set compaction_iterator earliest_snapshot to max if no snapshot

Summary:
It is a potential bug that will be triggered if we ingest files before inserting the first key into an empty db.
0 is a special value reserved to indicate the concept of non-existence. But not good for seqno in this case because 0 is a valid seqno for ingestion(bulk loading)
Closes https://github.com/facebook/rocksdb/pull/2183

Differential Revision: D4919827

Pulled By: lightmark

fbshipit-source-id: 237eea40f88bd6487b66806109d90065dc02c362

db/compaction_iterator.cc

@@ -82,7 +82,7 @@ CompactionIterator::CompactionIterator(
   if (snapshots_->size() == 0) {
     // optimize for fast path if there are no snapshots
     visible_at_tip_ = true;
-    earliest_snapshot_ = last_sequence;
+    earliest_snapshot_ = kMaxSequenceNumber;
     latest_snapshot_ = 0;
   } else {
     visible_at_tip_ = false;
@@ -543,7 +543,7 @@ void CompactionIterator::PrepareOutput() {
 
   // This is safe for TransactionDB write-conflict checking since transactions
   // only care about sequence number larger than any active snapshots.
-  if (bottommost_level_ && valid_ && ikey_.sequence < earliest_snapshot_ &&
+  if (bottommost_level_ && valid_ && ikey_.sequence <= earliest_snapshot_ &&
       ikey_.type != kTypeMerge &&
       !cmp_->Equal(compaction_->GetLargestUserKey(), ikey_.user_key)) {
     assert(ikey_.type != kTypeDeletion && ikey_.type != kTypeSingleDeletion);

db/db_compaction_filter_test.cc

@@ -626,7 +626,7 @@ TEST_F(DBTestCompactionFilter, CompactionFilterContextManual) {
       iter->Next();
     }
     ASSERT_EQ(total, 700);
-    ASSERT_EQ(count, 2);
+    ASSERT_EQ(count, 1);
   }
 }
 #endif  // ROCKSDB_LITE