Possibly bump NUMBER_OF_RESEEKS_IN_ITERATION (#8015)

Summary:
When changing db iterator direction, we may perform a reseek.
Therefore, we should bump the NUMBER_OF_RESEEKS_IN_ITERATION counter.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8015

Test Plan: make check

Reviewed By: ltamasi

Differential Revision: D26755415

Pulled By: riversand963

fbshipit-source-id: 211f51f1a454bcda768fc46c0dce51edeb7f05fe

db/db_iter.cc

@@ -681,6 +681,7 @@ bool DBIter::ReverseToForward() {
     last_key.SetInternalKey(ParsedInternalKey(
         saved_key_.GetUserKey(), kMaxSequenceNumber, kValueTypeForSeek));
     iter_.Seek(last_key.GetInternalKey());
+    RecordTick(statistics_, NUMBER_OF_RESEEKS_IN_ITERATION);
   }
 
   direction_ = kForward;
@@ -731,6 +732,7 @@ bool DBIter::ReverseToBackward() {
         iter_.SeekToLast();
       }
     }
+    RecordTick(statistics_, NUMBER_OF_RESEEKS_IN_ITERATION);
   }
 
   direction_ = kReverse;

db/db_iterator_test.cc

@@ -18,6 +18,7 @@
 #include "rocksdb/perf_context.h"
 #include "table/block_based/flush_block_policy.h"
 #include "util/random.h"
+#include "utilities/merge_operators/string_append/stringappend2.h"
 
 namespace ROCKSDB_NAMESPACE {
 
@@ -619,6 +620,40 @@ TEST_P(DBIteratorTest, IterReseek) {
   delete iter;
 }
 
+TEST_F(DBIteratorTest, ReseekUponDirectionChange) {
+  Options options = GetDefaultOptions();
+  options.create_if_missing = true;
+  options.prefix_extractor.reset(NewFixedPrefixTransform(1));
+  options.statistics = ROCKSDB_NAMESPACE::CreateDBStatistics();
+  options.merge_operator.reset(
+      new StringAppendTESTOperator(/*delim_char=*/' '));
+  DestroyAndReopen(options);
+  ASSERT_OK(Put("foo", "value"));
+  ASSERT_OK(Put("bar", "value"));
+  {
+    std::unique_ptr<Iterator> it(db_->NewIterator(ReadOptions()));
+    it->SeekToLast();
+    it->Prev();
+    it->Next();
+  }
+  ASSERT_EQ(1,
+            options.statistics->getTickerCount(NUMBER_OF_RESEEKS_IN_ITERATION));
+
+  const std::string merge_key("good");
+  ASSERT_OK(Put(merge_key, "orig"));
+  ASSERT_OK(Merge(merge_key, "suffix"));
+  {
+    std::unique_ptr<Iterator> it(db_->NewIterator(ReadOptions()));
+    it->Seek(merge_key);
+    ASSERT_TRUE(it->Valid());
+    const uint64_t prev_reseek_count =
+        options.statistics->getTickerCount(NUMBER_OF_RESEEKS_IN_ITERATION);
+    it->Prev();
+    ASSERT_EQ(prev_reseek_count + 1, options.statistics->getTickerCount(
+                                         NUMBER_OF_RESEEKS_IN_ITERATION));
+  }
+}
+
 TEST_P(DBIteratorTest, IterSmallAndLargeMix) {
   do {
     CreateAndReopenWithCF({"pikachu"}, CurrentOptions());
@@ -2997,18 +3032,18 @@ TEST_P(DBIteratorTest, Blob) {
   iter->Prev();
   ASSERT_EQ(TestGetTickerCount(options, NUMBER_OF_RESEEKS_IN_ITERATION), 5);
   iter->Next();
-  ASSERT_EQ(TestGetTickerCount(options, NUMBER_OF_RESEEKS_IN_ITERATION), 5);
+  ASSERT_EQ(TestGetTickerCount(options, NUMBER_OF_RESEEKS_IN_ITERATION), 6);
   ASSERT_EQ(IterStatus(iter), "b->vb3");
 
   // Switch from forward to reverse
   iter->SeekToFirst();
-  ASSERT_EQ(TestGetTickerCount(options, NUMBER_OF_RESEEKS_IN_ITERATION), 5);
+  ASSERT_EQ(TestGetTickerCount(options, NUMBER_OF_RESEEKS_IN_ITERATION), 6);
-  iter->Next();
-  ASSERT_EQ(TestGetTickerCount(options, NUMBER_OF_RESEEKS_IN_ITERATION), 5);
   iter->Next();
   ASSERT_EQ(TestGetTickerCount(options, NUMBER_OF_RESEEKS_IN_ITERATION), 6);
-  iter->Prev();
+  iter->Next();
   ASSERT_EQ(TestGetTickerCount(options, NUMBER_OF_RESEEKS_IN_ITERATION), 7);
+  iter->Prev();
+  ASSERT_EQ(TestGetTickerCount(options, NUMBER_OF_RESEEKS_IN_ITERATION), 8);
   ASSERT_EQ(IterStatus(iter), "b->vb3");
 }