Summary:
There are situations when RocksDB tries to recover, but the db is in an inconsistent state due to SST files referenced in the MANIFEST being missing. In this case, previous RocksDB will just fail the recovery and return a non-ok status.
This PR enables another possibility. During recovery, RocksDB checks possible MANIFEST files, and try to recover to the most recent state without missing table file. `VersionSet::Recover()` applies version edits incrementally and "materializes" a version only when this version does not reference any missing table file. After processing the entire MANIFEST, the version created last will be the latest version.
`DBImpl::Recover()` calls `VersionSet::Recover()`. Afterwards, WAL replay will *not* be performed.
To use this capability, set `options.best_efforts_recovery = true` when opening the db. Best-efforts recovery is currently incompatible with atomic flush.

Test plan (on devserver):
```
$make check
$COMPILE_WITH_ASAN=1 make all && make check
```
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6334

Reviewed By: anand1976

Differential Revision: D19778960

Pulled By: riversand963

fbshipit-source-id: c27ea80f29bc952e7d3311ecf5ee9c54393b40a8

Summary:
By supporting direct IO in RandomAccessFileReader::MultiRead, the benefits of parallel IO (IO uring) and direct IO can be combined.

In direct IO mode, read requests are aligned and merged together before being issued to RandomAccessFile::MultiRead, so blocks in the original requests might share the same underlying buffer, the shared buffers are returned in `aligned_bufs`, which is a new parameter of the `MultiRead` API.

For example, suppose alignment requirement for direct IO is 4KB, one request is (offset: 1KB, len: 1KB), another request is (offset: 3KB, len: 1KB), then since they all belong to page (offset: 0, len: 4KB), `MultiRead` only reads the page with direct IO into a buffer on heap, and returns 2 Slices referencing regions in that same buffer. See `random_access_file_reader_test.cc` for more examples.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6446

Test Plan: Added a new test `random_access_file_reader_test.cc`.

Reviewed By: anand1976

Differential Revision: D20097518

Pulled By: cheng-chang

fbshipit-source-id: ca48a8faf9c3af146465c102ef6b266a363e78d1

Summary:
When `use_direct_reads` and `use_direct_writes` are `false`, `logical_sector_size_` inside various `*File` implementations are not actually used, so `GetLogicalBlockSize` does not necessarily need to be called for `logical_sector_size_`, just set a default page size.

This is a follow up PR for https://github.com/facebook/rocksdb/pull/6457.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6522

Test Plan: make check

Reviewed By: siying

Differential Revision: D20408885

Pulled By: cheng-chang

fbshipit-source-id: f2d3808f41265237e7fa2c0be9f084f8fa97fe3d

Summary:
When building RocksDB on VS2015, an error shows up with

hash_map.h(39): error C2719: 'value': formal parameter with requested alignment of 8 won't be aligned

Making the reference a reference can solve the problem, and there isn't a reason we can't do that, at least for the current use of the hash map.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6567

Test Plan: See CI tests pass.

Reviewed By: pdillinger

Differential Revision: D20548543

fbshipit-source-id: 255b55d74cf68a0b324e6f504c56608a97ea6276

Summary:
This commit fixes an incorrect version of this change that was previously landed.

On recently adding ARM64 and PPC64LE builds to Travis, we
seem to have hit some parallel build limits that dramatically increased
queue times.

This change majorly limits the configurations for ARM64 and PPC64LE to
build on each pull request, but keeps the large matrix for branch
builds.

In the process, I changed some previously excluded osx build configurations
to happen in branch builds.

NB: we might want to move master branch Travis build to daily trigger
rather than push trigger to further reduce contention.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6557

Test Plan: Travis only

Reviewed By: siying

Differential Revision: D20563425

Pulled By: pdillinger

fbshipit-source-id: d619eb9f196486ed000364aa40de4661f0b1029d

Summary: TestPilot uses two flags to determine whether coverage is already instrumented: `fbcode_macros` and `coverage`.  Normally, these two tags are added automatically to cpp tests, but this is a fake cpp test, so we must manually add them.  The first is easy - `fbcode_macros` is added by the `custom_unittest` library, which is in `fbcode_macros`, so it is appropriate.  The second is harder - we need to verify that we should add the macro.  We do this using the `coverage.bzl` functions.

