Summary:
Cleaned up the public API to use the EncryptedEnv.  This change will allow providers to be developed and added to the system easier in the future.  It will also allow better integration in the future with the OPTIONS file.

- The internal classes were moved out of the public API into an internal "env_encryption_ctr.h" header.  Short-cut constructors were added to provide the original API functionality.
- The APIs to the constructors were changed to take shared_ptr, rather than raw pointers or references to allow better memory management and alternative implementations.
- CreateFromString methods were added to allow future expansion to other provider and cipher implementations through a standard API.

Additionally, there was a code duplication in the NewXXXFile methods.  This common code was moved under a templatized function.

A first-pass at structuring the code was made to potentially allow multiple EncryptionProviders in a single EncryptedEnv.  The idea was that different providers may use different cipher keys or different versions/algorithms.  The EncryptedEnv should have some means of picking different providers based on information.  The groundwork was started for this (the use of the provider_ member variable was localized) but the work has not been completed.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7279

Reviewed By: jay-zhuang

Differential Revision: D23709440

Pulled By: zhichao-cao

fbshipit-source-id: 0e845fff0e03a52603eb9672b4ade32d063ff2f2

Summary:
The patch adds support for writing blob files during flush by integrating
`BlobFileBuilder` with the flush logic, most importantly, `BuildTable` and
`CompactionIterator`. If `enable_blob_files` is set, large values are extracted
to blob files and replaced with references. The resulting blob files are then
logged to the MANIFEST as part of the flush job's `VersionEdit` and
added to the `Version`, similarly to table files. Errors related to writing
blob files fail the flush, and any blob files written by such jobs are immediately
deleted (again, similarly to how SST files are handled). In addition, the patch
extends the logging and statistics around flushes to account for the presence
of blob files (e.g. `InternalStats::CompactionStats::bytes_written`, which is
used for calculating write amplification, now considers the blob files as well).

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7345

Test Plan: Tested using `make check` and `db_bench`.

Reviewed By: riversand963

Differential Revision: D23506369

Pulled By: ltamasi

fbshipit-source-id: 646885f22dfbe063f650d38a1fedc132f499a159

Summary:
Introduced `valgrind_check_some`, which is analogous to the `check_some` target for non-valgrind tests. It simplifies the process for running a single valgrind test or subset of valgrind tests when trying to repro a failure.

I also added a `ROCKSDBTESTS_ONLY` parameter, which simplifies selecting a single test to run. Previously the user would have to use `ROCKSDBTESTS_START` and `ROCKSDBTESTS_END`, but it was difficult to determine the end variable since it is an exclusive endpoint and must match an actual test name.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7379

Reviewed By: pdillinger

Differential Revision: D23673608

Pulled By: ajkr

fbshipit-source-id: 87ed81f1a671d46c2dff6a701f85f1891c725b3f

Summary:
This PR merges the functionality of making the ColumnFamilyOptions, TableFactory, and DBOptions into Configurable into a single PR, resolving any merge conflicts

Pull Request resolved: https://github.com/facebook/rocksdb/pull/5753

Reviewed By: ajkr

Differential Revision: D23385030

Pulled By: zhichao-cao

fbshipit-source-id: 8b977a7731556230b9b8c5a081b98e49ee4f160a

Summary:
In a distributed file system, directory ownership is enforced by fencing
off the previous owner once they've been preempted by a new owner. This
PR adds a IOStatus subcode for ```StatusCode::IOError``` to indicate this.
Once this error is returned for a file write, the DB is put in read-only
mode and not allowed to resume in read-write mode.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7374

Test Plan: Add new unit tests in ```error_handler_fs_test```

Reviewed By: riversand963

Differential Revision: D23687777

Pulled By: anand1976

fbshipit-source-id: bef948642089dc0af399057864d9a8ca339e8b2f

Summary:
snapd update has been failing on ppc for ~a week. Disabling it
for now in pull requests.

Also, https://github.com/facebook/rocksdb/issues/6653 seems to be fixed, so re-enabling standard unit tests for
PPC on pull requests.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7381

Test Plan: CI

Reviewed By: jay-zhuang

Differential Revision: D23684962

Pulled By: pdillinger

fbshipit-source-id: 96ec9487b714c4741bb1653dae90b24118830cb5

Summary:
This is potentially the cause of failures:

    Failure in Destroy restore dir with: IO error: file rmdir: /dev/shm/rocksdb/rocksdb_crashtest_whitebox/.restore13: Directory not empty

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7384

Test Plan: smoke test blackbox_crash_test

Reviewed By: jay-zhuang

Differential Revision: D23685087

Pulled By: pdillinger

fbshipit-source-id: 55f62e9853ce84be1d5ca7d856de867f0f2596ee

Summary:
Covered methods:
- OldDefaults()
- OptimizeForSmallDb(std::shared_ptr<Cache>)

