Allow option string to get comparator from object registry (#5106)

Summary:
Even customized ldb may not be able to read data from some databases if
comparator is not standard. We modify option helper to get comparator from
object registry so that we can use customized ldb to read non-standard
comparator.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5106

Differential Revision: D14622107

Pulled By: siying

fbshipit-source-id: 151dcb295a35a4c7d54f919cd4e322a89dc601c9

options/options_helper.cc

@@ -19,6 +19,7 @@
 #include "rocksdb/rate_limiter.h"
 #include "rocksdb/slice_transform.h"
 #include "rocksdb/table.h"
+#include "rocksdb/utilities/object_registry.h"
 #include "table/block_based_table_factory.h"
 #include "table/plain_table_factory.h"
 #include "util/cast_util.h"
@@ -240,6 +241,8 @@ std::unordered_map<std::string, CompressionType>
         {"kDisableCompressionOption", kDisableCompressionOption}};
 #ifndef ROCKSDB_LITE
 
+const std::string kNameComparator = "comparator";
+
 template <typename T>
 Status GetStringFromStruct(
     std::string* opt_string, const T& options,
@@ -1013,6 +1016,16 @@ Status ParseColumnFamilyOption(const std::string& name,
         return s;
       }
     } else {
+      if (name == kNameComparator) {
+        // Try to get comparator from object registry first.
+        std::unique_ptr<const Comparator> comp_guard;
+        const Comparator* comp =
+            NewCustomObject<const Comparator>(value, &comp_guard);
+        // Only support static comparator for now.
+        if (comp != nullptr && !comp_guard) {
+          new_options->comparator = comp;
+        }
+      }
       auto iter = cf_options_type_info.find(name);
       if (iter == cf_options_type_info.end()) {
         return Status::InvalidArgument(
@@ -1876,7 +1889,7 @@ std::unordered_map<std::string, OptionTypeInfo>
          {offset_of(&ColumnFamilyOptions::bottommost_compression),
           OptionType::kCompressionType, OptionVerificationType::kNormal, false,
           0}},
-        {"comparator",
+        {kNameComparator,
          {offset_of(&ColumnFamilyOptions::comparator), OptionType::kComparator,
           OptionVerificationType::kByName, false, 0}},
         {"prefix_extractor",

options/options_test.cc

@@ -26,6 +26,7 @@
 #include "rocksdb/convenience.h"
 #include "rocksdb/memtablerep.h"
 #include "rocksdb/utilities/leveldb_options.h"
+#include "rocksdb/utilities/object_registry.h"
 #include "util/random.h"
 #include "util/stderr_logger.h"
 #include "util/string_util.h"
@@ -335,6 +336,18 @@ TEST_F(OptionsTest, GetColumnFamilyOptionsFromStringTest) {
               &new_cf_opt));
   ASSERT_OK(RocksDBOptionsParser::VerifyCFOptions(base_cf_opt, new_cf_opt));
 
+  // Comparator from object registry
+  std::string kCompName = "reverse_comp";
+  static Registrar<const Comparator> test_reg_a(
+      kCompName, [](const std::string& /*name*/,
+                    std::unique_ptr<const Comparator>* /*comparator_guard*/) {
+        return ReverseBytewiseComparator();
+      });
+
+  ASSERT_OK(GetColumnFamilyOptionsFromString(
+      base_cf_opt, "comparator=" + kCompName + ";", &new_cf_opt));
+  ASSERT_EQ(new_cf_opt.comparator, ReverseBytewiseComparator());
+
   // Wrong key/value pair
   ASSERT_NOK(GetColumnFamilyOptionsFromString(base_cf_opt,
              "write_buffer_size=13;max_write_buffer_number;", &new_cf_opt));