feat: Expose Last-Modified and ETag headers

Author: joachimvh

config/http/middleware/handlers/cors.json

@@ -17,6 +17,8 @@
       "options_credentials": true,
       "options_exposedHeaders": [
         "Accept-Patch",
+        "ETag",
+        "Last-Modified",
         "Link",
         "Location",
         "MS-Author-Via",

config/ldp/metadata-writer/default.json

@@ -4,6 +4,7 @@
     "files-scs:config/ldp/metadata-writer/writers/constant.json",
     "files-scs:config/ldp/metadata-writer/writers/link-rel.json",
     "files-scs:config/ldp/metadata-writer/writers/mapped.json",
+    "files-scs:config/ldp/metadata-writer/writers/modified.json",
     "files-scs:config/ldp/metadata-writer/writers/wac-allow.json",
     "files-scs:config/ldp/metadata-writer/writers/www-auth.json"
   ],
@@ -15,6 +16,7 @@
       "handlers": [
         { "@id": "urn:solid-server:default:MetadataWriter_Constant" },
         { "@id": "urn:solid-server:default:MetadataWriter_Mapped" },
+        { "@id": "urn:solid-server:default:MetadataWriter_Modified" },
         { "@id": "urn:solid-server:default:MetadataWriter_LinkRel" },
         { "@id": "urn:solid-server:default:MetadataWriter_WacAllow" },
         { "@id": "urn:solid-server:default:MetadataWriter_WwwAuth" }

config/ldp/metadata-writer/writers/modified.json

@@ -0,0 +1,10 @@
+{
+  "@context": "https://linkedsoftwaredependencies.org/bundles/npm/@solid/community-server/^1.0.0/components/context.jsonld",
+  "@graph": [
+    {
+      "comment": "Adds the Last-Modified and ETag headers.",
+      "@id": "urn:solid-server:default:MetadataWriter_Modified",
+      "@type": "ModifiedMetadataWriter"
+    }
+  ]
+}

src/index.ts

@@ -91,6 +91,7 @@ export * from './ldp/http/metadata/LinkTypeParser';
 export * from './ldp/http/metadata/MappedMetadataWriter';
 export * from './ldp/http/metadata/MetadataParser';
 export * from './ldp/http/metadata/MetadataWriter';
+export * from './ldp/http/metadata/ModifiedMetadataWriter';
 export * from './ldp/http/metadata/SlugParser';
 export * from './ldp/http/metadata/WacAllowMetadataWriter';
 export * from './ldp/http/metadata/WwwAuthMetadataWriter';

src/ldp/http/metadata/ModifiedMetadataWriter.ts

@@ -0,0 +1,23 @@
+import type { HttpResponse } from '../../../server/HttpResponse';
+import { getETag } from '../../../storage/Conditions';
+import { addHeader } from '../../../util/HeaderUtil';
+import { DC } from '../../../util/Vocabularies';
+import type { RepresentationMetadata } from '../../representation/RepresentationMetadata';
+import { MetadataWriter } from './MetadataWriter';
+
+/**
+ * A {@link MetadataWriter} that generates all the necessary headers related to the modification date of a resource.
+ */
+export class ModifiedMetadataWriter extends MetadataWriter {
+  public async handle(input: { response: HttpResponse; metadata: RepresentationMetadata }): Promise<void> {
+    const modified = input.metadata.get(DC.terms.modified);
+    if (modified) {
+      const date = new Date(modified.value);
+      addHeader(input.response, 'Last-Modified', date.toUTCString());
+    }
+    const etag = getETag(input.metadata);
+    if (etag) {
+      addHeader(input.response, 'ETag', etag);
+    }
+  }
+}

src/storage/Conditions.ts

@@ -1,4 +1,5 @@
 import type { RepresentationMetadata } from '../ldp/representation/RepresentationMetadata';
+import { DC } from '../util/Vocabularies';
 
 /**
  * The conditions of an HTTP conditional request.
@@ -7,11 +8,11 @@ export interface Conditions {
   /**
    * Valid if matching any of the given ETags.
    */
-  matchesEtag: string[];
+  matchesETag?: string[];
   /**
    * Valid if not matching any of the given ETags.
    */
-  notMatchesEtag: string[];
+  notMatchesETag?: string[];
   /**
    * Valid if modified since the given date.
    */
@@ -27,9 +28,23 @@ export interface Conditions {
    */
   matchesMetadata: (metadata: RepresentationMetadata) => boolean;
   /**
-   * Checks validity based on the given ETag and/org date.
+   * Checks validity based on the given ETag and/or date.
    * @param eTag - Condition based on ETag.
    * @param lastModified - Condition based on last modified date.
    */
   matches: (eTag?: string, lastModified?: Date) => boolean;
 }
+
+/**
+ * Generates an ETag based on the last modified date of a resource.
+ * @param metadata - Metadata of the resource.
+ *
+ * @returns the generated ETag. Undefined if no last modified date was found.
+ */
+export function getETag(metadata: RepresentationMetadata): string | undefined {
+  const modified = metadata.get(DC.terms.modified);
+  if (modified) {
+    const date = new Date(modified.value);
+    return `"${date.getTime()}"`;
+  }
+}

test/integration/Middleware.test.ts

@@ -99,6 +99,13 @@ describe('An http server with middleware', (): void => {
     expect(exposed.split(/\s*,\s*/u)).toContain('Accept-Patch');
   });
 
+  it('exposes the Last-Modified and ETag headers via CORS.', async(): Promise<void> => {
+    const res = await request(server).get('/').expect(200);
+    const exposed = res.header['access-control-expose-headers'];
+    expect(exposed.split(/\s*,\s*/u)).toContain('ETag');
+    expect(exposed.split(/\s*,\s*/u)).toContain('Last-Modified');
+  });
+
   it('exposes the Link header via CORS.', async(): Promise<void> => {
     const res = await request(server).get('/').expect(200);
     const exposed = res.header['access-control-expose-headers'];

test/unit/ldp/http/metadata/ModifiedMetadataWriter.test.ts

@@ -0,0 +1,29 @@
+import { createResponse } from 'node-mocks-http';
+import { ModifiedMetadataWriter } from '../../../../../src/ldp/http/metadata/ModifiedMetadataWriter';
+import { RepresentationMetadata } from '../../../../../src/ldp/representation/RepresentationMetadata';
+import type { HttpResponse } from '../../../../../src/server/HttpResponse';
+import { updateModifiedDate } from '../../../../../src/util/ResourceUtil';
+import { DC } from '../../../../../src/util/Vocabularies';
+
+describe('A ModifiedMetadataWriter', (): void => {
+  const writer = new ModifiedMetadataWriter();
+
+  it('adds the Last-Modified and ETag header if there is dc:modified metadata.', async(): Promise<void> => {
+    const response = createResponse() as HttpResponse;
+    const metadata = new RepresentationMetadata();
+    updateModifiedDate(metadata);
+    const dateTime = metadata.get(DC.terms.modified)!.value;
+    await expect(writer.handle({ response, metadata })).resolves.toBeUndefined();
+    expect(response.getHeaders()).toEqual({
+      'last-modified': new Date(dateTime).toUTCString(),
+      etag: `"${new Date(dateTime).getTime()}"`,
+    });
+  });
+
+  it('does nothing if there is no matching metadata.', async(): Promise<void> => {
+    const response = createResponse() as HttpResponse;
+    const metadata = new RepresentationMetadata();
+    await expect(writer.handle({ response, metadata })).resolves.toBeUndefined();
+    expect(response.getHeaders()).toEqual({});
+  });
+});