Covered fields:
- cf_paths

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7372

Reviewed By: pdillinger

Differential Revision: D23683449

Pulled By: jay-zhuang

fbshipit-source-id: 3e5a8b657cc382c19de3a48c666a3b0e8d96968d

Summary:
for release fork

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7378

Test Plan: make check + CI

Reviewed By: jay-zhuang

Differential Revision: D23669163

Pulled By: pdillinger

fbshipit-source-id: 14cbf95b32717c28418c71cc8e10f06733bbc49f

Summary:
During bottommost compaction, RocksDB cannot simply drop a tombstone if
this tombstone is not in the earliest snapshot. The current behavior is: RocksDB
skips other internal keys (of the same user key) in the same snapshot range. In
the meantime, RocksDB should check for the `shutting_down` flag. Otherwise, it
is possible for a bottommost compaction that has already started running to take
a long time to finish, even if the application has tried to cancel all background jobs.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7356

Test Plan: make check

Reviewed By: ltamasi

Differential Revision: D23663241

Pulled By: riversand963

fbshipit-source-id: 25f8e9b51bc3bfa3353cdf87557800f9d90ee0b5

Summary:
We've seen some segfaults in db_write_test, with at least one
suggesting corruption of a write group linked list. Adding an assertion
to have this fail in a more specific way if that is the broken
invariant.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7375

Test Plan: make check

Reviewed By: jay-zhuang

Differential Revision: D23638477

Pulled By: pdillinger

fbshipit-source-id: a76fd677cad60a3a516bd363947bfd9ce418edc1

Summary:
We can only check key on restored backup if in a stress test
configuration locking the key. (Fixes mismatch seen in backup/restore
with atomic flush.)

TestCheckpoint used a very ugly solution to the same problem: copy-paste
dozens of lines of code with some changes and removals. I removed the
unnecessary implementation and made the existing one simply adaptive,
like TestBackupRestore.

Also made TestBackupRestore clean up dead backup/restore directories on
success.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7373

Test Plan:
blackbox_crash_test_with_atomic_flush for a while,
blackbox_crash_test for a while, with backup and checkpoint 1 in 5k and
only 1k max_keys to stress this area

Reviewed By: ajkr

Differential Revision: D23629057

Pulled By: pdillinger

fbshipit-source-id: d7fe7e2be75aaf3cf974be9540a7c5c5de8b371b

Summary:
https://github.com/facebook/rocksdb/issues/3341 guaranteed that upon return of `GetSortedWalFiles` after
`DisableFileDeletions`, all pending purges of previously obsolete WAL
files will have finished. However, the addition of
avoid_unnecessary_blocking_io in https://github.com/facebook/rocksdb/issues/5043 opened a hole in the code making
that assurance, which can lead to files to be copied for checkpoint or
backup going missing before being copied, with that option enabled.

This change patches the hole.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7369

Test Plan:
apparent fix to backups in crash test observed. Will work
on a unit test for another commit

Reviewed By: ajkr

Differential Revision: D23620258

Pulled By: pdillinger

fbshipit-source-id: bea36b461a5b719c3e3ef802f967bc3e8ae71614

Summary:
The patch adds support for exposing the start of the expiration range
for TTL blob files through the `GetLiveFilesMetaData` API. This can be
used for monitoring purposes, i.e. to make sure TTL blob files are
deleted in a timely manner. The patch also fixes a couple of uninitialized
variable issues.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7365

Test Plan: `make check`

Reviewed By: pdillinger

Differential Revision: D23605465

Pulled By: ltamasi

fbshipit-source-id: 97a9612bf5f4b058423debdd3f28f576bb23a70f

Implement Java API for ConcurrentTaskLimiter class and compaction_thread_limiter field in ColumnFamilyOptions (#7347)

Summary:
as title

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7347

Test Plan: unit tests included

Reviewed By: jay-zhuang

Differential Revision: D23592552

Pulled By: pdillinger

fbshipit-source-id: 1c3571b6f42bfd0cfd723ff49d01fbc02a1be45b

Summary: Pull Request resolved: https://github.com/facebook/rocksdb/pull/7321

Reviewed By: ajkr

Differential Revision: D23590160

fbshipit-source-id: 35d106e732ac37f674222759cdb1dbb31e005ca7

Summary:
(a) Missed a case in updating handling of rand_keys
(b) Only opening restored db with DB::Open so don't (yet)
attempt to open restored BlobDB or TransactionDB.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7361

