Expose unix time in rocksdb::Snapshot (#9923)

Summary:
RocksDB snapshot already has a member unix_time_ set after
snapshot is taken. It is now exposed through GetSnapshotTime() API.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9923

Test Plan: Update unit tests

Reviewed By: riversand963

Differential Revision: D36048275

Pulled By: akankshamahajan15

fbshipit-source-id: 825210ec287deb0bc3aaa9b8e1f079f07ad686fa

HISTORY.md

 # Rocksdb Change Log
 ## Unreleased
+### Public API changes
+* Add new API GetUnixTime in Snapshot class which returns the unix time at which Snapshot is taken.
 
 ## 7.3.0 (05/20/2022)
 ### Bug Fixes

db/db_test.cc

@@ -1844,6 +1844,8 @@ TEST_F(DBTest, Snapshot) {
     uint64_t time_snap1 = GetTimeOldestSnapshots();
     ASSERT_GT(time_snap1, 0U);
     ASSERT_EQ(GetSequenceOldestSnapshots(), s1->GetSequenceNumber());
+    ASSERT_EQ(GetTimeOldestSnapshots(),
+              static_cast<uint64_t>(s1->GetUnixTime()));
     ASSERT_OK(Put(0, "foo", "0v2"));
     ASSERT_OK(Put(1, "foo", "1v2"));
 
@@ -1853,6 +1855,8 @@ TEST_F(DBTest, Snapshot) {
     ASSERT_EQ(2U, GetNumSnapshots());
     ASSERT_EQ(time_snap1, GetTimeOldestSnapshots());
     ASSERT_EQ(GetSequenceOldestSnapshots(), s1->GetSequenceNumber());
+    ASSERT_EQ(GetTimeOldestSnapshots(),
+              static_cast<uint64_t>(s1->GetUnixTime()));
     ASSERT_OK(Put(0, "foo", "0v3"));
     ASSERT_OK(Put(1, "foo", "1v3"));
 
@@ -1861,6 +1865,8 @@ TEST_F(DBTest, Snapshot) {
       ASSERT_EQ(3U, GetNumSnapshots());
       ASSERT_EQ(time_snap1, GetTimeOldestSnapshots());
       ASSERT_EQ(GetSequenceOldestSnapshots(), s1->GetSequenceNumber());
+      ASSERT_EQ(GetTimeOldestSnapshots(),
+                static_cast<uint64_t>(s1->GetUnixTime()));
 
       ASSERT_OK(Put(0, "foo", "0v4"));
       ASSERT_OK(Put(1, "foo", "1v4"));
@@ -1877,6 +1883,8 @@ TEST_F(DBTest, Snapshot) {
     ASSERT_EQ(2U, GetNumSnapshots());
     ASSERT_EQ(time_snap1, GetTimeOldestSnapshots());
     ASSERT_EQ(GetSequenceOldestSnapshots(), s1->GetSequenceNumber());
+    ASSERT_EQ(GetTimeOldestSnapshots(),
+              static_cast<uint64_t>(s1->GetUnixTime()));
     ASSERT_EQ("0v1", Get(0, "foo", s1));
     ASSERT_EQ("1v1", Get(1, "foo", s1));
     ASSERT_EQ("0v2", Get(0, "foo", s2));
@@ -1892,6 +1900,8 @@ TEST_F(DBTest, Snapshot) {
     ASSERT_EQ(1U, GetNumSnapshots());
     ASSERT_LT(time_snap1, GetTimeOldestSnapshots());
     ASSERT_EQ(GetSequenceOldestSnapshots(), s2->GetSequenceNumber());
+    ASSERT_EQ(GetTimeOldestSnapshots(),
+              static_cast<uint64_t>(s2->GetUnixTime()));
 
     db_->ReleaseSnapshot(s2);
     ASSERT_EQ(0U, GetNumSnapshots());
@@ -2859,6 +2869,12 @@ class ModelDB : public DB {
       assert(false);
       return 0;
     }
+
+    int64_t GetUnixTime() const override {
+      // no need to call this
+      assert(false);
+      return 0;
+    }
   };
 
   explicit ModelDB(const Options& options) : options_(options) {}

db/snapshot_impl.h

@@ -27,7 +27,9 @@ class SnapshotImpl : public Snapshot {
   // scope of queries to IsInSnapshot.
   SequenceNumber min_uncommitted_ = kMinUnCommittedSeq;
 
-  virtual SequenceNumber GetSequenceNumber() const override { return number_; }
+  SequenceNumber GetSequenceNumber() const override { return number_; }
+
+  int64_t GetUnixTime() const override { return unix_time_; }
 
  private:
   friend class SnapshotList;

include/rocksdb/snapshot.h

@@ -19,9 +19,12 @@ class DB;
 // To Destroy a Snapshot, call DB::ReleaseSnapshot(snapshot).
 class Snapshot {
  public:
-  // returns Snapshot's sequence number
   virtual SequenceNumber GetSequenceNumber() const = 0;
 
+  // Returns unix time i.e. the number of seconds since the Epoch, 1970-01-01
+  // 00:00:00 (UTC).
+  virtual int64_t GetUnixTime() const = 0;
+
  protected:
   virtual ~Snapshot();
 };