From: Steve Vinoski <vinoski@ieee.org>
Date: Thu, 20 Dec 2012 16:14:11 -0500
Subject: [PATCH] allow Get() calls to avoid copies into std::string

Add a new abstract base class leveldb::Value that applications can easily
derive from to supply their own memory management for values retrieved via
Get(). Add an internal class derived from Value that provides std::string
management to preserve backward compatibility. Overload DBImpl::Get() to
accept a Value*, and to preserve backward compatibility also keep the
original version taking a std::string*.

diff --git a/db/db_impl.cc b/db/db_impl.cc
index ae7b96d..5c3a05c 100644
--- a/db/db_impl.cc
+++ b/db/db_impl.cc
@@ -85,6 +85,22 @@ struct DBImpl::CompactionState {
   uint64_t total_bytes;
 };
 
+Value::~Value() {}
+
+class StringValue : public Value {
+ public:
+  explicit StringValue(std::string& val) : value_(val) {}
+  ~StringValue() {}
+
+  StringValue& assign(const char* data, size_t size) {
+    value_.assign(data, size);
+    return *this;
+  }
+
+ private:
+  std::string& value_;
+};
+
 // Fix user-supplied options to be reasonable
 template <class T, class V>
 static void ClipToRange(T* ptr, V minvalue, V maxvalue) {
@@ -1117,6 +1133,13 @@ int64_t DBImpl::TEST_MaxNextLevelOverlappingBytes() {
 
 Status DBImpl::Get(const ReadOptions& options, const Slice& key,
                    std::string* value) {
+  StringValue stringvalue(*value);
+  return DBImpl::Get(options, key, &stringvalue);
+}
+
+Status DBImpl::Get(const ReadOptions& options,
+                   const Slice& key,
+                   Value* value) {
   Status s;
   MutexLock l(&mutex_);
   SequenceNumber snapshot;
diff --git a/db/db_impl.h b/db/db_impl.h
index d955c2a..3127110 100644
--- a/db/db_impl.h
+++ b/db/db_impl.h
@@ -42,6 +42,9 @@ class DBImpl : public DB {
   Status Write(const WriteOptions& options, WriteBatch* updates) override;
   Status Get(const ReadOptions& options, const Slice& key,
              std::string* value) override;
+  virtual Status Get(const ReadOptions& options,
+	              const Slice& key,
+	              Value* value);
   Iterator* NewIterator(const ReadOptions&) override;
   const Snapshot* GetSnapshot() override;
   void ReleaseSnapshot(const Snapshot* snapshot) override;
diff --git a/db/db_test.cc b/db/db_test.cc
index 2e65370..db778d9 100644
--- a/db/db_test.cc
+++ b/db/db_test.cc
@@ -2065,6 +2065,11 @@ class ModelDB : public DB {
     assert(false);  // Not implemented
     return Status::NotFound(key);
   }
+  Status Get(const ReadOptions& options,
+	     const Slice& key, Value* value) override {
+    assert(false);      // Not implemented
+    return Status::NotFound(key);
+  }
   Iterator* NewIterator(const ReadOptions& options) override {
     if (options.snapshot == nullptr) {
       KVMap* saved = new KVMap;
diff --git a/db/memtable.cc b/db/memtable.cc
index f42774d..4689e2d 100644
--- a/db/memtable.cc
+++ b/db/memtable.cc
@@ -98,7 +98,7 @@ void MemTable::Add(SequenceNumber s, ValueType type, const Slice& key,
   table_.Insert(buf);
 }
 
-bool MemTable::Get(const LookupKey& key, std::string* value, Status* s) {
+bool MemTable::Get(const LookupKey& key, Value* value, Status* s) {
   Slice memkey = key.memtable_key();
   Table::Iterator iter(&table_);
   iter.Seek(memkey.data());
diff --git a/db/memtable.h b/db/memtable.h
index 9d986b1..85c4cce 100644
--- a/db/memtable.h
+++ b/db/memtable.h
@@ -60,7 +60,7 @@ class MemTable {
   // If memtable contains a deletion for key, store a NotFound() error
   // in *status and return true.
   // Else, return false.
-  bool Get(const LookupKey& key, std::string* value, Status* s);
+  bool Get(const LookupKey& key, Value* value, Status* s);
 
  private:
   friend class MemTableIterator;
diff --git a/db/version_set.cc b/db/version_set.cc
index 1963353..c83a4d2 100644
--- a/db/version_set.cc
+++ b/db/version_set.cc
@@ -256,7 +256,7 @@ struct Saver {
   SaverState state;
   const Comparator* ucmp;
   Slice user_key;
-  std::string* value;
+  Value* value;
 };
 }  // namespace
 static void SaveValue(void* arg, const Slice& ikey, const Slice& v) {
@@ -322,7 +322,7 @@ void Version::ForEachOverlapping(Slice user_key, Slice internal_key, void* arg,
 }
 
 Status Version::Get(const ReadOptions& options, const LookupKey& k,
-                    std::string* value, GetStats* stats) {
+                    Value* value, GetStats* stats) {
   stats->seek_file = nullptr;
   stats->seek_file_level = -1;
 
diff --git a/db/version_set.h b/db/version_set.h
index 69f3d70..0f0a463 100644
--- a/db/version_set.h
+++ b/db/version_set.h
@@ -72,7 +72,7 @@ class Version {
   // REQUIRES: This version has been saved (see VersionSet::SaveTo)
   void AddIterators(const ReadOptions&, std::vector<Iterator*>* iters);
 
-  Status Get(const ReadOptions&, const LookupKey& key, std::string* val,
+  Status Get(const ReadOptions&, const LookupKey& key, Value* val,
              GetStats* stats);
 
   // Adds "stats" into the current state.  Returns true if a new
diff --git a/include/leveldb/db.h b/include/leveldb/db.h
index 61c29c0..1a93feb 100644
--- a/include/leveldb/db.h
+++ b/include/leveldb/db.h
@@ -40,6 +40,17 @@ struct LEVELDB_EXPORT Range {
   Slice limit;  // Not included in the range
 };
 
+// Abstract holder for a DB value.
+// This allows callers to manage their own value buffers and have
+// DB values copied directly into those buffers.
+class Value {
+ public:
+  virtual Value& assign(const char* data, size_t size) = 0;
+
+ protected:
+  virtual ~Value();
+};
+
 // A DB is a persistent ordered map from keys to values.
 // A DB is safe for concurrent access from multiple threads without
 // any external synchronization.
