// when Mailu's renewal died behind this proxy on cutover day (2026-09-26). These tests pin the
+// three behaviours tokenOrRoute exists for.
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"golang.org/x/crypto/acme/autocert"
+)
+
+func routedTo(t *testing.T, marker string) http.Handler {
+	t.Helper()
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+		if _, err := w.Write([]byte(marker)); err != nil {
+			t.Fatal(err)
+		}
+	})
+}
+
+func TestATokenNoAuthorityHoldsIsRoutedNot404d(t *testing.T) {
+	m := &autocert.Manager{Prompt: autocert.AcceptTOS, Cache: autocert.DirCache(t.TempDir())}
+	h := tokenOrRoute(routedTo(t, "the workload answered"), m)
+
+	rec := httptest.NewRecorder()
+	h.ServeHTTP(rec, httptest.NewRequest("GET", "http://mail.example/.well-known/acme-challenge/somebody-elses-token", nil))
+
+	if rec.Code != http.StatusOK || rec.Body.String() != "the workload answered" {
+		t.Fatalf("a token no authority holds must reach plain routing; got %d %q", rec.Code, rec.Body.String())
+	}
+}
+
+func TestATokenAManagerHoldsIsAnsweredByIt(t *testing.T) {
+	// autocert reads a token it does not have in memory from its cache, under "<token>+http-01" —
+	// which is also how a token would survive the manager restarting mid-issuance.
+	dir := t.TempDir()
+	if err := os.WriteFile(filepath.Join(dir, "held-token+http-01"), []byte("the-key-authorization"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	m := &autocert.Manager{Prompt: autocert.AcceptTOS, Cache: autocert.DirCache(dir)}
+	h := tokenOrRoute(routedTo(t, "must not be reached"), m)
+
+	rec := httptest.NewRecorder()
+	h.ServeHTTP(rec, httptest.NewRequest("GET", "http://mail.example/.well-known/acme-challenge/held-token", nil))
+
+	if rec.Code != http.StatusOK || rec.Body.String() != "the-key-authorization" {
+		t.Fatalf("the manager holding a token answers it; got %d %q", rec.Code, rec.Body.String())
+	}
+}
+
+func TestASecondAuthorityIsProbedBeforeRouting(t *testing.T) {
+	first := &autocert.Manager{Prompt: autocert.AcceptTOS, Cache: autocert.DirCache(t.TempDir())}
+	dir := t.TempDir()
+	if err := os.WriteFile(filepath.Join(dir, "internal-token+http-01"), []byte("internal-key"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	second := &autocert.Manager{Prompt: autocert.AcceptTOS, Cache: autocert.DirCache(dir)}
+	h := tokenOrRoute(routedTo(t, "must not be reached"), first, second)
+
+	rec := httptest.NewRecorder()
+	h.ServeHTTP(rec, httptest.NewRequest("GET", "http://git.internal/.well-known/acme-challenge/internal-token", nil))
+
+	if rec.Code != http.StatusOK || rec.Body.String() != "internal-key" {
+		t.Fatalf("the second authority's token is found by probing past the first; got %d %q", rec.Code, rec.Body.String())
+	}
+}
+
+func TestAnOrdinaryPathNeverTouchesTheChallengeMachinery(t *testing.T) {
+	m := &autocert.Manager{Prompt: autocert.AcceptTOS, Cache: autocert.DirCache(t.TempDir())}
+	h := tokenOrRoute(routedTo(t, "routed"), m)
+
+	rec := httptest.NewRecorder()
+	h.ServeHTTP(rec, httptest.NewRequest("GET", "http://site.example/index.html", nil))
+
+	if rec.Code != http.StatusOK || rec.Body.String() != "routed" {
+		t.Fatalf("an ordinary path goes straight to routing; got %d %q", rec.Code, rec.Body.String())
+	}
+}