Fix regression bug of Prev() with upper bound (#3989)

Summary:
A recent change pushed down the upper bound checking to child iterators. However, this causes the logic of following sequence wrong:
  Seek(key);
  if (!Valid()) SeekToLast();
Because !Valid() may be caused by upper bounds, rather than the end of the iterator. In this case SeekToLast() points to totally wrong places. This can cause wrong results, infinite loops, or segfault in some cases.
This sequence is called when changing direction from forward to backward. And this by itself also implicitly happen during reseeking optimization in Prev().

Fix this bug by using SeekForPrev() rather than this sequuence, as what is already done in prefix extrator case.
Closes https://github.com/facebook/rocksdb/pull/3989

Differential Revision: D8385422

Pulled By: siying

fbshipit-source-id: 429e869990cfd2dc389421e0836fc496bed67bb4

db/db_iter_test.cc

@@ -36,7 +36,9 @@ class TestIterator : public InternalIterator {
         valid_(false),
         sequence_number_(0),
         iter_(0),
-        cmp(comparator) {}
+        cmp(comparator) {
+    data_.reserve(16);
+  }
 
   void AddPut(std::string argkey, std::string argvalue) {
     Add(argkey, kTypeValue, argvalue);
@@ -2660,7 +2662,7 @@ TEST_F(DBIterWithMergeIterTest, InnerMergeIteratorDataRace1) {
   // MergeIterator::Prev() realized the mem table iterator is at its end
   // and before an SeekToLast() is called.
   rocksdb::SyncPoint::GetInstance()->SetCallBack(
-      "MergeIterator::Prev:BeforeSeekToLast",
+      "MergeIterator::Prev:BeforePrev",
       [&](void* /*arg*/) { internal_iter2_->Add("z", kTypeValue, "7", 12u); });
   rocksdb::SyncPoint::GetInstance()->EnableProcessing();
 
@@ -2696,7 +2698,7 @@ TEST_F(DBIterWithMergeIterTest, InnerMergeIteratorDataRace2) {
   // mem table after MergeIterator::Prev() realized the mem tableiterator is at
   // its end and before an SeekToLast() is called.
   rocksdb::SyncPoint::GetInstance()->SetCallBack(
-      "MergeIterator::Prev:BeforeSeekToLast", [&](void* /*arg*/) {
+      "MergeIterator::Prev:BeforePrev", [&](void* /*arg*/) {
         internal_iter2_->Add("z", kTypeValue, "7", 12u);
         internal_iter2_->Add("z", kTypeValue, "7", 11u);
       });
@@ -2734,7 +2736,7 @@ TEST_F(DBIterWithMergeIterTest, InnerMergeIteratorDataRace3) {
   // mem table after MergeIterator::Prev() realized the mem table iterator is at
   // its end and before an SeekToLast() is called.
   rocksdb::SyncPoint::GetInstance()->SetCallBack(
-      "MergeIterator::Prev:BeforeSeekToLast", [&](void* /*arg*/) {
+      "MergeIterator::Prev:BeforePrev", [&](void* /*arg*/) {
         internal_iter2_->Add("z", kTypeValue, "7", 16u, true);
         internal_iter2_->Add("z", kTypeValue, "7", 15u, true);
         internal_iter2_->Add("z", kTypeValue, "7", 14u, true);

db/db_iterator_test.cc

@@ -2044,6 +2044,43 @@ TEST_P(DBIteratorTest, CreationFailure) {
   delete iter;
 }
 
