Add leveldb::Cache::Prune

Prune() drops on-memory read cache of the database, so that the client can
relief its memory shortage.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=101335710

include/leveldb/cache.h

@@ -81,6 +81,13 @@ class Cache {
   // its cache keys.
   virtual uint64_t NewId() = 0;
 
+  // Remove all cache entries that are not actively in use.  Memory-constrained
+  // applications may wish to call this method to reduce memory usage.
+  // Default implementation of Prune() does nothing.  Subclasses are strongly
+  // encouraged to override the default implementation.  A future release of
+  // leveldb may change Prune() to a pure abstract method.
+  virtual void Prune() {}
+
  private:
   void LRU_Remove(Handle* e);
   void LRU_Append(Handle* e);

util/cache.cc

@@ -147,6 +147,7 @@ class LRUCache {
   Cache::Handle* Lookup(const Slice& key, uint32_t hash);
   void Release(Cache::Handle* handle);
   void Erase(const Slice& key, uint32_t hash);
+  void Prune();
 
  private:
   void LRU_Remove(LRUHandle* e);
@@ -264,6 +265,19 @@ void LRUCache::Erase(const Slice& key, uint32_t hash) {
   }
 }
 
+void LRUCache::Prune() {
+  MutexLock l(&mutex_);
+  for (LRUHandle* e = lru_.next; e != &lru_; ) {
+    LRUHandle* next = e->next;
+    if (e->refs == 1) {
+      table_.Remove(e->key(), e->hash);
+      LRU_Remove(e);
+      Unref(e);
+    }
+    e = next;
+  }
+}
+
 static const int kNumShardBits = 4;
 static const int kNumShards = 1 << kNumShardBits;
 
@@ -314,6 +328,11 @@ class ShardedLRUCache : public Cache {
     MutexLock l(&id_mutex_);
     return ++(last_id_);
   }
+  virtual void Prune() {
+    for (int s = 0; s < kNumShards; s++) {
+      shard_[s].Prune();
+    }
+  }
 };
 
 }  // end anonymous namespace

util/cache_test.cc

@@ -179,6 +179,19 @@ TEST(CacheTest, NewId) {
   ASSERT_NE(a, b);
 }
 
+TEST(CacheTest, Prune) {
+  Insert(1, 100);
+  Insert(2, 200);
+
+  Cache::Handle* handle = cache_->Lookup(EncodeKey(1));
+  ASSERT_TRUE(handle);
+  cache_->Prune();
+  cache_->Release(handle);
+
+  ASSERT_EQ(100, Lookup(1));
+  ASSERT_EQ(-1, Lookup(2));
+}
+
 }  // namespace leveldb
 
 int main(int argc, char** argv) {