Serve both UI assets and REST API from handler

static.go

@@ -0,0 +1,100 @@
+package asynqmon
+
+import (
+	"embed"
+	"errors"
+	"html/template"
+	"io/fs"
+	"net/http"
+	"path/filepath"
+	"strings"
+)
+
+// uiAssetsHandler is a http.Handler.
+// The path to the static file directory and
+// the path to the index file within that static directory are used to
+// serve the SPA.
+type uiAssetsHandler struct {
+	rootPath      string
+	contents      embed.FS
+	staticDirPath string
+	indexFileName string
+}
+
+// ServeHTTP inspects the URL path to locate a file within the static dir
+// on the SPA handler.
+// If path '/' is requested, it will serve the index file, otherwise it will
+// serve the file specified by the URL path.
+func (h *uiAssetsHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	// Get the absolute path to prevent directory traversal.
+	path, err := filepath.Abs(r.URL.Path)
+	if err != nil {
+		http.Error(w, err.Error(), http.StatusBadRequest)
+		return
+	}
+
+	// Get the path relative to the root path.
+	if !strings.HasPrefix(path, h.rootPath) {
+		http.Error(w, "unexpected path prefix", http.StatusBadRequest)
+		return
+	}
+	path = strings.TrimPrefix(path, h.rootPath)
+
+	if code, err := h.serveFile(w, path); err != nil {
+		http.Error(w, err.Error(), code)
+		return
+	}
+}
+
+func (h *uiAssetsHandler) indexFilePath() string {
+	return filepath.Join(h.staticDirPath, h.indexFileName)
+}
+
+func (h *uiAssetsHandler) renderIndexFile(w http.ResponseWriter) error {
+	// Note: Replace the default delimiter ("{{") with a custom one
+	// since webpack escapes the '{' character when it compiles the index.html file.
+	// See the "homepage" field in package.json.
+	tmpl, err := template.New(h.indexFileName).Delims("/[[", "]]").ParseFS(h.contents, h.indexFilePath())
+	if err != nil {
+		return err
+	}
+	data := struct {
+		RootPath string
+	}{
+		RootPath: h.rootPath,
+	}
+	return tmpl.Execute(w, data)
+}
+
+// serveFile writes file requested at path and returns http status code and error if any.
+// If requested path is root, it serves the index file.
+// Otherwise, it looks for file requiested in the static content filesystem
+// and serves if a file is found.
+// If a requested file is not found in the filesystem, it serves the index file to
+// make sure when user refreshes the page in SPA things still work.
+func (h *uiAssetsHandler) serveFile(w http.ResponseWriter, path string) (code int, err error) {
+	if path == "/" || path == "" {
+		if err := h.renderIndexFile(w); err != nil {
+			return http.StatusInternalServerError, err
+		}
+		return http.StatusOK, nil
+	}
+	path = filepath.Join(h.staticDirPath, path)
+	bytes, err := h.contents.ReadFile(path)
+	if err != nil {
+		// If path is error (e.g. file not exist, path is a directory), serve index file.
+		var pathErr *fs.PathError
+		if errors.As(err, &pathErr) {
+			if err := h.renderIndexFile(w); err != nil {
+				return http.StatusInternalServerError, err
+			}
+			return http.StatusOK, nil
+		}
+		return http.StatusInternalServerError, err
+	}
+
+	if _, err := w.Write(bytes); err != nil {
+		return http.StatusInternalServerError, err
+	}
+	return http.StatusOK, nil
+}