Test Plan: better than being broken

Reviewed By: ajkr

Differential Revision: D23592570

Pulled By: pdillinger

fbshipit-source-id: dd1d999bcc0c852ee77cb6041964ec4abc0fd4fd

Fix the handling of the case when a blob file with a lower number gets added in VersionBuilder (#7349)

Summary:
When multiple background jobs are generating blob files in parallel, it is actually
possible for a blob file to be added with a file number that is lower than the
highest one in the base version. (This is a harmless race condition.) The patch
fixes the handling of this case and adds a unit test.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7349

Test Plan: `make check`

Reviewed By: riversand963

Differential Revision: D23542453

Pulled By: ltamasi

fbshipit-source-id: 4ff6f3654bc58c391d10b9870e1cc40b5e3fa8e4

Summary:
Was broken by https://github.com/facebook/rocksdb/issues/6660

Travis times before this change, after 6660:
platform_dependent: 17 min
group 1: 15 min
group 2: 44 min (often timeout on non-x86 or non-Linux)
group 3: 31 min
group 4: 21 min

After this change:
TODO

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7360

Test Plan: CI inspection

Reviewed By: ajkr

Differential Revision: D23586917

Pulled By: pdillinger

fbshipit-source-id: 4c67cf33180b0b833c39a817e6c1f128727941d2

Summary:
1.  Failed to compile because of use of FileSystem* instead of Env* to some methods;

2.  Failed to compile with addition of ConfigOptions to some methods

3.  Failed to run successfully because the database and/or db_bench would change some of the options, invalidating the comparison

4.  Failed to run successfully if Snappy was not available.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7344

Reviewed By: siying

Differential Revision: D23501093

Pulled By: jay-zhuang

fbshipit-source-id: 81fd947e95fff9db8a4c5ff419d69d4c36bef23f

Summary:
To fix the cmake build with third_party libs, like:
`mkdir build && cd build && cmake .. -DWITH_SNAPPY=1 && make`

Error:
```
Undefined symbols for architecture x86_64:
  "snappy::RawCompress(char const*, unsigned long, char*, unsigned long*)"
```

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7351

Reviewed By: pdillinger

Differential Revision: D23553705

Pulled By: jay-zhuang

fbshipit-source-id: 19b45c6763c7256107583e8af4c01d370ca06128

Summary:
Also enables a pull request to trigger all the Travis
configurations by writing FULL_CI in the commit message. (See what I did
there?)

First issue

    make: *** No rule to make target 'jl/util/crc32c_ppc_asm.o', needed by 'rocksdbjava'.  Stop.

Second issue

    tools/db_bench_tool.cc:5514:38: error: ‘gen_exp.rocksdb::Benchmark::GenerateTwoTermExpKeys::keyrange_size_’ may be used uninitialized in this function

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7359

Test Plan: CI

Reviewed By: zhichao-cao

Differential Revision: D23582132

Pulled By: pdillinger

fbshipit-source-id: 06d794673fd522ba11cf6398385387e6bd97ef89

Summary:
Replace FSRandomRWFile pointer with FSRandomRWFilePtr object in the rocksdb internal code.
This new object wraps FSRandomRWFile pointer.

Objective: If tracing is enabled, FSRandomRWFile object returns FSRandomRWFileTracingWrapper pointer that includes all necessary information in IORecord and calls underlying FileSystem and invokes IOTracer to dump that record in a binary file. If tracing is disabled then, underlying FileSystem pointer is returned directly.
FSRandomRWFilePtr wrapper class is added to bypass the FSRandomRWFileWrapper when
tracing is disabled.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7198

Test Plan: make check -j64

Reviewed By: anand1976

Differential Revision: D23421116

Pulled By: akankshamahajan15

fbshipit-source-id: 8a5ba0e7d9c1ba34c3a6f29829b107c5f09ab6a3

Summary:
gcc-4.8 returns error when using the constructor. Not sure if it's a compiler bug/limitation or code issue:
```
table/block_based/block_based_table_reader.cc:3183:67: error: use of deleted function ‘rocksdb::WritableFileStringStreamAdapter::WritableFileStringStreamAdapter(rocksdb::WritableFileStringStreamAdapter&&)’
```

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7358

Reviewed By: pdillinger

Differential Revision: D23577651

Pulled By: jay-zhuang

fbshipit-source-id: b0197e3d3538da61a6f3866410d88d2047fb9695

Summary:
As title.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7355

Reviewed By: pdillinger

Differential Revision: D23566635

Pulled By: riversand963

fbshipit-source-id: f8d846bcff637e7617b764b7bfb9a948ea18d195

Summary:
Replace FSWritableFile pointer with FSWritableFilePtr
    object in WritableFileWriter.
    This new object wraps FSWritableFile pointer.

