vpp-maglev/cmd/frontend/handlers.go

// Copyright (c) 2026, Pim van Pelt <pim@ipng.ch>

package main

import (
	"context"
	"crypto/subtle"
	"encoding/json"
	"fmt"
	"io/fs"
	"log/slog"
	"net/http"
	"strings"
	"time"

	buildinfo "git.ipng.ch/ipng/vpp-maglev/cmd"
)

// adminCreds holds the basic-auth credentials for the /admin/ surface.
// Enabled is true when both the user and password env vars were set
// and non-empty at startup; when false, /admin/ is hidden entirely
// (returns 404) so operators who never intended to expose it don't
// see a teasing "unauthorized" response.
type adminCreds struct {
	User     string
	Password string
	Enabled  bool
}

func registerHandlers(mux *http.ServeMux, clients []*maglevClient, broker *Broker, admin adminCreds) {
	byName := make(map[string]*maglevClient, len(clients))
	for _, c := range clients {
		byName[c.name] = c
	}

	mux.HandleFunc("/healthz", func(w http.ResponseWriter, _ *http.Request) {
		_, _ = w.Write([]byte("ok\n"))
	})

	mux.HandleFunc("/view/api/version", func(w http.ResponseWriter, _ *http.Request) {
		writeJSON(w, VersionInfo{
			Version:      buildinfo.Version(),
			Commit:       buildinfo.Commit(),
			Date:         buildinfo.Date(),
			AdminEnabled: admin.Enabled,
		})
	})

	mux.HandleFunc("/view/api/maglevds", func(w http.ResponseWriter, _ *http.Request) {
		infos := make([]MaglevdInfo, 0, len(clients))
		for _, c := range clients {
			infos = append(infos, c.Info())
		}
		writeJSON(w, infos)
	})

	mux.HandleFunc("/view/api/state", func(w http.ResponseWriter, _ *http.Request) {
		out := make([]*StateSnapshot, 0, len(clients))
		for _, c := range clients {
			out = append(out, c.Snapshot())
		}
		writeJSON(w, out)
	})

	mux.HandleFunc("/view/api/state/", func(w http.ResponseWriter, r *http.Request) {
		name := strings.TrimPrefix(r.URL.Path, "/view/api/state/")
		c, ok := byName[name]
		if !ok {
			http.NotFound(w, r)
			return
		}
		writeJSON(w, c.Snapshot())
	})

	mux.HandleFunc("/view/api/events", func(w http.ResponseWriter, r *http.Request) {
		serveSSE(w, r, broker)
	})

	// Static SPA served from the embedded dist fs, mounted under /view/.
	staticFS, err := fs.Sub(webFS, "web/dist")
	if err != nil {
		slog.Error("embed-subfs", "err", err)
		return
	}
	fileServer := http.FileServer(http.FS(staticFS))
	mux.Handle("/view/", http.StripPrefix("/view/", fileServer))

	// /admin/ serves the same SPA shell behind basic auth when the
	// credentials are configured. Only the index.html is served here —
	// all JS, CSS, and assets are referenced via absolute /view/assets/
	// URLs baked in by Vite, so they continue to load from the
	// unauthenticated /view/ tree. Read-only API calls also go to
	// /view/api/* unchanged. Mutation endpoints live under /admin/api/
	// so the same basic-auth middleware covers every writing path.
	if admin.Enabled {
		indexBytes, ierr := fs.ReadFile(staticFS, "index.html")
		if ierr != nil {
			slog.Error("embed-index", "err", ierr)
			return
		}
		serveIndex := http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
			w.Header().Set("Content-Type", "text/html; charset=utf-8")
			w.Header().Set("Cache-Control", "no-store")
			_, _ = w.Write(indexBytes)
		})
		adminAPI := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
			handleAdminAPI(w, r, byName)
		})
		realm := "maglevd-frontend admin"
		// Register /admin/api/ before /admin/ so the more specific
		// pattern wins in net/http's ServeMux.
		mux.Handle("/admin/api/", basicAuth(realm, admin.User, admin.Password, adminAPI))
		mux.Handle("/admin/", basicAuth(realm, admin.User, admin.Password, serveIndex))
	}

	mux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
		if r.URL.Path == "/" {
			http.Redirect(w, r, "/view/", http.StatusFound)
			return
		}
		http.NotFound(w, r)
	})
}