Reviewed By: siying

Differential Revision: D20549040

fbshipit-source-id: d2732b3ec26f3dff065efdf398abe3241075bb2f

Summary:
On recently adding ARM64 and PPC64LE builds to Travis, we
seem to have hit some parallel build limits that dramatically increased
queue times.

This change majorly limits the configurations for ARM64 and PPC64LE to
build on each pull request, but keeps the large matrix for branch
builds.

In the process, I changed some previously excluded osx build configurations
to happen in branch builds.

NB: we might want to move master branch Travis build to daily trigger
rather than push trigger to further reduce contention.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6557

Test Plan: Travis only

Reviewed By: siying

Differential Revision: D20524575

Pulled By: pdillinger

fbshipit-source-id: babcb2c64e195679e472473a1cbdf42de47231ff

Summary:
Each time RocksDB switches to a new MANIFEST file from old one, it calls WriteCurrentStateToManifest() which writes a 'snapshot' of the current in-memory state of versions to the beginning of the new manifest as a bunch of version edits. We can distinguish these version edits from other version edits written during normal operations with a custom, safe-to-ignore tag.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6530

Test Plan: added test to version_edit_test, pass make asan_check

Reviewed By: riversand963

Differential Revision: D20524516

Pulled By: zhichao-cao

fbshipit-source-id: f1de102f5499bfa88dae3caa2f32c7f42cf904db

Summary:
The whole point of the pimpl idiom is to hide implementation details.
Internal helper methods like `CheckConsistency`, `CheckConsistencyForDeletes`,
and `MaybeAddFile` do not belong in the public interface of the class.
In addition, the patch switches to `unique_ptr` for the implementation
object instead of using a raw `delete`.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6556

Test Plan: `make check`

Reviewed By: riversand963

Differential Revision: D20523568

Pulled By: ltamasi

fbshipit-source-id: 5bbb0ccebd0c47a33b815398c7f9cfe13bd775ac

Summary:
Currently, `db_stress` tests a randomly picked one of `GetLiveFiles`,
`GetSortedWalFiles`, and `GetCurrentWalFile` with a 1/N chance when the
command line parameter `get_live_files_and_wal_files_one_in` is specified.
The problem is that `GetSortedWalFiles` and `GetCurrentWalFile` are unreliable
in the sense that they can return errors if another thread removes a WAL file
while they are executing (which is a perfectly plausible and legitimate scenario).
The patch splits this command line parameter into three (one for each API),
and changes the crash test script so that only `GetLiveFiles` is tested during
our continuous crash test runs.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6491

Test Plan:
```
make check
python tools/db_crashtest.py whitebox
```

Reviewed By: siying

Differential Revision: D20312200

Pulled By: ltamasi

fbshipit-source-id: e7c3481eddfe3bd3d5349476e34abc9eee5b7dc8

Summary:
ldb and sst_dump are most important tools and they don't dependend on gflags. In cmake, we don't have an way to only build these two tools and exclude other tools. This is inconvenient if the environment has a problem with gflags. Add such an option WITH_CORE_TOOLS.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6506

Test Plan: cmake and build with WITH_TOOLS and without.

