Fix pprof endpoints not working with --web.route-prefix

When using --web.route-prefix or --web.external-url with a path component,
the pprof debug endpoints were not accessible. The handlers were directly
passing requests to http.DefaultServeMux without adjusting the URL path
to match what the pprof handlers expect.

For example, with --web.route-prefix=/prometheus/alertmanager, a request
to /prometheus/alertmanager/debug/pprof/ was being forwarded with the full
path intact, but http.DefaultServeMux only has handlers registered for
paths starting with /debug/pprof/.

This fix reconstructs the request path by extracting the subpath parameter
from the route and prepending /debug to it, ensuring the path matches what
http.DefaultServeMux expects. The fix also preserves trailing slashes which
are required by some pprof handlers.

Added unit tests to verify pprof endpoints work correctly both with and
without route prefix configuration.

Co-authored-by: Claude <[email protected]>
Signed-off-by: Will Hegedus <[email protected]>

Author: wbh1

ui/web.go

@@ -19,6 +19,7 @@ import (
 	"net/http"
 	_ "net/http/pprof" // Comment this line to disable pprof endpoint.
 	"path"
+	"strings"
 
 	"github.com/prometheus/client_golang/prometheus/promhttp"
 	"github.com/prometheus/common/route"
@@ -87,8 +88,17 @@ func Register(r *route.Router, reloadCh chan<- chan error, logger *slog.Logger)
 		w.WriteHeader(http.StatusOK)
 	})
 
-	r.Get("/debug/*subpath", http.DefaultServeMux.ServeHTTP)
-	r.Post("/debug/*subpath", http.DefaultServeMux.ServeHTTP)
+	debugHandlerFunc := func(w http.ResponseWriter, req *http.Request) {
+		subpath := route.Param(req.Context(), "subpath")
+		req.URL.Path = path.Join("/debug", subpath)
+		// path.Join removes trailing slashes, but some pprof handlers expect them.
+		if strings.HasSuffix(subpath, "/") && !strings.HasSuffix(req.URL.Path, "/") {
+			req.URL.Path += "/"
+		}
+		http.DefaultServeMux.ServeHTTP(w, req)
+	}
+	r.Get("/debug/*subpath", debugHandlerFunc)
+	r.Post("/debug/*subpath", debugHandlerFunc)
 }
 
 func disableCaching(w http.ResponseWriter) {

ui/web_test.go

@@ -0,0 +1,67 @@
+// Copyright 2015 Prometheus Team
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package ui
+
+import (
+	"log/slog"
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"testing"
+
+	"github.com/prometheus/common/route"
+)
+
+func TestDebugHandlersWithRoutePrefix(t *testing.T) {
+	logger := slog.New(slog.NewTextHandler(os.Stderr, nil))
+	reloadCh := make(chan chan error)
+
+	// Test with route prefix
+	routePrefix := "/prometheus/alertmanager"
+	router := route.New().WithPrefix(routePrefix)
+	Register(router, reloadCh, logger)
+
+	// Test GET request to pprof index (note: pprof index returns text/html)
+	req := httptest.NewRequest("GET", routePrefix+"/debug/pprof/", nil)
+	w := httptest.NewRecorder()
+	router.ServeHTTP(w, req)
+
+	if w.Code != http.StatusOK {
+		t.Errorf("Expected status %d for %s, got %d. Body: %s", http.StatusOK, req.URL.Path, w.Code, w.Body.String())
+	}
+	if w.Body.Len() == 0 {
+		t.Error("Expected non-empty response body for pprof index")
+	}
+
+	// Test GET request to pprof heap endpoint
+	req = httptest.NewRequest("GET", routePrefix+"/debug/pprof/heap", nil)
+	w = httptest.NewRecorder()
+	router.ServeHTTP(w, req)
+
+	if w.Code != http.StatusOK {
+		t.Errorf("Expected status %d for %s, got %d", http.StatusOK, req.URL.Path, w.Code)
+	}
+
+	// Test without route prefix (should also work)
+	router2 := route.New()
+	Register(router2, reloadCh, logger)
+
+	req = httptest.NewRequest("GET", "/debug/pprof/", nil)
+	w = httptest.NewRecorder()
+	router2.ServeHTTP(w, req)
+
+	if w.Code != http.StatusOK {
+		t.Errorf("Expected status %d for %s without prefix, got %d", http.StatusOK, req.URL.Path, w.Code)
+	}
+}