// handleAdminAPI dispatches mutation requests under /admin/api/.
//
// Supported shapes:
//
//	POST /admin/api/{maglevd}/backend/{name}/{pause|resume|enable|disable}
//	  → fresh BackendSnapshot as JSON
//
//	POST /admin/api/{maglevd}/frontend/{fe}/pool/{pool}/backend/{name}/weight
//	  body: {"weight": 0-100, "flush": bool}
//	  → fresh FrontendSnapshot as JSON
//
// The WatchEvents stream also delivers a backend-transition (and, for
// the weight case, no event — since the config mutation doesn't flip
// the health state). The POST response is primarily for the
// originating SPA to learn about failures and to refresh effective
// weights immediately. Errors from the gRPC side are surfaced as
// 400 (bad request) or 502 (maglevd returned an error).
func handleAdminAPI(w http.ResponseWriter, r *http.Request, byName map[string]*maglevClient) {
	if r.Method != http.MethodPost {
		w.Header().Set("Allow", "POST")
		http.Error(w, "method not allowed", http.StatusMethodNotAllowed)
		return
	}
	parts := strings.Split(strings.TrimPrefix(r.URL.Path, "/admin/api/"), "/")
	// Peel off the maglevd name (always the first segment).
	if len(parts) < 2 {
		http.NotFound(w, r)
		return
	}
	maglevd := parts[0]
	c, ok := byName[maglevd]
	if !ok {
		http.NotFound(w, r)
		return
	}
	rest := parts[1:]

	switch {
	// {maglevd}/backend/{name}/{action}
	case len(rest) == 3 && rest[0] == "backend":
		handleBackendLifecycle(w, r, c, rest[1], rest[2])
	// {maglevd}/frontend/{fe}/pool/{pool}/backend/{name}/weight
	case len(rest) == 7 && rest[0] == "frontend" && rest[2] == "pool" && rest[4] == "backend" && rest[6] == "weight":
		handleBackendWeight(w, r, c, rest[1], rest[3], rest[5])
	default:
		http.NotFound(w, r)
	}
}

func handleBackendLifecycle(w http.ResponseWriter, r *http.Request, c *maglevClient, name, action string) {
	switch action {
	case "pause", "resume", "enable", "disable":
	default:
		http.Error(w, fmt.Sprintf("unknown action %q", action), http.StatusBadRequest)
		return
	}
	ctx, cancel := context.WithTimeout(r.Context(), 10*time.Second)
	defer cancel()
	snap, err := c.BackendAction(ctx, name, action)
	if err != nil {
		slog.Warn("admin-backend-action", "maglevd", c.name, "backend", name, "action", action, "err", err)
		http.Error(w, err.Error(), http.StatusBadGateway)
		return
	}
	slog.Info("admin-backend-action",
		"maglevd", c.name, "backend", name, "action", action, "state", snap.State)
	writeJSON(w, snap)
}

type setWeightBody struct {
	Weight int32 `json:"weight"`
	Flush  bool  `json:"flush"`
}

func handleBackendWeight(w http.ResponseWriter, r *http.Request, c *maglevClient, frontend, pool, backend string) {
	var body setWeightBody
	if err := json.NewDecoder(r.Body).Decode(&body); err != nil {
		http.Error(w, fmt.Sprintf("bad json: %v", err), http.StatusBadRequest)
		return
	}
	if body.Weight < 0 || body.Weight > 100 {
		http.Error(w, fmt.Sprintf("weight %d out of range [0, 100]", body.Weight), http.StatusBadRequest)
		return
	}
	ctx, cancel := context.WithTimeout(r.Context(), 10*time.Second)
	defer cancel()
	snap, err := c.SetBackendWeight(ctx, frontend, pool, backend, body.Weight, body.Flush)
	if err != nil {
		slog.Warn("admin-set-weight",
			"maglevd", c.name, "frontend", frontend, "pool", pool, "backend", backend,
			"weight", body.Weight, "flush", body.Flush, "err", err)
		http.Error(w, err.Error(), http.StatusBadGateway)
		return
	}
	slog.Info("admin-set-weight",
		"maglevd", c.name, "frontend", frontend, "pool", pool, "backend", backend,
		"weight", body.Weight, "flush", body.Flush)
	writeJSON(w, snap)
}

