C++ migration: add a C++ implementation of `FSTExponentialBackoff` (#1465)

This is a pretty close port of `FSTExponentialBackoff`. The changes are pretty minor:

* delay is calculated using <chrono> duration types, not plain numbers, which should be a little more type-safe;
* split a piece of code into a ClampDelay function, because it's reasonably close to std::clamp;
* rephrased the class-level comment to make it clearer that the first attempt always has delay = 0;
* added simple tests (other platforms don't have tests for this).

Also make sure that canceling a DelayedOperation is always valid.

1 files changed, 24 insertions, 0 deletions

diff --git a/Firestore/core/test/firebase/firestore/util/executor_test.cc b/Firestore/core/test/firebase/firestore/util/executor_test.cc
index 99bddce..e983bfe 100644
--- a/Firestore/core/test/firebase/firestore/util/executor_test.cc
+++ b/Firestore/core/test/firebase/firestore/util/executor_test.cc
@@ -115,6 +115,30 @@ TEST_P(ExecutorTest, DelayedOperationIsValidAfterTheOperationHasRun) {
   EXPECT_NO_THROW(delayed_operation.Cancel());
 }
 
+TEST_P(ExecutorTest, CancelingEmptyDelayedOperationIsValid) {
+  DelayedOperation delayed_operation;
+  EXPECT_NO_THROW(delayed_operation.Cancel());
+}
+
+TEST_P(ExecutorTest, DoubleCancelingDelayedOperationIsValid) {
+  std::string steps;
+
+  executor->Execute([&] {
+    DelayedOperation delayed_operation = Schedule(
+        executor.get(), Executor::Milliseconds(1), [&steps] { steps += '1'; });
+    Schedule(executor.get(), Executor::Milliseconds(5), [&] {
+      steps += '2';
+      signal_finished();
+    });
+
+    delayed_operation.Cancel();
+    delayed_operation.Cancel();
+  });
+
+  EXPECT_TRUE(WaitForTestToFinish());
+  EXPECT_EQ(steps, "2");
+}
+
 TEST_P(ExecutorTest, IsCurrentExecutor) {
   EXPECT_FALSE(executor->IsCurrentExecutor());
   EXPECT_NE(executor->Name(), executor->CurrentExecutorName());