Fix a backward compatibility problem with table_properties being nullptr

Summary:
Currently when ldb built from master tries to open a DB from version 2.2, there will be a segfault because table_properties didn't exist back then.
Closes https://github.com/facebook/rocksdb/pull/3890

Differential Revision: D8100914

Pulled By: miasantreble

fbshipit-source-id: b255e8aedc54695432be2e704839c857dabdd65a

table/block_based_table_reader.cc

@@ -176,16 +176,20 @@ Cache::Handle* GetEntryFromCache(Cache* block_cache, const Slice& key,
 // For hash based index, return true if prefix_extractor and
 // prefix_extractor_block mismatch, false otherwise. This flag will be used
 // as total_order_seek via NewIndexIterator
-bool PrefixExtractorChanged(std::string prefix_extractor_block,
+bool PrefixExtractorChanged(
+    std::shared_ptr<const TableProperties> table_properties,
     const SliceTransform* prefix_extractor) {
   // BlockBasedTableOptions::kHashSearch requires prefix_extractor to be set.
   // Turn off hash index in prefix_extractor is not set; if  prefix_extractor
   // is set but prefix_extractor_block is not set, also disable hash index
-  if (prefix_extractor == nullptr || prefix_extractor_block.empty()) {
+  if (prefix_extractor == nullptr || table_properties == nullptr ||
+      table_properties->prefix_extractor_name.empty()) {
     return true;
   }
+
   // prefix_extractor and prefix_extractor_block are both non-empty
-  if (prefix_extractor_block.compare(prefix_extractor->Name()) != 0) {
+  if (table_properties->prefix_extractor_name.compare(
+          prefix_extractor->Name()) != 0) {
     return true;
   } else {
     return false;
@@ -887,8 +891,8 @@ Status BlockBasedTable::Open(const ImmutableCFOptions& ioptions,
       bool prefix_extractor_changed = false;
       // check prefix_extractor match only if hash based index is used
       if (rep->index_type == BlockBasedTableOptions::kHashSearch) {
-        prefix_extractor_changed = PrefixExtractorChanged(
-            rep->table_properties->prefix_extractor_name, prefix_extractor);
+        prefix_extractor_changed =
+            PrefixExtractorChanged(rep->table_properties, prefix_extractor);
       }
       unique_ptr<InternalIterator> iter(new_table->NewIndexIterator(
           ReadOptions(), prefix_extractor_changed, nullptr, &index_entry));
@@ -2018,8 +2022,8 @@ void BlockBasedTableIterator::FindKeyBackward() {
 InternalIterator* BlockBasedTable::NewIterator(
     const ReadOptions& read_options, const SliceTransform* prefix_extractor,
     Arena* arena, bool skip_filters) {
-  bool prefix_extractor_changed = PrefixExtractorChanged(
-      rep_->table_properties->prefix_extractor_name, prefix_extractor);
+  bool prefix_extractor_changed =
+      PrefixExtractorChanged(rep_->table_properties, prefix_extractor);
   if (arena == nullptr) {
     return new BlockBasedTableIterator(
         this, read_options, rep_->internal_comparator,
@@ -2124,8 +2128,8 @@ Status BlockBasedTable::Get(const ReadOptions& read_options, const Slice& key,
     // BlockPrefixIndex. Only do this check when index_type is kHashSearch.
     bool prefix_extractor_changed = false;
     if (rep_->index_type == BlockBasedTableOptions::kHashSearch) {
-      prefix_extractor_changed = PrefixExtractorChanged(
-          rep_->table_properties->prefix_extractor_name, prefix_extractor);
+      prefix_extractor_changed =
+          PrefixExtractorChanged(rep_->table_properties, prefix_extractor);
     }
     auto iiter = NewIndexIterator(read_options, prefix_extractor_changed,
                                   &iiter_on_stack, /* index_entry */ nullptr,