From e2f7bc79cd4326f769f370f230041101a099d98c Mon Sep 17 00:00:00 2001
From: Florian Dold <florian.dold@gmail.com>
Date: Tue, 18 Aug 2020 18:23:06 +0530
Subject: introduce locking to avoid certain simultaneous requests to the
 exchange

---
 packages/taler-wallet-core/src/operations/state.ts | 53 +++++++++++++++++++++-
 1 file changed, 52 insertions(+), 1 deletion(-)

(limited to 'packages/taler-wallet-core/src/operations/state.ts')

diff --git a/packages/taler-wallet-core/src/operations/state.ts b/packages/taler-wallet-core/src/operations/state.ts
index cfec85d0f..582dd92d3 100644
--- a/packages/taler-wallet-core/src/operations/state.ts
+++ b/packages/taler-wallet-core/src/operations/state.ts
@@ -22,11 +22,15 @@ import { Logger } from "../util/logging";
 import { PendingOperationsResponse } from "../types/pending";
 import { WalletNotification } from "../types/notifications";
 import { Database } from "../util/query";
+import { openPromise, OpenedPromise } from "../util/promiseUtils";
 
 type NotificationListener = (n: WalletNotification) => void;
 
 const logger = new Logger("state.ts");
 
+export const EXCHANGE_COINS_LOCK = "exchange-coins-lock";
+export const EXCHANGE_RESERVES_LOCK = "exchange-reserves-lock";
+
 export class InternalWalletState {
   cachedNextUrl: { [fulfillmentUrl: string]: NextUrlResult } = {};
   memoProcessReserve: AsyncOpMemoMap<void> = new AsyncOpMemoMap();
@@ -41,6 +45,16 @@ export class InternalWalletState {
 
   listeners: NotificationListener[] = [];
 
+  /**
+   * Promises that are waiting for a particular resource.
+   */
+  private resourceWaiters: Record<string, OpenedPromise<void>[]> = {};
+
+  /**
+   * Resources that are currently locked.
+   */
+  private resourceLocks: Set<string> = new Set();
+
   constructor(
     public db: Database,
     public http: HttpRequestLibrary,
@@ -49,7 +63,7 @@ export class InternalWalletState {
     this.cryptoApi = new CryptoApi(cryptoWorkerFactory);
   }
 
-  public notify(n: WalletNotification): void {
+  notify(n: WalletNotification): void {
     logger.trace("Notification", n);
     for (const l of this.listeners) {
       const nc = JSON.parse(JSON.stringify(n));
@@ -62,4 +76,41 @@ export class InternalWalletState {
   addNotificationListener(f: (n: WalletNotification) => void): void {
     this.listeners.push(f);
   }
+
+  /**
+   * Run an async function after acquiring a list of locks, identified
+   * by string tokens.
+   */
+  async runSequentialized<T>(tokens: string[], f: () => Promise<T>) {
+    // Make sure locks are always acquired in the same order
+    tokens = [... tokens].sort();
+
+    for (const token of tokens) {
+      if (this.resourceLocks.has(token)) {
+        const p = openPromise<void>();
+        let waitList = this.resourceWaiters[token];
+        if (!waitList) {
+          waitList = this.resourceWaiters[token] = [];
+        }
+        waitList.push(p);
+        await p.promise;
+      }
+      this.resourceLocks.add(token);
+    }
+
+    try {
+      logger.trace(`begin exclusive execution on ${JSON.stringify(tokens)}`);
+      const result = await f();
+      logger.trace(`end exclusive execution on ${JSON.stringify(tokens)}`);
+      return result;
+    } finally {
+      for (const token of tokens) {
+        this.resourceLocks.delete(token);
+        let waiter = (this.resourceWaiters[token] ?? []).shift();
+        if (waiter) {
+          waiter.resolve();
+        }
+      }
+    }
+  }
 }
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