Differential Revision: D20473029

fbshipit-source-id: 3d730fd14bbae6eeeae7f9cc9aec50a4e488ad72

Summary: Pull Request resolved: https://github.com/facebook/rocksdb/pull/6553

Test Plan:
```
$ make valgrind_test -j24
$ ./folly_synchronization_distributed_mutex_test
DistributedMutex is not supported in ROCKSDB_LITE, on ARM, or in valgrind_test runs
```

Reviewed By: pdillinger

Differential Revision: D20501966

Pulled By: ltamasi

fbshipit-source-id: 386ec5f258f89d0781a36c5b390c665787093a74

Summary:
https://github.com/facebook/rocksdb/pull/6531 removed some code in partitioned index seek logic. By mistake the logic of storing previous index offset is removed, while the logic of using it is preserved, so that the code might use wrong value to determine reseeking condition.
This will trigger a bug, if following a Seek() not going to the last block, SeekToLast() is called, and then Seek() is called which should position the cursor to the block before SeekToLast().
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6551

Test Plan: Add a unit test that reproduces the bug. In the same unit test, also some reseek cases are covered to avoid regression.

Reviewed By: pdillinger

Differential Revision: D20493990

fbshipit-source-id: 3919aa4861c0481ec96844e053048da1a934b91d

Summary:
Allowing table/sst_file_reader_test.cc to use custom Env specified by TEST_ENV_URI.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6536

Reviewed By: riversand963

Differential Revision: D20448525

Pulled By: akankshamahajan15

fbshipit-source-id: 74e4d34c8ac4c2743741e78bf599571a4a465459

Summary:
Reduce runtime by reducing test scale to avoid test time-outs.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6546

Test Plan:
time ./db_with_timestamp_basic_test
and watch internal tests.

Reviewed By: zhichao-cao

Differential Revision: D20479292

Pulled By: riversand963

fbshipit-source-id: c9e4a155be7699dd4de60fa531de86d442a3ba0a

Summary:
Define a private member variable only in debug mode. Without fix, build will fail
```
In file included from table/block_based/partitioned_index_iterator.cc:9:
./table/block_based/partitioned_index_iterator.h:125:32: error: private field 'icomp_' is not used [-Werror,-Wunused-private-field]
  const InternalKeyComparator& icomp_;
```

Test plan (dev server)
1. make check
2. Make sure fixed in Travis
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6547

Reviewed By: siying

Differential Revision: D20480027

Pulled By: pdillinger

fbshipit-source-id: 288bc94280e240c3136335b6c73eb1ccb0db459d

Summary:
For s390x support, some updates in newer version of Align.h are
needed. Upgrading just that file as best we can, with one addition to
Portability.h and tweaking new code in Align.h to use C++11 only (no
non-trivial constexpr functions).
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6534

Test Plan: CI, further work in PR https://github.com/facebook/rocksdb/issues/6168

Differential Revision: D20445942

Pulled By: pdillinger

fbshipit-source-id: 0cef3c367463c71f3123d12cdf287c573af5e342

Summary:
There was an alignment bug in our copy of the streaming APIs
for XXH3 (which we dubbed "XXH3p" for "preview" release). Since those
APIs are unused and some values for XXH3 have changed since XXH3p, I'm
simply removing those APIs, expecting it's better to use finalized XXH3
function if/when we decide to use those APIs (e.g. for checksums).

Fixes https://github.com/facebook/rocksdb/issues/6508
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6540

Test Plan: make check

Differential Revision: D20479271

Pulled By: pdillinger

fbshipit-source-id: 246cf1690d614d3b31042b563d249de32dec1e0d

Summary:
When using custom Env, trying to call DestroyDB() with default Options will
fail.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6539

Test Plan: ./db_stress

Differential Revision: D20476204

Pulled By: riversand963

fbshipit-source-id: 612c6754660cc9b5bb3e9c2dbb2f6ecd7f648797

Summary:
I start to see following failures:

