From d56bdba3ff35e8203131cfd2a43c0499e5d3e2ce Mon Sep 17 00:00:00 2001
From: fiatjaf <fiatjaf@gmail.com>
Date: Wed, 15 Apr 2026 21:19:03 -0300
Subject: [PATCH] khatru: WithServiceURL() subhandlers.

---
 khatru/handlers.go   |  4 ++--
 khatru/relay.go      | 19 ++++++++++++++--
 khatru/relay_test.go | 54 ++++++++++++++++++++++++++++++++++++++++++++
 khatru/utils.go      |  1 +
 4 files changed, 74 insertions(+), 4 deletions(-)

diff --git a/khatru/handlers.go b/khatru/handlers.go
index df093eb..e954413 100644
--- a/khatru/handlers.go
+++ b/khatru/handlers.go
@@ -43,8 +43,8 @@ func (rl *Relay) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	})
 
 	relayPathMatches := true
-	if rl.ServiceURL != "" {
-		p, err := url.Parse(rl.ServiceURL)
+	if serviceURL := rl.getServiceURL(r); serviceURL != "" {
+		p, err := url.Parse(serviceURL)
 		if err == nil {
 			relayPathMatches = strings.TrimSuffix(r.URL.Path, "/") == strings.TrimSuffix(p.Path, "/")
 		}
diff --git a/khatru/relay.go b/khatru/relay.go
index 23a337c..9e22b0b 100644
--- a/khatru/relay.go
+++ b/khatru/relay.go
@@ -166,8 +166,8 @@ func (rl *Relay) UseEventstore(store eventstore.Store, maxQueryLimit int) {
 }
 
 func (rl *Relay) getBaseURL(r *http.Request) string {
-	if rl.ServiceURL != "" {
-		return rl.ServiceURL
+	if serviceURL := rl.getServiceURL(r); serviceURL != "" {
+		return serviceURL
 	}
 
 	host := r.Header.Get("X-Forwarded-Host")
@@ -192,6 +192,14 @@ func (rl *Relay) getBaseURL(r *http.Request) string {
 	return proto + "://" + host + r.URL.Path
 }
 
+func (rl *Relay) getServiceURL(r *http.Request) string {
+	if serviceURL, ok := r.Context().Value(serviceURLOverrideKey).(string); ok {
+		return serviceURL
+	}
+
+	return rl.ServiceURL
+}
+
 // Stats returns the current number of connected clients and open listeners.
 func (rl *Relay) Stats() (clients, listeners int) {
 	rl.clientsMutex.Lock()
@@ -211,3 +219,10 @@ func (rl *Relay) Router() *http.ServeMux {
 func (rl *Relay) SetRouter(mux *http.ServeMux) {
 	rl.serveMux = mux
 }
+
+func (rl *Relay) WithServiceURL(serviceURL string) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		ctx := context.WithValue(r.Context(), serviceURLOverrideKey, serviceURL)
+		rl.ServeHTTP(w, r.WithContext(ctx))
+	})
+}
diff --git a/khatru/relay_test.go b/khatru/relay_test.go
index baceb4b..bcaed80 100644
--- a/khatru/relay_test.go
+++ b/khatru/relay_test.go
@@ -2,6 +2,9 @@ package khatru
 
 import (
 	"context"
+	"encoding/json"
+	"io"
+	"net/http"
 	"net/http/httptest"
 	"strconv"
 	"testing"
@@ -9,9 +12,60 @@ import (
 
 	"fiatjaf.com/nostr"
 	"fiatjaf.com/nostr/eventstore/slicestore"
+	"fiatjaf.com/nostr/nip11"
 	"github.com/stretchr/testify/require"
 )
 
+func TestWithServiceURL(t *testing.T) {
+	relay := NewRelay()
+	relay.Info.Icon = "icon.png"
+	relay.Info.Banner = "banner.png"
+	relay.Router().HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusAccepted)
+		io.WriteString(w, "fallback")
+	})
+
+	handlerA := relay.WithServiceURL("https://a.example/relay")
+	handlerB := relay.WithServiceURL("https://b.example/relay")
+
+	t.Run("uses override for nip11 base url", func(t *testing.T) {
+		for _, tc := range []struct {
+			name         string
+			handler      http.Handler
+			expectedBase string
+		}{
+			{name: "first interface", handler: handlerA, expectedBase: "https://a.example/relay"},
+			{name: "second interface", handler: handlerB, expectedBase: "https://b.example/relay"},
+		} {
+			t.Run(tc.name, func(t *testing.T) {
+				req := httptest.NewRequest(http.MethodGet, "http://internal/relay", nil)
+				req.Header.Set("Accept", "application/nostr+json")
+				rr := httptest.NewRecorder()
+
+				tc.handler.ServeHTTP(rr, req)
+
+				require.Equal(t, http.StatusOK, rr.Code)
+
+				var info nip11.RelayInformationDocument
+				require.NoError(t, json.NewDecoder(rr.Body).Decode(&info))
+				require.Equal(t, tc.expectedBase+"/icon.png", info.Icon)
+				require.Equal(t, tc.expectedBase+"/banner.png", info.Banner)
+			})
+		}
+	})
+
+	t.Run("uses override for relay path matching", func(t *testing.T) {
+		req := httptest.NewRequest(http.MethodGet, "http://internal/not-relay", nil)
+		req.Header.Set("Accept", "application/nostr+json")
+		rr := httptest.NewRecorder()
+
+		handlerA.ServeHTTP(rr, req)
+
+		require.Equal(t, http.StatusAccepted, rr.Code)
+		require.Equal(t, "fallback", rr.Body.String())
+	})
+}
+
 func TestBasicRelayFunctionality(t *testing.T) {
 	// setup relay with in-memory store
 	relay := NewRelay()
diff --git a/khatru/utils.go b/khatru/utils.go
index 800d0c0..b11f138 100644
--- a/khatru/utils.go
+++ b/khatru/utils.go
@@ -11,6 +11,7 @@ const (
 	subscriptionIdKey
 	nip86HeaderAuthKey
 	internalCallKey
+	serviceURLOverrideKey
 )
 
 func RequestAuth(ctx context.Context) {