Fix race due to use of TryLock where

grpc_event_engine::experimental::TimerList::TimerCheck
could miss an existing timer and leave the wakeup time to InfiniteFuture.

src/core/lib/event_engine/posix_engine/timer.cc

@@ -298,12 +298,10 @@ TimerList::TimerCheck(grpc_core::Timestamp* next) {
     return std::vector<experimental::EventEngine::Closure*>();
   }
 
-  if (!checker_mu_.TryLock()) return absl::nullopt;
-  std::vector<experimental::EventEngine::Closure*> run =
-      FindExpiredTimers(now, next);
-  checker_mu_.Unlock();
-
-  return std::move(run);
+  {
+    absl::MutexLock lock(&checker_mu_);
+    return FindExpiredTimers(now, next);
+  }
 }
 
 }  // namespace experimental

src/core/lib/event_engine/posix_engine/timer.h

@@ -149,8 +149,8 @@ class TimerList {
   grpc_core::Mutex mu_;
   // The deadline of the next timer due across all timer shards
   std::atomic<uint64_t> min_timer_;
-  // Allow only one FindExpiredTimers at once (used as a TryLock, protects no
-  // fields but ensures limits on concurrency)
+  // Allow only one FindExpiredTimers at once (protects no fields but ensures
+  // limits on concurrency)
   grpc_core::Mutex checker_mu_;
   // Array of timer shards. Whenever a timer (Timer *) is added, its address
   // is hashed to select the timer shard to add the timer to