Hack to load OPTIONS file for read_amp_bytes_per_bit (facebook#7659)

Summary:
A temporary hack to work around a bug in 6.10, 6.11, 6.12, 6.13 and
6.14. The bug will write out 8 bytes to OPTIONS file from the starting
address of BlockBasedTableOptions.read_amp_bytes_per_bit which is
actually a uint32. Consequently, the value of read_amp_bytes_per_bit
written in the OPTIONS file is wrong. From 6.15, RocksDB will
try to parse the read_amp_bytes_per_bit from OPTIONS file as a uint32.
To be able to load OPTIONS file generated by affected releases before
the fix, we need to manually parse read_amp_bytes_per_bit with this hack.

Pull Request resolved: facebook#7659

Test Plan:
Generate a db with current 6.14.fb (head at facebook@b6db05d). Maybe use db_stress.

Checkout this PR, run
```
 ~/rocksdb/ldb --db=. --try_load_options --ignore_unknown_options idump --count_only
```
Expect success, and should not see
```
Failed: Invalid argument: Error parsing read_amp_bytes_per_bit:17179869184
```

Also
make check

Reviewed By: anand1976

Differential Revision: D24954752

Pulled By: riversand963

fbshipit-source-id: c7b802fc3e52acd050a4fc1cd475016122234394

options/options_test.cc

@@ -846,7 +846,12 @@ TEST_F(OptionsTest, GetBlockBasedTableOptionsFromString) {
       "block_cache=1M;block_cache_compressed=1k;block_size=1024;"
       "block_size_deviation=8;block_restart_interval=4;"
       "format_version=5;whole_key_filtering=1;"
-      "filter_policy=bloomfilter:4.567:false;",
+      "filter_policy=bloomfilter:4.567:false;"
+      // A bug caused read_amp_bytes_per_bit to be a large integer in OPTIONS
+      // file generated by 6.10 to 6.14. Though bug is fixed in these releases,
+      // we need to handle the case of loading OPTIONS file generated before the
+      // fix.
+      "read_amp_bytes_per_bit=17179869185;",
       &new_opt));
   ASSERT_TRUE(new_opt.cache_index_and_filter_blocks);
   ASSERT_EQ(new_opt.index_type, BlockBasedTableOptions::kHashSearch);
@@ -867,6 +872,9 @@ TEST_F(OptionsTest, GetBlockBasedTableOptionsFromString) {
   EXPECT_EQ(bfp->GetMillibitsPerKey(), 4567);
   EXPECT_EQ(bfp->GetWholeBitsPerKey(), 5);
   EXPECT_EQ(bfp->GetMode(), BloomFilterPolicy::kAutoBloom);
+  // Verify that only the lower 32bits are stored in
+  // new_opt.read_amp_bytes_per_bit.
+  EXPECT_EQ(1U, new_opt.read_amp_bytes_per_bit);
 
   // unknown option
   Status s = GetBlockBasedTableOptionsFromString(

table/block_based/block_based_table_factory.cc

@@ -360,7 +360,22 @@ static std::unordered_map<std::string, OptionTypeInfo>
         {"read_amp_bytes_per_bit",
          {offsetof(struct BlockBasedTableOptions, read_amp_bytes_per_bit),
           OptionType::kUInt32T, OptionVerificationType::kNormal,
-          OptionTypeFlags::kNone}},
+          OptionTypeFlags::kNone,
+          [](const ConfigOptions& /*opts*/, const std::string& /*name*/,
+             const std::string& value, char* addr) {
+            // A workaround to fix a bug in 6.10, 6.11, 6.12, 6.13
+            // and 6.14. The bug will write out 8 bytes to OPTIONS file from the
+            // starting address of BlockBasedTableOptions.read_amp_bytes_per_bit
+            // which is actually a uint32. Consequently, the value of
+            // read_amp_bytes_per_bit written in the OPTIONS file is wrong.
+            // From 6.15, RocksDB will try to parse the read_amp_bytes_per_bit
+            // from OPTIONS file as a uint32. To be able to load OPTIONS file
+            // generated by affected releases before the fix, we need to
+            // manually parse read_amp_bytes_per_bit with this special hack.
+            uint64_t read_amp_bytes_per_bit = ParseUint64(value);
+            EncodeFixed32(addr, static_cast<uint32_t>(read_amp_bytes_per_bit));
+            return Status::OK();
+          }}},
         {"enable_index_compression",
          {offsetof(struct BlockBasedTableOptions, enable_index_compression),
           OptionType::kBoolean, OptionVerificationType::kNormal,