feat: Cache static assets.

Closes https://github.com/solid/community-server/issues/861

config/http/static/default.json

@@ -5,6 +5,7 @@
       "comment": "Servers static files on fixed URLs.",
       "@id": "urn:solid-server:default:StaticAssetHandler",
       "@type": "StaticAssetHandler",
+      "options_expires": 86400,
       "assets": [
         {
           "StaticAssetHandler:_assets_key": "/favicon.ico",

src/server/middleware/StaticAssetHandler.ts

@@ -20,20 +20,23 @@ import type { HttpRequest } from '../HttpRequest';
 export class StaticAssetHandler extends HttpHandler {
   private readonly mappings: Record<string, string>;
   private readonly pathMatcher: RegExp;
+  private readonly expires: number;
   private readonly logger = getLoggerFor(this);
 
   /**
    * Creates a handler for the provided static resources.
    * @param assets - A mapping from URL paths to paths,
    *  where URL paths ending in a slash are interpreted as entire folders.
+   * @param options - Cache expiration time in seconds.
    */
-  public constructor(assets: Record<string, string>) {
+  public constructor(assets: Record<string, string>, options: { expires?: number } = {}) {
     super();
     this.mappings = {};
     for (const [ url, path ] of Object.entries(assets)) {
       this.mappings[url] = resolveAssetPath(path);
     }
     this.pathMatcher = this.createPathMatcher(assets);
+    this.expires = Number.isInteger(options.expires) ? Math.max(0, options.expires!) : 0;
   }
 
   /**
@@ -90,7 +93,10 @@ export class StaticAssetHandler extends HttpHandler {
       // Write a 200 response when the asset becomes readable
       asset.once('readable', (): void => {
         const contentType = mime.lookup(filePath) || APPLICATION_OCTET_STREAM;
-        response.writeHead(200, { 'content-type': contentType });
+        response.writeHead(200, {
+          'content-type': contentType,
+          ...this.getCacheHeaders(),
+        });
 
         // With HEAD, only write the headers
         if (request.method === 'HEAD') {
@@ -120,4 +126,13 @@ export class StaticAssetHandler extends HttpHandler {
       });
     });
   }
+
+  private getCacheHeaders(): Record<string, string> {
+    return this.expires <= 0 ?
+      {} :
+      {
+        'cache-control': `max-age=${this.expires}`,
+        expires: new Date(Date.now() + (this.expires * 1000)).toUTCString(),
+      };
+  }
 }

test/unit/server/middleware/StaticAssetHandler.test.ts

@@ -213,4 +213,21 @@ describe('A StaticAssetHandler', (): void => {
     const response = createResponse({ eventEmitter: EventEmitter });
     await expect(handler.canHandle({ request, response } as any)).rejects.toThrow();
   });
+
+  it('caches responses when the expires option is set.', async(): Promise<void> => {
+    jest.spyOn(Date, 'now').mockReturnValue(0);
+    const cachedHandler = new StaticAssetHandler({
+      '/foo/bar/style': '/assets/styles/bar.css',
+    }, {
+      expires: 86400,
+    });
+    const request = { method: 'GET', url: '/foo/bar/style' };
+    const response = createResponse();
+    await cachedHandler.handleSafe({ request, response } as any);
+    jest.restoreAllMocks();
+
+    expect(response.statusCode).toBe(200);
+    expect(response.getHeaders()).toHaveProperty('cache-control', 'max-age=86400');
+    expect(response.getHeaders()).toHaveProperty('expires', 'Fri, 02 Jan 1970 00:00:00 GMT');
+  });
 });