web + ui: Pass init cookie from server to client

pkg/server/web/app_json.go

@@ -0,0 +1,5 @@
+package web
+
+type jsonAppConfig struct {
+	MonitorVisible bool `json:"monitor-visible"`
+}

pkg/server/web/handlers.go

@@ -31,6 +31,16 @@ func (h Handler) ServeHTTP(w http.ResponseWriter, req *http.Request) {
 	}
 }
 
+// cookieHandler injects an HTTP cookie into the response.
+func cookieHandler(cookie *http.Cookie, next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
+		log.Debug().Str("module", "web").Str("remote", req.RemoteAddr).Str("proto", req.Proto).
+			Str("method", req.Method).Str("path", req.RequestURI).Msg("Injecting cookie")
+		http.SetCookie(w, cookie)
+		next.ServeHTTP(w, req)
+	})
+}
+
 // fileHandler creates a handler that sends the named file regardless of the requested URL.
 func fileHandler(name string) http.Handler {
 	return http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {

pkg/server/web/server.go

@@ -3,10 +3,12 @@ package web
 
 import (
 	"context"
+	"encoding/json"
 	"expvar"
 	"net"
 	"net/http"
 	"net/http/pprof"
+	"net/url"
 	"path/filepath"
 	"time"
 
@@ -75,7 +77,8 @@ func Initialize(
 		fileHandler(filepath.Join(conf.Web.UIDir, "favicon.png")))
 
 	// SPA managed paths.
-	spaHandler := fileHandler(filepath.Join(conf.Web.UIDir, "index.html"))
+	spaHandler := cookieHandler(appConfigCookie(conf.Web),
+		fileHandler(filepath.Join(conf.Web.UIDir, "index.html")))
 	Router.Path("/").Handler(spaHandler)
 	Router.Path("/monitor").Handler(spaHandler)
 	Router.Path("/status").Handler(spaHandler)
@@ -126,6 +129,22 @@ func Start(ctx context.Context) {
 	}
 }
 
+func appConfigCookie(webConfig config.Web) *http.Cookie {
+	o := &jsonAppConfig{
+		MonitorVisible: webConfig.MonitorVisible,
+	}
+	b, err := json.Marshal(o)
+	if err != nil {
+		log.Error().Str("module", "web").Str("phase", "startup").Err(err).
+			Msg("Failed to convert app-config to JSON")
+	}
+	return &http.Cookie{
+		Name:  "app-config",
+		Value: url.PathEscape(string(b)),
+		Path:  "/",
+	}
+}
+
 // serve begins serving HTTP requests
 func serve(ctx context.Context) {
 	// server.Serve blocks until we close the listener