// basicAuth wraps a handler in an HTTP basic-auth check. Uses
// subtle.ConstantTimeCompare to avoid leaking credential length or
// content via response-timing side channels.
func basicAuth(realm, user, password string, next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		u, p, ok := r.BasicAuth()
		// Compare fixed-length byte slices so a wrong username takes
		// the same time as a wrong password; only the boolean result
		// matters.
		uOK := subtle.ConstantTimeCompare([]byte(u), []byte(user)) == 1
		pOK := subtle.ConstantTimeCompare([]byte(p), []byte(password)) == 1
		if !ok || !uOK || !pOK {
			w.Header().Set("WWW-Authenticate", fmt.Sprintf(`Basic realm=%q`, realm))
			http.Error(w, "unauthorized", http.StatusUnauthorized)
			return
		}
		next.ServeHTTP(w, r)
	})
}

func writeJSON(w http.ResponseWriter, v any) {
	w.Header().Set("Content-Type", "application/json")
	enc := json.NewEncoder(w)
	enc.SetEscapeHTML(false)
	if err := enc.Encode(v); err != nil {
		slog.Error("json-encode", "err", err)
	}
}

// serveSSE handles the long-lived /view/api/events stream. The operational
// requirements (retry hint, heartbeat, flush-after-write, X-Accel-Buffering,
// context-done teardown) are documented in PLAN_FRONTEND.md §SSE operational
// requirements.
func serveSSE(w http.ResponseWriter, r *http.Request, broker *Broker) {
	flusher, ok := w.(http.Flusher)
	if !ok {
		http.Error(w, "streaming unsupported", http.StatusInternalServerError)
		return
	}

	h := w.Header()
	h.Set("Content-Type", "text/event-stream")
	h.Set("Cache-Control", "no-cache")
	h.Set("Connection", "keep-alive")
	h.Set("X-Accel-Buffering", "no")
	w.WriteHeader(http.StatusOK)

	// Reconnect hint: EventSource default is 3–5s; 2s feels livelier.
	fmt.Fprintf(w, "retry: 2000\n\n")
	flusher.Flush()

	result := broker.Subscribe(r.Header.Get("Last-Event-ID"))
	defer broker.Unsubscribe(result.Channel)

	if result.NeedResync {
		// No id: line — the browser keeps whatever Last-Event-ID it had,
		// so subsequent reconnects compare against a real event ID.
		fmt.Fprintf(w, "event: resync\ndata: {}\n\n")
		flusher.Flush()
	}
	for _, ev := range result.ReplayEvents {
		if err := writeEvent(w, ev); err != nil {
			return
		}
		flusher.Flush()
	}

	heartbeat := time.NewTicker(15 * time.Second)
	defer heartbeat.Stop()

	for {
		select {
		case <-r.Context().Done():
			return
		case ev, ok := <-result.Channel:
			if !ok {
				return
			}
			if err := writeEvent(w, ev); err != nil {
				return
			}
			flusher.Flush()
		case <-heartbeat.C:
			if _, err := fmt.Fprintf(w, ": ping\n\n"); err != nil {
				return
			}
			flusher.Flush()
		}
	}
}

func writeEvent(w http.ResponseWriter, ev deliveredEvent) error {
	// "resync" goes out as a named SSE event so the SPA's existing
	// addEventListener("resync", ...) handler fires (and not the
	// default onmessage path). Every other event type keeps the
	// default onmessage path with a JSON body. We still emit an id
	// so a reconnecting browser can replay from the right point in
	// the ring; the resync handler is idempotent (a duplicate
	// replay just triggers a redundant fetchState).
	if ev.Event.Type == "resync" {
		_, err := fmt.Fprintf(w, "id: %s\nevent: resync\ndata: {}\n\n", ev.ID)
		return err
	}
	body, err := json.Marshal(ev.Event)
	if err != nil {
		return err
	}
	_, err = fmt.Fprintf(w, "id: %s\ndata: %s\n\n", ev.ID, body)
	return err
}