+TEST_P(DBIteratorTest, UpperBoundWithChangeDirection) {
+  Options options = CurrentOptions();
+  options.max_sequential_skip_in_iterations = 3;
+  DestroyAndReopen(options);
+
+  // write a bunch of kvs to the database.
+  ASSERT_OK(Put("a", "1"));
+  ASSERT_OK(Put("y", "1"));
+  ASSERT_OK(Put("y1", "1"));
+  ASSERT_OK(Put("y2", "1"));
+  ASSERT_OK(Put("y3", "1"));
+  ASSERT_OK(Put("z", "1"));
+  ASSERT_OK(Flush());
+  ASSERT_OK(Put("a", "1"));
+  ASSERT_OK(Put("z", "1"));
+  ASSERT_OK(Put("bar", "1"));
+  ASSERT_OK(Put("foo", "1"));
+
+  std::string upper_bound = "x";
+  Slice ub_slice(upper_bound);
+  ReadOptions ro;
+  ro.iterate_upper_bound = &ub_slice;
+  ro.max_skippable_internal_keys = 1000;
+
+  Iterator* iter = NewIterator(ro);
+  iter->Seek("foo");
+  ASSERT_TRUE(iter->Valid());
+  ASSERT_EQ("foo", iter->key().ToString());
+
+  iter->Prev();
+  ASSERT_TRUE(iter->Valid());
+  ASSERT_OK(iter->status());
+  ASSERT_EQ("bar", iter->key().ToString());
+
+  delete iter;
+}
+
 TEST_P(DBIteratorTest, TableFilter) {
   ASSERT_OK(Put("a", "1"));
   dbfull()->Flush(FlushOptions());
@@ -2110,6 +2147,47 @@ TEST_P(DBIteratorTest, TableFilter) {
   }
 }
 
+TEST_P(DBIteratorTest, UpperBoundWithPrevReseek) {
+  Options options = CurrentOptions();
+  options.max_sequential_skip_in_iterations = 3;
+  DestroyAndReopen(options);
+
+  // write a bunch of kvs to the database.
+  ASSERT_OK(Put("a", "1"));
+  ASSERT_OK(Put("y", "1"));
+  ASSERT_OK(Put("z", "1"));
+  ASSERT_OK(Flush());
+  ASSERT_OK(Put("a", "1"));
+  ASSERT_OK(Put("z", "1"));
+  ASSERT_OK(Put("bar", "1"));
+  ASSERT_OK(Put("foo", "1"));
+  ASSERT_OK(Put("foo", "2"));
+
+  ASSERT_OK(Put("foo", "3"));
+  ASSERT_OK(Put("foo", "4"));
+  ASSERT_OK(Put("foo", "5"));
+  const Snapshot* snapshot = db_->GetSnapshot();
+  ASSERT_OK(Put("foo", "6"));
+
+  std::string upper_bound = "x";
+  Slice ub_slice(upper_bound);
+  ReadOptions ro;
+  ro.snapshot = snapshot;
+  ro.iterate_upper_bound = &ub_slice;
+
+  Iterator* iter = NewIterator(ro);
+  iter->SeekForPrev("goo");
+  ASSERT_TRUE(iter->Valid());
+  ASSERT_EQ("foo", iter->key().ToString());
+  iter->Prev();
+
+  ASSERT_TRUE(iter->Valid());
+  ASSERT_EQ("bar", iter->key().ToString());
+
+  delete iter;
+  db_->ReleaseSnapshot(snapshot);
+}
+
 TEST_P(DBIteratorTest, SkipStatistics) {
   Options options = CurrentOptions();
   options.statistics = rocksdb::CreateDBStatistics();

table/merging_iterator.cc

@@ -228,28 +228,12 @@ class MergingIterator : public InternalIterator {
       Slice target = key();
       for (auto& child : children_) {
         if (&child != current_) {
-          if (!prefix_seek_mode_) {
-            child.Seek(target);
-            if (child.Valid()) {
-              // Child is at first entry >= key().  Step back one to be < key()
-              TEST_SYNC_POINT_CALLBACK("MergeIterator::Prev:BeforePrev",
-                                       &child);
-              assert(child.status().ok());
-              child.Prev();
-            } else {
-              // Child has no entries >= key().  Position at last entry.
-              TEST_SYNC_POINT("MergeIterator::Prev:BeforeSeekToLast");
-              considerStatus(child.status());
-              child.SeekToLast();
-            }
+          child.SeekForPrev(target);
+          TEST_SYNC_POINT_CALLBACK("MergeIterator::Prev:BeforePrev", &child);
+          considerStatus(child.status());
+          if (child.Valid() && comparator_->Equal(target, child.key())) {
+            child.Prev();
             considerStatus(child.status());
-          } else {
-            child.SeekForPrev(target);
-            considerStatus(child.status());
-            if (child.Valid() && comparator_->Equal(target, child.key())) {
-              child.Prev();
-              considerStatus(child.status());
-            }
           }
         }
         if (child.Valid()) {