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diff --git a/packages/taler-util/src/promises.ts b/packages/taler-util/src/promises.ts
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+++ b/packages/taler-util/src/promises.ts
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+/*
+ This file is part of GNU Taler
+ (C) 2019 GNUnet e.V.
+
+ TALER is free software; you can redistribute it and/or modify it under the
+ terms of the GNU General Public License as published by the Free Software
+ Foundation; either version 3, or (at your option) any later version.
+
+ TALER is distributed in the hope that it will be useful, but WITHOUT ANY
+ WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
+ A PARTICULAR PURPOSE.  See the GNU General Public License for more details.
+
+ You should have received a copy of the GNU General Public License along with
+ TALER; see the file COPYING.  If not, see <http://www.gnu.org/licenses/>
+ */
+
+/**
+ * An opened promise.
+ *
+ * @see {@link openPromise}
+ */
+export interface OpenedPromise<T> {
+  promise: Promise<T>;
+  resolve: (val: T) => void;
+  reject: (err: any) => void;
+  lastError?: any;
+}
+
+/**
+ * Get an unresolved promise together with its extracted resolve / reject
+ * function.
+ *
+ * Recent ECMAScript proposals also call this a promise capability.
+ */
+export function openPromise<T>(): OpenedPromise<T> {
+  let resolve: ((x?: any) => void) | null = null;
+  let promiseReject: ((reason?: any) => void) | null = null;
+  const promise = new Promise<T>((res, rej) => {
+    resolve = res;
+    promiseReject = rej;
+  });
+  if (!(resolve && promiseReject)) {
+    // Never happens, unless JS implementation is broken
+    throw Error("JS implementation is broken");
+  }
+  const result: OpenedPromise<T> = { resolve, reject: promiseReject, promise };
+  function saveLastError(reason?: any) {
+    result.lastError = reason;
+    promiseReject!(reason);
+  }
+  result.reject = saveLastError;
+  return result;
+}
+
+export class AsyncCondition {
+  private promCap?: OpenedPromise<void> = undefined;
+  constructor() {}
+
+  wait(): Promise<void> {
+    if (!this.promCap) {
+      this.promCap = openPromise<void>();
+    }
+    return this.promCap.promise;
+  }
+
+  trigger(): void {
+    if (this.promCap) {
+      this.promCap.resolve();
+    }
+    this.promCap = undefined;
+  }
+}
+
+/**
+ * Flag that can be raised to notify asynchronous waiters.
+ *
+ * You can think of it as a promise that can
+ * be un-resolved.
+ */
+export class AsyncFlag {
+  private promCap?: OpenedPromise<void> = undefined;
+  private internalFlagRaised: boolean = false;
+
+  constructor() {}
+
+  /**
+   * Wait until the flag is raised.
+   *
+   * Reset if before returning.
+   */
+  wait(): Promise<void> {
+    if (this.internalFlagRaised) {
+      return Promise.resolve();
+    }
+    if (!this.promCap) {
+      this.promCap = openPromise<void>();
+    }
+    return this.promCap.promise;
+  }
+
+  raise(): void {
+    this.internalFlagRaised = true;
+    if (this.promCap) {
+      this.promCap.resolve();
+    }
+  }
+
+  reset(): void {
+    this.internalFlagRaised = false;
+    this.promCap = undefined;
+  }
+}