Fix lower bound check error when iterate across file boundary (#5540)

Summary:
Since https://github.com/facebook/rocksdb/issues/5468 `LevelIterator` compare lower bound and file smallest key on `NewFileIterator` and cache the result to reduce per key lower bound check. However when iterate across file boundary, it doesn't update the cached result since `Valid()=false` because `Valid()` still reflect the status of the previous file iterator. Fixing it by remove the `Valid()` check from `CheckMayBeOutOfLowerBound()`.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5540

Test Plan:
See the new test.

Signed-off-by: Yi Wu <yiwu@pingcap.com>

Differential Revision: D16127653

fbshipit-source-id: a0691e1164658d485c17971aaa97028812f74678

db/db_iterator_test.cc

@@ -2821,6 +2821,32 @@ TEST_P(DBIteratorTest, IterateBoundChangedBeforeSeek) {
   delete iter;
 }
 
+TEST_P(DBIteratorTest, IterateWithLowerBoundAcrossFileBoundary) {
+  ASSERT_OK(Put("aaa", "v"));
+  ASSERT_OK(Put("bbb", "v"));
+  ASSERT_OK(Flush());
+  ASSERT_OK(Put("ccc", "v"));
+  ASSERT_OK(Put("ddd", "v"));
+  ASSERT_OK(Flush());
+  // Move both files to bottom level.
+  ASSERT_OK(dbfull()->CompactRange(CompactRangeOptions(), nullptr, nullptr));
+  Slice lower_bound("b");
+  ReadOptions read_opts;
+  read_opts.iterate_lower_bound = &lower_bound;
+  std::unique_ptr<Iterator> iter(NewIterator(read_opts));
+  iter->SeekForPrev("d");
+  ASSERT_TRUE(iter->Valid());
+  ASSERT_OK(iter->status());
+  ASSERT_EQ("ccc", iter->key());
+  iter->Prev();
+  ASSERT_TRUE(iter->Valid());
+  ASSERT_OK(iter->status());
+  ASSERT_EQ("bbb", iter->key());
+  iter->Prev();
+  ASSERT_FALSE(iter->Valid());
+  ASSERT_OK(iter->status());
+}
+
 INSTANTIATE_TEST_CASE_P(DBIteratorTestInstance, DBIteratorTest,
                         testing::Values(true, false));
 

db/version_set.cc

@@ -984,7 +984,8 @@ class LevelIterator final : public InternalIterator {
   // Note MyRocks may update iterate bounds between seek. To workaround it,
   // we need to check and update may_be_out_of_lower_bound_ accordingly.
   void CheckMayBeOutOfLowerBound() {
-    if (Valid() && read_options_.iterate_lower_bound != nullptr) {
+    if (read_options_.iterate_lower_bound != nullptr &&
+        file_index_ < flevel_->num_files) {
       may_be_out_of_lower_bound_ =
           user_comparator_.Compare(
               ExtractUserKey(file_smallest_key(file_index_)),