Fix another bug caused by recent hash index fix (#6305)

Summary:
Recent bug fix related to hash index introduced a new bug: hash index can return NotFound but it is not handled by BlockBasedTable::Get(). The end result is that Get() stops being executed too early. Fix it by ignoring NotFound code in Get().
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6305

Test Plan: A problematic DB used to return NotFound incorrectly, and now able to return correct result. Will try to construct a unit test too.0

Differential Revision: D19438925

fbshipit-source-id: e751afa8c13728d56511cfeb1bc811ecb99f3217

db/db_test2.cc

@@ -4362,6 +4362,48 @@ TEST_F(DBTest2, BlockBasedTablePrefixIndexSeekForPrev) {
   }
 }
 
+TEST_F(DBTest2, BlockBasedTablePrefixGetIndexNotFound) {
+  // create a DB with block prefix index
+  BlockBasedTableOptions table_options;
+  Options options = CurrentOptions();
+  table_options.block_size = 300;
+  table_options.index_type = BlockBasedTableOptions::kHashSearch;
+  table_options.index_shortening =
+      BlockBasedTableOptions::IndexShorteningMode::kNoShortening;
+  options.table_factory.reset(NewBlockBasedTableFactory(table_options));
+  options.prefix_extractor.reset(NewFixedPrefixTransform(1));
+  options.level0_file_num_compaction_trigger = 8;
+
+  Reopen(options);
+
+  ASSERT_OK(Put("b1", "ok"));
+  Flush();
+
+  // Flushing several files so that the chance that hash bucket
+  // is empty fo "b" in at least one of the files is high.
+  ASSERT_OK(Put("a1", ""));
+  ASSERT_OK(Put("c1", ""));
+  Flush();
+
+  ASSERT_OK(Put("a2", ""));
+  ASSERT_OK(Put("c2", ""));
+  Flush();
+
+  ASSERT_OK(Put("a3", ""));
+  ASSERT_OK(Put("c3", ""));
+  Flush();
+
+  ASSERT_OK(Put("a4", ""));
+  ASSERT_OK(Put("c4", ""));
+  Flush();
+
+  ASSERT_OK(Put("a5", ""));
+  ASSERT_OK(Put("c5", ""));
+  Flush();
+
+  ASSERT_EQ("ok", Get("b1"));
+}
+
 }  // namespace rocksdb
 
 #ifdef ROCKSDB_UNITTESTS_WITH_CUSTOM_OBJECTS_FROM_STATIC_LIBS

table/block_based/block_based_table_reader.cc

@@ -3467,7 +3467,7 @@ Status BlockBasedTable::Get(const ReadOptions& read_options, const Slice& key,
       PERF_COUNTER_BY_LEVEL_ADD(bloom_filter_full_true_positive, 1,
                                 rep_->level);
     }
-    if (s.ok()) {
+    if (s.ok() && !iiter->status().IsNotFound()) {
       s = iiter->status();
     }
   }