Fix underflow in sizeBucketsPowerAndLocksPower()

Summary: If `cacheEntries` is too small, `bucketsPower_` will be less than 10. Since `bucketsPower_` is unsigned, it will underflow if we subtract 10 from it.

Reviewed By: jaesoo-fb

Differential Revision: D50994522

fbshipit-source-id: 55151fdab1982442a706267b3dd3a0bb6a6fd6e7

Author: Alex Schneidman

cachelib/allocator/ChainedHashTable.h

@@ -286,7 +286,8 @@ class ChainedHashTable {
       }
 
       // 1 lock per 1000 buckets.
-      locksPower_ = std::max<unsigned int>(1, bucketsPower_ - 10);
+      locksPower_ =
+          (bucketsPower_ <= 20) ? (bucketsPower_ / 2) + 1 : bucketsPower_ - 10;
     }
 
     unsigned int getBucketsPower() const noexcept { return bucketsPower_; }

cachelib/allocator/tests/ChainedHashTest.cpp

@@ -33,6 +33,10 @@ TEST(ChainedHashTableConfigTest, Size) {
 
   ASSERT_THROW(config.sizeBucketsPowerAndLocksPower(2700000000),
                std::invalid_argument);
+
+  config.sizeBucketsPowerAndLocksPower(1);
+  EXPECT_EQ(config.getBucketsPower(), 1);
+  EXPECT_EQ(config.getLocksPower(), 1);
 }
 
 TEST_F(ChainedHashTest, Insert) { testInsert(); }