Retry outbound read syncs for up to 24 hours

ts/util/exponentialBackoff.ts

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+// Copyright 2021 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+import * as moment from 'moment';
+
+const BACKOFF_FACTOR = 1.9;
+const MAX_BACKOFF = moment.duration(15, 'minutes').asMilliseconds();
+
+/**
+ * For a given attempt, how long should we sleep (in milliseconds)?
+ *
+ * The attempt should be a positive integer, and it is 1-indexed. The first attempt is 1,
+ * the second is 2, and so on.
+ *
+ * This is modified from [iOS's codebase][0].
+ *
+ * [0]: https://github.com/signalapp/Signal-iOS/blob/6069741602421744edfb59923d2fb3a66b1b23c1/SignalServiceKit/src/Util/OWSOperation.swift
+ */
+export function exponentialBackoffSleepTime(attempt: number): number {
+  const failureCount = attempt - 1;
+  if (failureCount === 0) {
+    return 0;
+  }
+  return Math.min(MAX_BACKOFF, 100 * BACKOFF_FACTOR ** failureCount);
+}
+
+/**
+ * If I want to retry for X milliseconds, how many attempts is that, roughly? For example,
+ * 24 hours (86,400,000 milliseconds) is 111 attempts.
+ *
+ * `desiredDurationMs` should be at least 1.
+ */
+export function exponentialBackoffMaxAttempts(
+  desiredDurationMs: number
+): number {
+  let attempts = 0;
+  let total = 0;
+  // There's probably some algebra we could do here instead of this loop, but this is
+  //   fast even for giant numbers, and is typically called just once at startup.
+  do {
+    attempts += 1;
+    total += exponentialBackoffSleepTime(attempts);
+  } while (total < desiredDurationMs);
+  return attempts;
+}