properly intercept not found requests for single page app

When a request comes in for an unknown path looking for an HTML content
type, we need to render the index.html page so that the single page app
can do its thing.

main.go

@@ -55,6 +55,9 @@ func initAndServe(ctx context.Context) int {
 		logger.ErrorContext(ctx, "error getting frontend index", slog.String("error", err.Error()))
 		return 1
 	}
+	httpFS := http.FS(frontendIndex)
+	fileServer := http.FileServer(httpFS)
+	serveIndex := serveFileContents("index.html", httpFS)
 
 	dbPool, err := getDBPool(ctx, dbURL)
 	if err != nil {
@@ -87,7 +90,7 @@ func initAndServe(ctx context.Context) int {
 	mux.HandleFunc("GET /api/workflows/{id}", handler.WorkflowGet)
 	mux.HandleFunc("GET /api/states", handler.StatesAndCounts)
 	mux.HandleFunc("/api", http.NotFound)
-	mux.Handle("/", http.FileServer(http.FS(frontendIndex)))
+	mux.Handle("/", intercept404(fileServer, serveIndex))
 
 	logHandler := sloghttp.Recovery(mux)
 	config := sloghttp.Config{

spa_response_writer.go

@@ -0,0 +1,73 @@
+package main
+
+import (
+	"fmt"
+	"net/http"
+	"strings"
+)
+
+func intercept404(handler, on404 http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		hookedWriter := &spaResponseWriter{ResponseWriter: w}
+		handler.ServeHTTP(hookedWriter, r)
+
+		if hookedWriter.got404 {
+			on404.ServeHTTP(w, r)
+		}
+	})
+}
+
+func serveFileContents(file string, files http.FileSystem) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		// Restrict only to instances where the browser is looking for an HTML file
+		if !strings.Contains(r.Header.Get("Accept"), "text/html") {
+			w.WriteHeader(http.StatusNotFound)
+			fmt.Fprint(w, "404 not found")
+
+			return
+		}
+
+		// Open the file and return its contents using http.ServeContent
+		index, err := files.Open(file)
+		if err != nil {
+			w.WriteHeader(http.StatusNotFound)
+			fmt.Fprintf(w, "%s not found", file)
+
+			return
+		}
+
+		fi, err := index.Stat()
+		if err != nil {
+			w.WriteHeader(http.StatusNotFound)
+			fmt.Fprintf(w, "%s not found", file)
+
+			return
+		}
+
+		w.Header().Set("Content-Type", "text/html; charset=utf-8")
+		http.ServeContent(w, r, fi.Name(), fi.ModTime(), index)
+	}
+}
+
+type spaResponseWriter struct {
+	http.ResponseWriter
+	got404 bool
+}
+
+func (srw *spaResponseWriter) WriteHeader(status int) {
+	if status == http.StatusNotFound {
+		// Don't actually write the 404 header, just set a flag.
+		srw.got404 = true
+	} else {
+		srw.ResponseWriter.WriteHeader(status)
+	}
+}
+
+func (hrw *spaResponseWriter) Write(p []byte) (int, error) {
+	if hrw.got404 {
+		// No-op, but pretend that we wrote len(p) bytes to the writer.
+		return len(p), nil
+	}
+
+	return hrw.ResponseWriter.Write(p)
+}