tools/db_bench_tool.cc: In constructor ‘rocksdb::NormalDistribution::NormalDistribution(unsigned int, unsigned int)’:
tools/db_bench_tool.cc:1528:58: error: declaration of ‘max’ shadows a member of 'this' [-Werror=shadow]
   NormalDistribution(unsigned int min, unsigned int max) :
                                                          ^
tools/db_bench_tool.cc:1528:58: error: declaration of ‘min’ shadows a member of 'this' [-Werror=shadow]
tools/db_bench_tool.cc: In constructor ‘rocksdb::UniformDistribution::UniformDistribution(unsigned int, unsigned int)’:
tools/db_bench_tool.cc:1546:59: error: declaration of ‘max’ shadows a member of 'this' [-Werror=shadow]
   UniformDistribution(unsigned int min, unsigned int max) :
                                                           ^
tools/db_bench_tool.cc:1546:59: error: declaration of ‘min’ shadows a member of 'this' [-Werror=shadow]

when I build from GCC 4.8. Rename those variables to fix the problem.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6537

Test Plan: make all with the compiler that used to show the failure.

Differential Revision: D20448741

fbshipit-source-id: 18bcf012dbe020f22f79038a9b08f447befa2574

Summary:
Right now block based table iterator is used as both of iterating data for block based table, and for the index iterator for partitioend index. This was initially convenient for introducing a new iterator and block type for new index format, while reducing code change. However, these two usage doesn't go with each other very well. For example, Prev() is never called for partitioned index iterator, and some other complexity is maintained in block based iterators, which is not needed for index iterator but maintainers will always need to reason about it. Furthermore, the template usage is not following Google C++ Style which we are following, and makes a large chunk of code tangled together. This commit separate the two iterators. Right now, here is what it is done:
1. Copy the block based iterator code into partitioned index iterator, and de-template them.
2. Remove some code not needed for partitioned index. The upper bound check and tricks are removed. We never tested performance for those tricks when partitioned index is enabled in the first place. It's unlikelyl to generate performance regression, as creating new partitioned index block is much rarer than data blocks.
3. Separate out the prefetch logic to a helper class and both classes call them.

This commit will enable future follow-ups. One direction is that we might separate index iterator interface for data blocks and index blocks, as they are quite different.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6531

Test Plan: build using make and cmake. And build release

Differential Revision: D20473108

fbshipit-source-id: e48011783b339a4257c204cc07507b171b834b0f

Summary:
CircleCI is the new recommended CI system internally.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6518

Test Plan: Watch https://app.circleci.com/pipelines/github/facebook/rocksdb

Differential Revision: D20454743

Pulled By: cheng-chang

fbshipit-source-id: 39031568d6c1d3d25b7fbd78fa9a0e6067ddc47c

Summary:
If DB is not deleted, in concurrent test, the tests might fail because of the previously existing DB.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6532

Test Plan:
make clean && make -j24 LITE=1  db_logical_block_size_cache_test && ./db_logical_block_size_cache_test
make clean && make -j24 db_logical_block_size_cache_test && ./db_logical_block_size_cache_test

Differential Revision: D20454734

Pulled By: cheng-chang

fbshipit-source-id: 8abede2ec1d79c1a4fe1bc95fbda489f8f7ee052

Summary:
Fix the build corruption caused by PR https://github.com/facebook/rocksdb/issues/6516

Testing plan: make make asan_check
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6535

Differential Revision: D20448614

Pulled By: zhichao-cao

fbshipit-source-id: 4d2a3dae6cdd781fcfe8e28a84ac3f536db1b067

Summary:
After we had a lot of failures with maven.org downloads, we
wanted an alternative location for downloading binary dependencies.
Hosting them through github would have been good in terms of
organizational and network dependencies, but that approach seems to be
awkward (fake releases, so would need a 'rocksdb-deps' repo) and
strangely complicated for Facebook policy on open source repositories.

This commit moves the downloads (that are not officially hosted by
others on github) from my personal rocksdb fork to an S3 bucket owned
by the Facebook RocksDB AWS account. Facebook employees can access
this through an internal tool, and we should be able to grant permission
to outside collaborators.

