[core] Customized hash and eq for LRU cache (ray-project#48954)

As titled, allow heterogeneous lookup and insertion; also add customized
hash and eq.

---------

Signed-off-by: hjiang <dentinyhao@gmail.com>
Signed-off-by: Connor Sanders <connor@elastiflow.com>

src/ray/util/shared_lru.h

@@ -27,19 +27,17 @@
 // // Check and consume `val`.
 //
 // TODO(hjiang):
-// 1. Add template arguments for key hash and key equal, to pass into absl::flat_hash_map.
-// 2. Provide a key hash wrapper to save a copy.
-// 3. flat hash map supports heterogeneous lookup, expose `KeyLike` templated interface.
-// 4. Add a `GetOrCreate` interface, which takes factory function to creation value.
-// 5. For thread-safe cache, add a sharded container wrapper to reduce lock contention.
+// 1. Write a wrapper around KeyHash and KeyEq, which takes std::reference_wrapper<Key>,
+// so we could store keys only in std::list, and reference in absl::flat_hash_map.
+// 2. Add a `GetOrCreate` interface, which takes factory function to creation value.
+// 3. For thread-safe cache, add a sharded container wrapper to reduce lock contention.
 
 #pragma once
 
 #include <cstdint>
 #include <list>
 #include <memory>
 #include <mutex>
-#include <optional>
 #include <string>
 #include <utility>
 
@@ -90,7 +88,8 @@ class SharedLruCache final {
   // Delete the entry with key `key`. Return true if the entry was found for
   // `key`, false if the entry was not found. In both cases, there is no entry
   // with key `key` existed after the call.
-  bool Delete(const Key &key) {
+  template <typename KeyLike>
+  bool Delete(KeyLike &&key) {
     auto it = cache_.find(key);
     if (it == cache_.end()) {
       return false;
@@ -100,8 +99,10 @@ class SharedLruCache final {
     return true;
   }
 
-  // Look up the entry with key `key`. Return nullptr if key doesn't exist.
-  std::shared_ptr<Val> Get(const Key &key) {
+  // Look up the entry with key `key`. Return std::nullopt if key doesn't exist.
+  // If found, return a copy for the value.
+  template <typename KeyLike>
+  std::shared_ptr<Val> Get(KeyLike &&key) {
     const auto cache_iter = cache_.find(key);
     if (cache_iter == cache_.end()) {
       return nullptr;
@@ -173,16 +174,18 @@ class ThreadSafeSharedLruCache final {
   // Delete the entry with key `key`. Return true if the entry was found for
   // `key`, false if the entry was not found. In both cases, there is no entry
   // with key `key` existed after the call.
-  bool Delete(const Key &key) {
+  template <typename KeyLike>
+  bool Delete(KeyLike &&key) {
     std::lock_guard lck(mu_);
-    return cache_.Delete(key);
+    return cache_.Delete(std::forward<KeyLike>(key));
   }
 
   // Look up the entry with key `key`. Return std::nullopt if key doesn't exist.
   // If found, return a copy for the value.
-  std::shared_ptr<Val> Get(const Key &key) {
+  template <typename KeyLike>
+  std::shared_ptr<Val> Get(KeyLike &&key) {
     std::lock_guard lck(mu_);
-    return cache_.Get(key);
+    return cache_.Get(std::forward<KeyLike>(key));
   }
 
   // Clear the cache.

src/ray/util/tests/shared_lru_test.cc

@@ -23,6 +23,19 @@ namespace ray::utils::container {
 
 namespace {
 constexpr size_t kTestCacheSz = 1;
+
+class TestClassWithHashAndEq {
+ public:
+  TestClassWithHashAndEq(std::string data) : data_(std::move(data)) {}
+  bool operator==(const TestClassWithHashAndEq &rhs) const { return data_ == rhs.data_; }
+  template <typename H>
+  friend H AbslHashValue(H h, const TestClassWithHashAndEq &obj) {
+    return H::combine(std::move(h), obj.data_);
+  }
+
+ private:
+  std::string data_;
+};
 }  // namespace
 
 TEST(SharedLruCache, PutAndGet) {
@@ -34,22 +47,21 @@ TEST(SharedLruCache, PutAndGet) {
 
   // Check put and get.
   cache.Put("1", std::make_shared<std::string>("1"));
-  val = cache.Get("1");
+  val = cache.Get(std::string_view{"1"});
   EXPECT_NE(val, nullptr);
   EXPECT_EQ(*val, "1");
 
   // Check key eviction.
   cache.Put("2", std::make_shared<std::string>("2"));
-  val = cache.Get("1");
+  val = cache.Get(std::string_view{"1"});
   EXPECT_EQ(val, nullptr);
-  val = cache.Get("2");
+  val = cache.Get(std::string_view{"2"});
   EXPECT_NE(val, nullptr);
   EXPECT_EQ(*val, "2");
 
   // Check deletion.
-  EXPECT_FALSE(cache.Delete("1"));
-  EXPECT_TRUE(cache.Delete("2"));
-  val = cache.Get("2");
+  EXPECT_FALSE(cache.Delete(std::string_view{"1"}));
+  val = cache.Get(std::string_view{"1"});
   EXPECT_EQ(val, nullptr);
 }
 
@@ -73,4 +85,13 @@ TEST(SharedLruConstCache, TypeAliasAssertion) {
       std::is_same_v<SharedLruConstCache<int, int>, SharedLruCache<int, const int>>);
 }
 
+TEST(SharedLruConstCache, CustomizedKey) {
+  TestClassWithHashAndEq obj1{"hello"};
+  TestClassWithHashAndEq obj2{"hello"};
+  SharedLruCache<TestClassWithHashAndEq, std::string> cache{2};
+  cache.Put(obj1, std::make_shared<std::string>("val"));
+  auto val = cache.Get(obj2);
+  EXPECT_EQ(*val, "val");
+}
+
 }  // namespace ray::utils::container