feat: Create ConditionalHandler

This handler rejects all inputs once a certain condition is met

src/index.ts

@@ -307,6 +307,7 @@ export * from './util/errors/UnsupportedMediaTypeHttpError';
 // Util/Handlers
 export * from './util/handlers/AsyncHandler';
 export * from './util/handlers/BooleanHandler';
+export * from './util/handlers/ConditionalHandler';
 export * from './util/handlers/ParallelHandler';
 export * from './util/handlers/SequenceHandler';
 export * from './util/handlers/UnsupportedAsyncHandler';

src/util/handlers/ConditionalHandler.ts

@@ -0,0 +1,57 @@
+import type { KeyValueStorage } from '../../storage/keyvalue/KeyValueStorage';
+import { NotImplementedHttpError } from '../errors/NotImplementedHttpError';
+import { AsyncHandler } from './AsyncHandler';
+
+/**
+ * This handler will pass all requests to the wrapped handler,
+ * until a specific value has been set in the given storage.
+ * After that all input will be rejected.
+ * Once the value has been matched this behaviour will be cached,
+ * so changing the value again afterwards will not enable this handler again.
+ */
+export class ConditionalHandler<TIn, TOut> extends AsyncHandler<TIn, TOut> {
+  private readonly source: AsyncHandler<TIn, TOut>;
+  private readonly storage: KeyValueStorage<string, unknown>;
+  private readonly storageKey: string;
+  private readonly storageValue: unknown;
+
+  private finished: boolean;
+
+  public constructor(source: AsyncHandler<TIn, TOut>, storage: KeyValueStorage<string, unknown>, storageKey: string,
+    storageValue: unknown) {
+    super();
+    this.source = source;
+    this.storage = storage;
+    this.storageKey = storageKey;
+    this.storageValue = storageValue;
+
+    this.finished = false;
+  }
+
+  public async canHandle(input: TIn): Promise<void> {
+    await this.checkCondition();
+    await this.source.canHandle(input);
+  }
+
+  public async handleSafe(input: TIn): Promise<TOut> {
+    await this.checkCondition();
+    return this.source.handleSafe(input);
+  }
+
+  public async handle(input: TIn): Promise<TOut> {
+    return this.source.handle(input);
+  }
+
+  /**
+   * Checks if the condition has already been fulfilled.
+   */
+  private async checkCondition(): Promise<void> {
+    if (!this.finished) {
+      this.finished = await this.storage.get(this.storageKey) === this.storageValue;
+    }
+
+    if (this.finished) {
+      throw new NotImplementedHttpError('The condition has been fulfilled.');
+    }
+  }
+}