Fix crash in BlockBasedTableIterator::Seek() (#5291)

Summary:
https://github.com/facebook/rocksdb/pull/5256 broke it: `block_iter_.user_key()` may not be valid even if `block_iter_points_to_real_block_` is true. E.g. if there was an IO error or Status::Incomplete.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5291

Differential Revision: D15273324

Pulled By: al13n321

fbshipit-source-id: 442e5b09f9884a58f92a6ac1ca93af719c219886

table/block_based_table_reader.cc

@@ -2325,7 +2325,7 @@ void BlockBasedTableIterator<TBlockIter, TValue>::Seek(const Slice& target) {
   }
 
   bool need_seek_index = true;
-  if (block_iter_points_to_real_block_) {
+  if (block_iter_points_to_real_block_ && block_iter_.Valid()) {
     // Reseek.
     prev_index_value_ = index_iter_->value();
     // We can avoid an index seek if:

table/table_test.cc

@@ -1798,6 +1798,44 @@ TEST_P(BlockBasedTableTest, PartitionIndexTest) {
   }
 }
 
+TEST_P(BlockBasedTableTest, IndexSeekOptimizationIncomplete) {
+  std::unique_ptr<InternalKeyComparator> comparator(
+      new InternalKeyComparator(BytewiseComparator()));
+  BlockBasedTableOptions table_options = GetBlockBasedTableOptions();
+  Options options;
+  options.table_factory.reset(NewBlockBasedTableFactory(table_options));
+  const ImmutableCFOptions ioptions(options);
+  const MutableCFOptions moptions(options);
+
+  TableConstructor c(BytewiseComparator());
+  AddInternalKey(&c, "pika");
+
+  std::vector<std::string> keys;
+  stl_wrappers::KVMap kvmap;
+  c.Finish(options, ioptions, moptions, table_options, *comparator, &keys,
+           &kvmap);
+  ASSERT_EQ(1, keys.size());
+
+  auto reader = c.GetTableReader();
+  ReadOptions ropt;
+  ropt.read_tier = ReadTier::kBlockCacheTier;
+  std::unique_ptr<InternalIterator> iter(
+      reader->NewIterator(ropt, /* prefix_extractor */ nullptr));
+
+  auto ikey = [](Slice user_key) {
+    return InternalKey(user_key, 0, kTypeValue).Encode().ToString();
+  };
+
+  iter->Seek(ikey("pika"));
+  ASSERT_FALSE(iter->Valid());
+  ASSERT_TRUE(iter->status().IsIncomplete());
+
+  // This used to crash at some point.
+  iter->Seek(ikey("pika"));
+  ASSERT_FALSE(iter->Valid());
+  ASSERT_TRUE(iter->status().IsIncomplete());
+}
+
 // It's very hard to figure out the index block size of a block accurately.
 // To make sure we get the index size, we just make sure as key number
 // grows, the filter block size also grows.