folly::ConcurrentHashMap::reserve() - avoid setting count to 0

Summary: folly::ConcurrentHashMap::reserve() - avoid setting count to 0

Reviewed By: magedm

Differential Revision: D25398175

fbshipit-source-id: fc61b27d145f0d800cba9d35c942c0a5c8b53e2a

folly/concurrency/ConcurrentHashMap.h

@@ -464,6 +464,8 @@ class ConcurrentHashMap {
 
   void reserve(size_t count) {
     count = count >> ShardBits;
+    if (!count)
+      return;
     for (uint64_t i = 0; i < NumShards; i++) {
       auto seg = segments_[i].load(std::memory_order_acquire);
       if (seg) {

folly/concurrency/test/ConcurrentHashMapTest.cpp

@@ -193,6 +193,7 @@ TYPED_TEST_P(ConcurrentHashMapTest, MapResizeTest) {
   if (res != foomap.cend()) {
     EXPECT_EQ(0, res->second);
   }
+  foomap.reserve(0);
 }
 
 // Ensure we can insert objects without copy constructors.