Assuming this works out, I will back-port to older branches to stabilize
their CI testing as well.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6526

Test Plan: CI

Differential Revision: D20430130

Pulled By: pdillinger

fbshipit-source-id: df52394a65e0a57942db3039bdaade8a4d520cb2

Summary:
In some of the test, db_basic_test may cause time out due to its long running time. Separate the timestamp related test from db_basic_test to avoid the potential issue.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6516

Test Plan: pass make asan_check

Differential Revision: D20423922

Pulled By: zhichao-cao

fbshipit-source-id: d6306f89a8de55b07bf57233e4554c09ef1fe23a

Summary:
block_based_table_reader.cc is a giant file, which makes it hard for users to navigate the code. Divide the files to multiple files.
Some class templates cannot be moved to .cc file. They are moved to .h files. It is still better than including them all in block_based_table_reader.cc.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6527

Test Plan: "make all check" and "make release". Also build using cmake.

Differential Revision: D20428455

fbshipit-source-id: ca713c698469f07f35bc0c271358c0874ed4eb28

Summary:
This patch based on https://github.com/facebook/rocksdb/issues/5932

 offers a better solution to add arm64 to TravisCI matrix.
Really thank adamretter for initiating Arm CI setup.

Difference comparing to amd64:
1. For CMake, as no official arm64 release ready on Kitware page,
a third party (conda-forge) released one is used instead of
building from source. The main reason is to save CI time.
2. Explicit export JAVA_HOME on arm64
3. Disable mingw test

Signed-off-by: Yuqi Gu <yuqi.gu@arm.com>
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6436

Differential Revision: D20428505

Pulled By: pdillinger

fbshipit-source-id: 81ef02435e41480bb71710b783d85ebf452ce926

Summary:
tcc gtest runner need to know the location of the binary in order to collect coverage.  We can give them the location in an environment variable.

Note that all these tests will break in tpx currently, though this is a bug in rocksdb's wrapper script, not tpx.

Reviewed By: siying

Differential Revision: D20430043

fbshipit-source-id: c77d5f70bbc28f6011c6f91906bce2ceecc2f167

Summary:
Update example usage of closing column family.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6505

Test Plan: cd examples && make column_families_example && ./column_families_example

Differential Revision: D20362100

Pulled By: cheng-chang

fbshipit-source-id: 493c5e0068a40b4f237f8f8511cddd22dc15ea5c

Summary:
In DBLogicalBlockSizeCacheTest, do not test OpenForReadOnly in LITE mode.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6523

Test Plan: watch test for LITE mode

Differential Revision: D20420321

Pulled By: cheng-chang

fbshipit-source-id: e45bf6f2800206d6f8ce9af7308e76a08de80643

Summary:
In the `.travis.yml` file the `jdk: openjdk7` element is ignored when `language: cpp`. So whatever version of the JDK that was installed in the Travis container was used - typically JDK 11.

To ensure our RocksJava builds are working, we now instead install and use OpenJDK 8. Ideally we would use OpenJDK 7, as RocksJava supports Java 7, but many of the newer Travis containers don't support Java 7, so Java 8 is the next best thing.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6512

Differential Revision: D20388296

Pulled By: pdillinger

fbshipit-source-id: 8bbe6b59b70cfab7fe81ff63867d907fefdd2df1

Summary: Pull Request resolved: https://github.com/facebook/rocksdb/pull/6519

Test Plan:
```
make all
make check
```

Differential Revision: D20400691

Pulled By: ltamasi

fbshipit-source-id: 20ef911cf1c2c92c7f71ef0b493f9be64f2eef94

Summary:
fix a few build warnings that are treated as failures with more strict MSVC warning settings
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6517

Differential Revision: D20401325

Pulled By: pdillinger

fbshipit-source-id: b44979dfaafdc7b3b8cb44a565400a99b331dd30

Summary:
Remove copy of pairs from the for range loop
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6514

Test Plan: make check

Differential Revision: D20389688

Pulled By: cheng-chang

fbshipit-source-id: 1c772091f955be33267514010f3596c61a6f46b5

Summary:
In Linux, when reopening DB with many SST files, profiling shows that 100% system cpu time spent for a couple of seconds for `GetLogicalBufferSize`. This slows down MyRocks' recovery time when site is down.

