Fix flakiness in //test/cpp/common:timer_test (#30606)

test/cpp/common/timer_test.cc

@@ -140,18 +140,24 @@ TEST_F(TimerTest, CancelSomeTimers) {
   grpc_core::ExecCtx exec_ctx;
   const int kNumTimers = 10;
   grpc_timer timers[kNumTimers];
-  int timer_fired = 0;
+  std::atomic<int> timer_fired{0};
+  grpc_core::ExecCtx::Get()->InvalidateNow();
   for (int i = 0; i < kNumTimers; ++i) {
-    grpc_timer_init(&timers[i],
+    // Set a large firing time for timers which are bound to be cancelled
+    // and set a small firing time for timers which need to execute.
+    grpc_timer_init(
+        &timers[i],
         grpc_core::ExecCtx::Get()->Now() +
-                        grpc_core::Duration::Milliseconds(500) +
+            ((i < kNumTimers / 2) ? grpc_core ::Duration::Milliseconds(60000)
-                        grpc_core::Duration::Milliseconds(i),
+                                  : grpc_core ::Duration::Milliseconds(100) +
+                                        grpc_core::Duration::Milliseconds(i)),
         GRPC_CLOSURE_CREATE(
             [](void* arg, grpc_error_handle error) {
               if (error == GRPC_ERROR_CANCELLED) {
                 return;
               }
-                          int* timer_fired = static_cast<int*>(arg);
+              std::atomic<int>* timer_fired =
+                  static_cast<std::atomic<int>*>(arg);
               ++*timer_fired;
             },
             &timer_fired, grpc_schedule_on_exec_ctx));