Fix GenericRateLimiter hanging bug (#11763)

Summary:
Fixes #11742

Even after performing duty (1) ("Waiting for the next refill time"), it is possible the remaining threads are all in `Wait()`. Waking up at least one thread is enough to ensure progress continues, even if no new requests arrive.

The repro unit test (bb54245e6) is not included as it depends on an unlanded PR (#11753)

Pull Request resolved: #11763

Reviewed By: jaykorean

Differential Revision: D48710130

Pulled By: ajkr

fbshipit-source-id: 9d166bd577ea3a96ccd81dde85871fec5e85a4eb

unreleased_history/bug_fixes/fixed_generic_rate_limiter_hang.md

@@ -0,0 +1 @@
+Fixed a race condition in `GenericRateLimiter` that could cause it to stop granting requests

util/rate_limiter.cc

@@ -179,16 +179,16 @@ void GenericRateLimiter::Request(int64_t bytes, const Env::IOPriority pri,
       // Whichever thread reaches here first performs duty (2) as described
       // above.
       RefillBytesAndGrantRequestsLocked();
-      if (r.request_bytes == 0) {
-        // If there is any remaining requests, make sure there exists at least
-        // one candidate is awake for future duties by signaling a front request
-        // of a queue.
-        for (int i = Env::IO_TOTAL - 1; i >= Env::IO_LOW; --i) {
-          std::deque<Req*> queue = queue_[i];
-          if (!queue.empty()) {
-            queue.front()->cv.Signal();
-            break;
-          }
+    }
+    if (r.request_bytes == 0) {
+      // If there is any remaining requests, make sure there exists at least
+      // one candidate is awake for future duties by signaling a front request
+      // of a queue.
+      for (int i = Env::IO_TOTAL - 1; i >= Env::IO_LOW; --i) {
+        auto& queue = queue_[i];
+        if (!queue.empty()) {
+          queue.front()->cv.Signal();
+          break;
         }
       }
     }