feat: Add required ACP headers

src/server/middleware/AcpHeaderHandler.ts

@@ -0,0 +1,39 @@
+import type { AuxiliaryIdentifierStrategy } from '../../http/auxiliary/AuxiliaryIdentifierStrategy';
+import type { TargetExtractor } from '../../http/input/identifier/TargetExtractor';
+import { addHeader } from '../../util/HeaderUtil';
+import { ACP } from '../../util/Vocabularies';
+import type { HttpHandlerInput } from '../HttpHandler';
+import { HttpHandler } from '../HttpHandler';
+
+/**
+ * Handles all the required ACP headers as defined at
+ * https://solid.github.io/authorization-panel/acp-specification/#conforming-acp-server
+ */
+export class AcpHeaderHandler extends HttpHandler {
+  private readonly targetExtractor: TargetExtractor;
+  private readonly strategy: AuxiliaryIdentifierStrategy;
+  private readonly modes: string[];
+  private readonly attributes: string[];
+
+  public constructor(targetExtractor: TargetExtractor, strategy: AuxiliaryIdentifierStrategy,
+    modes: string[], attributes: string[]) {
+    super();
+    this.targetExtractor = targetExtractor;
+    this.strategy = strategy;
+    this.modes = modes;
+    this.attributes = attributes;
+  }
+
+  public async handle({ request, response }: HttpHandlerInput): Promise<void> {
+    const identifier = await this.targetExtractor.handleSafe({ request });
+    if (!this.strategy.isAuxiliaryIdentifier(identifier)) {
+      return;
+    }
+    const linkValues = [
+      `<${ACP.AccessControlResource}>; rel="type"`,
+      ...this.modes.map((mode): string => `<${mode}>; rel="${ACP.grant}"`),
+      ...this.attributes.map((attribute): string => `<${attribute}>; rel="${ACP.attribute}"`),
+    ];
+    addHeader(response, 'Link', linkValues);
+  }
+}