fix assertion failure in Prev()

Summary:
fix assertion failure in db_stress.
It happens because of prefix seek key is larger than merge iterator key when they have the same user key

Test Plan: ./db_stress --max_background_compactions=1 --max_write_buffer_number=3 --sync=0 --reopen=20 --write_buffer_size=33554432 --delpercent=5 --log2_keys_per_lock=10 --block_size=16384 --allow_concurrent_memtable_write=0 --test_batches_snapshots=0 --max_bytes_for_level_base=67108864 --progress_reports=0 --mmap_read=0 --writepercent=35 --disable_data_sync=0 --readpercent=50 --subcompactions=4 --ops_per_thread=20000000 --memtablerep=skip_list --prefix_size=0 --target_file_size_multiplier=1 --column_families=1 --threads=32 --disable_wal=0 --open_files=500000 --destroy_db_initially=0 --target_file_size_base=16777216 --nooverwritepercent=1 --iterpercent=10 --max_key=100000000 --prefixpercent=0 --use_clock_cache=false --kill_random_test=888887 --cache_size=1048576 --verify_checksum=1

Reviewers: sdong, andrewkr, yiwu, yhchiang

Reviewed By: yhchiang

Subscribers: andrewkr, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D65025

db/db_iter.cc

@@ -512,8 +512,7 @@ void DBIter::ReverseToForward() {
     IterKey last_key;
     last_key.SetInternalKey(ParsedInternalKey(
         saved_key_.GetKey(), kMaxSequenceNumber, kValueTypeForSeek));
-    Slice db_iter_key = last_key.GetKey();
-    iter_->ResetPrefixSeekKey(&db_iter_key);
+    iter_->Seek(last_key.GetKey());
   }
   FindNextUserKey();
   direction_ = kForward;
@@ -527,8 +526,7 @@ void DBIter::ReverseToBackward() {
     IterKey last_key;
     last_key.SetInternalKey(
         ParsedInternalKey(saved_key_.GetKey(), 0, kValueTypeForSeekForPrev));
-    Slice db_iter_key = last_key.GetKey();
-    iter_->ResetPrefixSeekKey(&db_iter_key);
+    iter_->SeekForPrev(last_key.GetKey());
   }
   if (current_entry_is_merged_) {
     // Not placed in the same key. Need to call Prev() until finding the

db/db_iterator_test.cc

@@ -666,7 +666,8 @@ TEST_F(DBIteratorTest, IterMultiWithDelete) {
       // TODO: merge operator does not support backward iteration yet
       if (kPlainTableAllBytesPrefix != option_config_ &&
           kBlockBasedTableWithWholeKeyHashIndex != option_config_ &&
-          kHashLinkList != option_config_) {
+          kHashLinkList != option_config_ &&
+          kHashSkipList != option_config_) {  // doesn't support SeekToLast
         iter->Prev();
         ASSERT_EQ(IterStatus(iter), "ka->va");
       }
@@ -732,7 +733,7 @@ TEST_F(DBIteratorTest, IterWithSnapshot) {
       // TODO: merge operator does not support backward iteration yet
       if (kPlainTableAllBytesPrefix != option_config_ &&
           kBlockBasedTableWithWholeKeyHashIndex != option_config_ &&
-          kHashLinkList != option_config_) {
+          kHashLinkList != option_config_ && kHashSkipList != option_config_) {
         iter->Prev();
         ASSERT_EQ(IterStatus(iter), "key4->val4");
         iter->Prev();
@@ -751,7 +752,7 @@ TEST_F(DBIteratorTest, IterWithSnapshot) {
       // TODO(gzh): merge operator does not support backward iteration yet
       if (kPlainTableAllBytesPrefix != option_config_ &&
           kBlockBasedTableWithWholeKeyHashIndex != option_config_ &&
-          kHashLinkList != option_config_) {
+          kHashLinkList != option_config_ && kHashSkipList != option_config_) {
         iter->SeekForPrev("key1");
         ASSERT_EQ(IterStatus(iter), "key1->val1");
         iter->Next();

table/internal_iterator.h

@@ -94,12 +94,6 @@ class InternalIterator : public Cleanable {
   virtual Status GetProperty(std::string prop_name, std::string* prop) {
     return Status::NotSupported("");
   }
-  // Reset the key used for Seek() in merge iterator, especially for prefix
-  // seek mode
-  // When in prefix_seek_mode, there is inconsistency between db_iterator and
-  // merge iterator. This inconsistency can cause problem when do Seek() in
-  // merge iterator in prefix mode.
-  virtual void ResetPrefixSeekKey(const Slice* prefix_seek_key = nullptr) {}
 
  protected:
   void SeekForPrevImpl(const Slice& target, const Comparator* cmp) {

table/merger.cc

@@ -158,7 +158,7 @@ class MergingIterator : public InternalIterator {
       ClearHeaps();
       for (auto& child : children_) {
         if (&child != current_) {
-          child.Seek(prefix_seek_key_ ? *prefix_seek_key_ : key());
+          child.Seek(key());
           if (child.Valid() && comparator_->Equal(key(), child.key())) {
             child.Next();
           }
@@ -204,7 +204,7 @@ class MergingIterator : public InternalIterator {
       InitMaxHeap();
       for (auto& child : children_) {
         if (&child != current_) {
-          child.SeekForPrev(prefix_seek_key_ ? *prefix_seek_key_ : key());
+          child.SeekForPrev(key());
           if (child.Valid() && comparator_->Equal(key(), child.key())) {
             child.Prev();
           }
@@ -277,17 +277,6 @@ class MergingIterator : public InternalIterator {
            current_->IsValuePinned();
   }
 
-  virtual void ResetPrefixSeekKey(const Slice* db_iter_key) override {
-    if (db_iter_key == nullptr) {
-      prefix_seek_key_.reset();
-      return;
-    }
-    if (!prefix_seek_key_) {
-      prefix_seek_key_.reset(new std::string);
-    }
-    *prefix_seek_key_ = db_iter_key->ToString();
-  }
-
  private:
   // Clears heaps for both directions, used when changing direction or seeking
   void ClearHeaps();
@@ -314,7 +303,6 @@ class MergingIterator : public InternalIterator {
   // forward.  Lazily initialize it to save memory.
   std::unique_ptr<MergerMaxIterHeap> maxHeap_;
   PinnedIteratorsManager* pinned_iters_mgr_;
-  std::unique_ptr<std::string> prefix_seek_key_;
 
   IteratorWrapper* CurrentForward() const {
     assert(direction_ == kForward);