feat: Add acl link header writer

This writer will add a link to the corresponding acl file
for all LDP requests.

src/index.ts

@@ -23,6 +23,7 @@ export * from './init/RootContainerInitializer';
 export * from './init/ServerInitializer';
 
 // LDP/HTTP/Metadata
+export * from './ldp/http/metadata/AclLinkMetadataWriter';
 export * from './ldp/http/metadata/BasicMetadataExtractor';
 export * from './ldp/http/metadata/ConstantMetadataWriter';
 export * from './ldp/http/metadata/ContentTypeParser';

src/ldp/http/metadata/AclLinkMetadataWriter.ts

@@ -0,0 +1,25 @@
+import type { AclManager } from '../../../authorization/AclManager';
+import type { HttpResponse } from '../../../server/HttpResponse';
+import { addHeader } from '../../../util/HeaderUtil';
+import type { RepresentationMetadata } from '../../representation/RepresentationMetadata';
+import { MetadataWriter } from './MetadataWriter';
+
+/**
+ * A MetadataWriter that always adds a rel="acl" link header to a response.
+ * The `rel` parameter can be used if a different `rel` value is needed (such as http://www.w3.org/ns/solid/terms#acl).
+ */
+export class AclLinkMetadataWriter extends MetadataWriter {
+  private readonly aclManager: AclManager;
+  private readonly rel: string;
+
+  public constructor(aclManager: AclManager, rel = 'acl') {
+    super();
+    this.aclManager = aclManager;
+    this.rel = rel;
+  }
+
+  public async handle(input: { response: HttpResponse; metadata: RepresentationMetadata }): Promise<void> {
+    const identifier = await this.aclManager.getAclDocument({ path: input.metadata.identifier.value });
+    addHeader(input.response, 'Link', `<${identifier.path}>; rel="${this.rel}"`);
+  }
+}