This PR introduces two new APIs:
1. `Env::RegisterDbPaths` and `Env::UnregisterDbPaths` lets `DB` tell the env when it starts or stops using its database directories . The `PosixFileSystem` takes this opportunity to set up a cache from database directories to the corresponding logical block sizes.
2. `LogicalBlockSizeCache` is defined only for OS_LINUX to cache the logical block sizes.

Other modifications:
1. rename `logical buffer size` to `logical block size` to be consistent with Linux terms.
2. declare `GetLogicalBlockSize` in `PosixHelper` to expose it to `PosixFileSystem`.
3. change the functions `IOError` and `IOStatus` in `env/io_posix.h` to have external linkage since they are used in other translation units too.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6457

Test Plan:
1. A new unit test is added for `LogicalBlockSizeCache` in `env/io_posix_test.cc`.
2. A new integration test is added for `DB` operations related to the cache in `db/db_logical_block_size_cache_test.cc`.

`make check`

Differential Revision: D20131243

Pulled By: cheng-chang

fbshipit-source-id: 3077c50f8065c0bffb544d8f49fb10bba9408d04

Summary:
When users fail to open a DB with file lock failure, it is sometimes hard for users to debug. We now include the time the lock is acquired and the thread ID that acquired the lock, to help users debug problems like this. Default Env's thread ID is used.

Since type of lockedFiles is changed, rename it to follow naming convention too.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6507

Test Plan: Add a unit test and improve an existing test to validate the case.

Differential Revision: D20378333

fbshipit-source-id: 312fe0e9733fd1d1e9969c321b90ce523cf4708a

Summary: Pull Request resolved: https://github.com/facebook/rocksdb/pull/6513

Test Plan: `make unity_test`

Differential Revision: D20388919

Pulled By: ltamasi

fbshipit-source-id: 88dbceab0723a54ee3939e1644e13dc9a4c70420

Summary:
Let's see how this goes...
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6144

Differential Revision: D20387515

Pulled By: pdillinger

fbshipit-source-id: ba2669348c267141dfddff910b4c2224a22cbb38

Summary:
* **macOS version:** 10.15.2 (Catalina)
* **XCode/Clang version:** Apple clang version 11.0.0 (clang-1100.0.33.16)

Before this bugfix the error generated is:

```
In file included from ./util/compression.h:23:
./snappy-1.1.7/snappy.h:76:59: error: unknown type name 'string'; did you mean 'std::string'?
  size_t Compress(const char* input, size_t input_length, string* output);
                                                          ^~~~~~
                                                          std::string
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/iosfwd:211:65: note: 'std::string' declared here
typedef basic_string<char, char_traits<char>, allocator<char> > string;
                                                                ^
In file included from db/builder.cc:10:
In file included from ./db/builder.h:12:
In file included from ./db/range_tombstone_fragmenter.h:15:
In file included from ./db/pinned_iterators_manager.h:12:
In file included from ./table/internal_iterator.h:13:
In file included from ./table/format.h:25:
In file included from ./options/cf_options.h:14:
In file included from ./util/compression.h:23:
./snappy-1.1.7/snappy.h:85:19: error: unknown type name 'string'; did you mean 'std::string'?
                  string* uncompressed);
                  ^~~~~~
                  std::string
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/iosfwd:211:65: note: 'std::string' declared here
typedef basic_string<char, char_traits<char>, allocator<char> > string;
                                                                ^
2 errors generated.
make: *** [jls/db/builder.o] Error 1
```
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6496

Differential Revision: D20389254

Pulled By: pdillinger

fbshipit-source-id: 2864245c8d0dba7b2ab81294241a62f2adf02e20