memcpy empty string issue

Summary:
memcpying from a nullptr is a UB.

UBSan caught it on one of the rollout tests

This unfortunately has an overhead of an extra if but given that most strings are bigger than 8 chars, hopefully not a big one.

Reviewed By: udippant, elalfer

Differential Revision: D51754836

fbshipit-source-id: 50069195a3ac8121fc42f9310d43c49c17831917

mcrouter/lib/fbi/cpp/LowerBoundPrefixMap.h

@@ -40,8 +40,10 @@ class SmallPrefix {
       data_ = folly::Endian::swap(data_);
       return;
     }
-    std::memcpy(&data_, s.data(), s.size());
-    data_ = folly::Endian::swap(data_);
+    if (FOLLY_LIKELY(s.data() != nullptr)) {
+      std::memcpy(&data_, s.data(), s.size());
+      data_ = folly::Endian::swap(data_);
+    }
   }
 
   // default operator== and operator<=> are ok here

mcrouter/lib/fbi/cpp/test/LowerBoundPrefixMapTest.cpp

@@ -205,5 +205,10 @@ TEST(LowerBoundPrefixMapTest, EmptyMap) {
   ASSERT_TRUE(lbMap.empty());
 }
 
+TEST(LowerBoundPrefixMapTest, NullKey) {
+  LowerBoundPrefixMap<int> lbMap;
+  ASSERT_EQ(lbMap.findPrefix(std::string_view{}), lbMap.end());
+}
+
 } // namespace
 } // namespace facebook::memcache