simple ManagedSnapshot wrapper

Summary: Implemented this simple wrapper for something else I was working on.  Seemed like it makes sense to expose it instead of burying it in some random code.

Test Plan: added test

Reviewers: rven, kradhakrishnan, sdong, yhchiang

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D43293

db/db_impl.h

@@ -19,11 +19,11 @@
 #include <string>
 
 #include "db/dbformat.h"
-#include "db/log_writer.h"
-#include "db/snapshot.h"
 #include "db/column_family.h"
 #include "db/compaction_job.h"
 #include "db/flush_job.h"
+#include "db/log_writer.h"
+#include "db/snapshot_impl.h"
 #include "db/version_edit.h"
 #include "db/wal_manager.h"
 #include "db/writebuffer.h"

db/db_test.cc

@@ -39,11 +39,12 @@
 #include "rocksdb/env.h"
 #include "rocksdb/experimental.h"
 #include "rocksdb/filter_policy.h"
+#include "rocksdb/options.h"
 #include "rocksdb/perf_context.h"
 #include "rocksdb/slice.h"
 #include "rocksdb/slice_transform.h"
+#include "rocksdb/snapshot.h"
 #include "rocksdb/table.h"
-#include "rocksdb/options.h"
 #include "rocksdb/table_properties.h"
 #include "rocksdb/thread_status.h"
 #include "rocksdb/utilities/write_batch_with_index.h"
@@ -3026,22 +3027,23 @@ TEST_F(DBTest, Snapshot) {
     Put(0, "foo", "0v3");
     Put(1, "foo", "1v3");
 
-    const Snapshot* s3 = db_->GetSnapshot();
-    ASSERT_EQ(3U, GetNumSnapshots());
-    ASSERT_EQ(time_snap1, GetTimeOldestSnapshots());
-
-    Put(0, "foo", "0v4");
-    Put(1, "foo", "1v4");
-    ASSERT_EQ("0v1", Get(0, "foo", s1));
-    ASSERT_EQ("1v1", Get(1, "foo", s1));
-    ASSERT_EQ("0v2", Get(0, "foo", s2));
-    ASSERT_EQ("1v2", Get(1, "foo", s2));
-    ASSERT_EQ("0v3", Get(0, "foo", s3));
-    ASSERT_EQ("1v3", Get(1, "foo", s3));
-    ASSERT_EQ("0v4", Get(0, "foo"));
-    ASSERT_EQ("1v4", Get(1, "foo"));
+    {
+      ManagedSnapshot s3(db_);
+      ASSERT_EQ(3U, GetNumSnapshots());
+      ASSERT_EQ(time_snap1, GetTimeOldestSnapshots());
+
+      Put(0, "foo", "0v4");
+      Put(1, "foo", "1v4");
+      ASSERT_EQ("0v1", Get(0, "foo", s1));
+      ASSERT_EQ("1v1", Get(1, "foo", s1));
+      ASSERT_EQ("0v2", Get(0, "foo", s2));
+      ASSERT_EQ("1v2", Get(1, "foo", s2));
+      ASSERT_EQ("0v3", Get(0, "foo", s3.snapshot()));
+      ASSERT_EQ("1v3", Get(1, "foo", s3.snapshot()));
+      ASSERT_EQ("0v4", Get(0, "foo"));
+      ASSERT_EQ("1v4", Get(1, "foo"));
+    }
 
-    db_->ReleaseSnapshot(s3);
     ASSERT_EQ(2U, GetNumSnapshots());
     ASSERT_EQ(time_snap1, GetTimeOldestSnapshots());
     ASSERT_EQ("0v1", Get(0, "foo", s1));

db/flush_job.h

@@ -17,11 +17,11 @@
 #include <string>
 
 #include "db/dbformat.h"
-#include "db/log_writer.h"
-#include "db/snapshot.h"
 #include "db/column_family.h"
-#include "db/version_edit.h"
+#include "db/log_writer.h"
 #include "db/memtable_list.h"
+#include "db/snapshot_impl.h"
+#include "db/version_edit.h"
 #include "port/port.h"
 #include "rocksdb/db.h"
 #include "rocksdb/env.h"

