1 files changed, 73 insertions, 0 deletions
diff --git a/packages/backend/src/remote/activitypub/resolver.ts b/packages/backend/src/remote/activitypub/resolver.ts
new file mode 100644
index 0000000000..f392a65e3a
--- /dev/null
+++ b/packages/backend/src/remote/activitypub/resolver.ts
@@ -0,0 +1,73 @@
+import config from '@/config/index';
+import { getJson } from '@/misc/fetch';
+import { ILocalUser } from '@/models/entities/user';
+import { getInstanceActor } from '@/services/instance-actor';
+import { signedGet } from './request';
+import { IObject, isCollectionOrOrderedCollection, ICollection, IOrderedCollection } from './type';
+import { fetchMeta } from '@/misc/fetch-meta';
+import { extractDbHost } from '@/misc/convert-host';
+
+export default class Resolver {
+	private history: Set<string>;
+	private user?: ILocalUser;
+
+	constructor() {
+		this.history = new Set();
+	}
+
+	public getHistory(): string[] {
+		return Array.from(this.history);
+	}
+
+	public async resolveCollection(value: string | IObject): Promise<ICollection | IOrderedCollection> {
+		const collection = typeof value === 'string'
+			? await this.resolve(value)
+			: value;
+
+		if (isCollectionOrOrderedCollection(collection)) {
+			return collection;
+		} else {
+			throw new Error(`unrecognized collection type: ${collection.type}`);
+		}
+	}
+
+	public async resolve(value: string | IObject): Promise<IObject> {
+		if (value == null) {
+			throw new Error('resolvee is null (or undefined)');
+		}
+
+		if (typeof value !== 'string') {
+			return value;
+		}
+
+		if (this.history.has(value)) {
+			throw new Error('cannot resolve already resolved one');
+		}
+
+		this.history.add(value);
+
+		const meta = await fetchMeta();
+		const host = extractDbHost(value);
+		if (meta.blockedHosts.includes(host)) {
+			throw new Error('Instance is blocked');
+		}
+
+		if (config.signToActivityPubGet && !this.user) {
+			this.user = await getInstanceActor();
+		}
+
+		const object = this.user
+			? await signedGet(value, this.user)
+			: await getJson(value, 'application/activity+json, application/ld+json');
+
+		if (object == null || (
+			Array.isArray(object['@context']) ?
+				!object['@context'].includes('https://www.w3.org/ns/activitystreams') :
+				object['@context'] !== 'https://www.w3.org/ns/activitystreams'
+		)) {
+			throw new Error('invalid response');
+		}
+
+		return object;
+	}
+}