[xray] Fix UniqueID hashing for object and task IDs. (#2017)

* Skip object prefix in UniqueIDHasher, choose shard based on hash

* lint

src/ray/constants.h

@@ -1,12 +1,17 @@
 #ifndef RAY_CONSTANTS_H_
 #define RAY_CONSTANTS_H_
 
+#include <limits.h>
+
 /// Length of Ray IDs in bytes.
 constexpr int64_t kUniqueIDSize = 20;
 
 /// An ObjectID's bytes are split into the task ID itself and the index of the
 /// object's creation. This is the maximum width of the object index in bits.
 constexpr int kObjectIdIndexSize = 32;
+static_assert(kObjectIdIndexSize % CHAR_BIT == 0,
+              "ObjectID prefix not a multiple of bytes");
+
 /// The maximum number of objects that can be returned by a task when finishing
 /// execution. An ObjectID's bytes are split into the task ID itself and the
 /// index of the object's creation. A positive index indicates an object

src/ray/id.cc

@@ -1,5 +1,6 @@
 #include "ray/id.h"
 
+#include <limits.h>
 #include <random>
 
 #include "ray/constants.h"
@@ -83,7 +84,8 @@ bool UniqueID::operator==(const UniqueID &rhs) const {
 
 size_t UniqueID::hash() const {
   size_t result;
-  std::memcpy(&result, id_, sizeof(size_t));
+  // Skip the bytes for the object prefix.
+  std::memcpy(&result, id_ + (kObjectIdIndexSize / CHAR_BIT), sizeof(size_t));
   return result;
 }