Expose GetTtl() API in TTL DB (facebook#13790)

Summary:
As title

Pull Request resolved: facebook#13790

Test Plan:
```
./ttl_test --gtest_filter="*TtlTest.ChangeTtlOnOpenDb*"
```

Reviewed By: cbi42

Differential Revision: D78670347

Pulled By: jaykorean

fbshipit-source-id: 1b2538d6cd0f2a0fbf397a5d2f677852f97272c4

Author: jaykorean

include/rocksdb/utilities/db_ttl.h

@@ -63,6 +63,8 @@ class DBWithTTL : public StackableDB {
 
   virtual void SetTtl(ColumnFamilyHandle* h, int32_t ttl) = 0;
 
+  virtual Status GetTtl(ColumnFamilyHandle* h, int32_t* ttl) = 0;
+
  protected:
   explicit DBWithTTL(DB* db) : StackableDB(db) {}
 };

unreleased_history/public_api_changes/get_ttl_in_ttl_db.md

@@ -0,0 +1 @@
+GetTtl() API is now available in TTL DB

utilities/ttl/db_ttl_impl.cc

@@ -635,4 +635,22 @@ void DBWithTTLImpl::SetTtl(ColumnFamilyHandle* h, int32_t ttl) {
   filter->SetTtl(ttl);
 }
 
+Status DBWithTTLImpl::GetTtl(ColumnFamilyHandle* h, int32_t* ttl) {
+  if (h == nullptr || ttl == nullptr) {
+    return Status::InvalidArgument(
+        "column family handle or ttl cannot be null");
+  }
+  std::shared_ptr<TtlCompactionFilterFactory> filter;
+  Options opts;
+  opts = GetOptions(h);
+  filter = std::static_pointer_cast<TtlCompactionFilterFactory>(
+      opts.compaction_filter_factory);
+  if (!filter) {
+    return Status::InvalidArgument(
+        "TTLCompactionFilterFactory is not set for TTLDB");
+  }
+  *ttl = filter->GetTtl();
+  return Status::OK();
+}
+
 }  // namespace ROCKSDB_NAMESPACE

utilities/ttl/db_ttl_impl.h

@@ -100,6 +100,8 @@ class DBWithTTLImpl : public DBWithTTL {
 
   void SetTtl(ColumnFamilyHandle* h, int32_t ttl) override;
 
+  Status GetTtl(ColumnFamilyHandle* h, int32_t* ttl) override;
+
  private:
   // remember whether the Close completes or not
   bool closed_;
@@ -184,6 +186,7 @@ class TtlCompactionFilterFactory : public CompactionFilterFactory {
   std::unique_ptr<CompactionFilter> CreateCompactionFilter(
       const CompactionFilter::Context& context) override;
   void SetTtl(int32_t ttl) { ttl_ = ttl; }
+  int32_t GetTtl() { return ttl_; }
 
   const char* Name() const override { return kClassName(); }
   static const char* kClassName() { return "TtlCompactionFilterFactory"; }

utilities/ttl/ttl_test.cc

@@ -720,6 +720,9 @@ TEST_F(TtlTest, ChangeTtlOnOpenDb) {
 
   OpenTtl(1);  // T=0:Open the db with ttl = 2
   SetTtl(3);
+  int32_t ttl = 0;
+  ASSERT_OK(db_ttl_->GetTtl(db_ttl_->DefaultColumnFamily(), &ttl));
+  ASSERT_EQ(ttl, 3);
   PutValues(0, kSampleSize_);  // T=0:Insert Set1. Delete at t=2
   SleepCompactCheck(2, 0, kSampleSize_, true);  // T=2:Set1 should be there
   CloseTtl();