db/snapshot_impl.cc

@@ -0,0 +1,23 @@
+//  Copyright (c) 2015, Facebook, Inc.  All rights reserved.
+//  This source code is licensed under the BSD-style license found in the
+//  LICENSE file in the root directory of this source tree. An additional grant
+//  of patent rights can be found in the PATENTS file in the same directory.
+
+#include "rocksdb/snapshot.h"
+
+#include "rocksdb/db.h"
+
+namespace rocksdb {
+
+ManagedSnapshot::ManagedSnapshot(DB* db) : db_(db),
+                                           snapshot_(db->GetSnapshot()) {}
+
+ManagedSnapshot::~ManagedSnapshot() {
+  if (snapshot_) {
+    db_->ReleaseSnapshot(snapshot_);
+  }
+}
+
+const Snapshot* ManagedSnapshot::snapshot() { return snapshot_;}
+
+}  // namespace rocksdb

db/write_batch.cc

@@ -32,7 +32,7 @@
 #include "db/db_impl.h"
 #include "db/column_family.h"
 #include "db/memtable.h"
-#include "db/snapshot.h"
+#include "db/snapshot_impl.h"
 #include "db/write_batch_internal.h"
 #include "util/coding.h"
 #include "util/statistics.h"

include/rocksdb/db.h

@@ -22,6 +22,7 @@
 #include "rocksdb/types.h"
 #include "rocksdb/transaction_log.h"
 #include "rocksdb/listener.h"
+#include "rocksdb/snapshot.h"
 #include "rocksdb/thread_status.h"
 
 #ifdef _WIN32
@@ -68,18 +69,6 @@ struct ColumnFamilyDescriptor {
 static const int kMajorVersion = __ROCKSDB_MAJOR__;
 static const int kMinorVersion = __ROCKSDB_MINOR__;
 
-// Abstract handle to particular state of a DB.
-// A Snapshot is an immutable object and can therefore be safely
-// accessed from multiple threads without any external synchronization.
-class Snapshot {
- public:
-  // returns Snapshot's sequence number
-  virtual SequenceNumber GetSequenceNumber() const = 0;
-
- protected:
-  virtual ~Snapshot();
-};
-
 // A range of keys
 struct Range {
   Slice start;          // Included in the range

include/rocksdb/snapshot.h

@@ -0,0 +1,45 @@
+//  Copyright (c) 2015, Facebook, Inc.  All rights reserved.
+//  This source code is licensed under the BSD-style license found in the
+//  LICENSE file in the root directory of this source tree. An additional grant
+//  of patent rights can be found in the PATENTS file in the same directory.
+
+#pragma once
+
+#include "rocksdb/types.h"
+
+namespace rocksdb {
+
+class DB;
+
+// Abstract handle to particular state of a DB.
+// A Snapshot is an immutable object and can therefore be safely
+// accessed from multiple threads without any external synchronization.
+//
+// To Create a Snapshot, call DB::GetSnapshot().
+// To Destroy a Snapshot, call DB::ReleaseSnapshot(snapshot).
+class Snapshot {
+ public:
+  // returns Snapshot's sequence number
+  virtual SequenceNumber GetSequenceNumber() const = 0;
+
+ protected:
+  virtual ~Snapshot();
+};
+
+// Simple RAII wrapper class for Snapshot.
+// Constructing this object will create a snapshot.  Destructing will
+// release the snapshot.
+class ManagedSnapshot {
+ public:
+  explicit ManagedSnapshot(DB* db);
+
+  ~ManagedSnapshot();
+
+  const Snapshot* snapshot();
+
+ private:
+  DB* db_;
+  const Snapshot* snapshot_;
+};
+
+}  // namespace rocksdb

src.mk

@@ -33,6 +33,7 @@ LIB_SOURCES =                                                   \
   db/merge_operator.cc                                          \
   db/repair.cc                                                  \
   db/slice.cc                                                   \
+  db/snapshot_impl.cc                                           \
   db/table_cache.cc                                             \
   db/table_properties_collector.cc                              \
   db/transaction_log_impl.cc                                    \