    Objective: If tracing is enabled, FSWritableFile Ptr returns
    FSWritableFileTracingWrapper pointer that includes all necessary
    information in IORecord and calls underlying FileSystem and invokes
    IOTracer to dump that record in a binary file. If tracing is disabled
    then, underlying FileSystem pointer is returned directly.
    FSWritableFilePtr wrapper class is added to bypass the
    FSWritableFileWrapper when
    tracing is disabled.

    Test Plan: make check -j64

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7193

Reviewed By: anand1976

Differential Revision: D23355915

Pulled By: akankshamahajan15

fbshipit-source-id: e62a27a13c1fd77e36a6dbafc7006d969bed25cf

Summary:
(1) Skip check on specific key if restoring an old backup
(small minority of cases) because it can fail in those cases. (2) Remove
an old assertion about number of column families and number of keys
passed in, which is broken by atomic flush (cf_consistency) test. Like
other code (for better or worse) assume a single key and iterate over
column families. (3) Apply mock_direct_io to NewSequentialFile so that
db_stress backup works on /dev/shm.

Also add more context to output in case of backup/restore db_stress
failure.

Also a minor fix to BackupEngine to report first failure status in
creating new backup, and drop another clue about the potential
source of a "Backup failed" status.

Reverts "Disable backup/restore stress test (https://github.com/facebook/rocksdb/issues/7350)"

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7357

Test Plan:
Using backup_one_in=10000,
"USE_CLANG=1 make crash_test_with_atomic_flush" for 30+ minutes
"USE_CLANG=1 make blackbox_crash_test" for 30+ minutes
And with use_direct_reads with TEST_TMPDIR=/dev/shm/rocksdb

Reviewed By: riversand963

Differential Revision: D23567244

Pulled By: pdillinger

fbshipit-source-id: e77171c2e8394d173917e36898c02dead1c40b77

Summary:
The patch cleans up a few things in `CompactionJob::SubcompactionState`:

* Instead of using both the member initializer list and in-class initializers (and
sometimes both at the same time for the same member), the struct now uniformly
uses the latter to initialize integer members.
* The default parameter value for the constructor parameter `size` is removed.
* The explicitly deleted copy operations are removed, since they are implicitly deleted
anyways because of the `unique_ptr` members.
* The handwritten move operations, which did not move the member `c_iter` and
were not declared `nothrow`, are removed. Note that with the user-declared copy
operations gone (see the previous item), we can rely on the compiler to (correctly)
generate these methods.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7322

Test Plan: `make check`

Reviewed By: siying

Differential Revision: D23382408

Pulled By: ltamasi

fbshipit-source-id: a4ae5af150161c50ff7bdc07fa145482d0150bfe

Summary:
Currently, application may pass a statistics object to db but later
wants to reduce stats tracking overhead by setting stats level to
kExceptHistogramOrTimers (the current lowest level). Tickers will still
be incremented, causing up to 1% CPU. We can add a new lowest stats
level `kExceptTickers` to disable ticker incrementing as well, thus
reducing CPU cycles spent on tickers.

Test Plan (devserver):
```
make check
make clean
DEBUG_LEVEL=0 make db_bench
./db_bench -perf_level=1 -stats_level=0 -statistics -benchmarks=fillseq,readrandom -duration=120
```

Measure CPU util (%) before and after change:
CPU util by rocksdb::RecordTick: 1.1 vs (<0.1)

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7329

Reviewed By: pdillinger

Differential Revision: D23434014

Pulled By: riversand963

fbshipit-source-id: 72ff0f02a192ac476d4b0044b9f37fd4a22ff0d4

Summary: Pull Request resolved: https://github.com/facebook/rocksdb/pull/7315

Test Plan:
`ASSERT_STATUS_CHECKED=1 make sst_dump_test && ./sst_dump_test`
And manually run `./sst_dump --file=*.sst` before and after the change.

Reviewed By: pdillinger

Differential Revision: D23361669

Pulled By: jay-zhuang

fbshipit-source-id: 5bf51a2a90ee35c8c679e5f604732ec2aef5949a

Summary:
Seems it's causing some tests failures.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7350

Reviewed By: riversand963

Differential Revision: D23544109

Pulled By: jay-zhuang

fbshipit-source-id: 798a0ca374a20b6c2d0f29582729ff101c6a2e99

Summary:
This change has the crash test randomly select from a few file
checksum implementations, or nullptr, for DB file_checksum_gen_factory.
For compatibility across runs on same DB, each non-null factory can
understand all the other functions, but the default changes.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7343

Test Plan:
'make blackbox_crash_test' for a while, including with some
debug output to ensure code is being exercised.

Reviewed By: zhichao-cao

Differential Revision: D23494580

Pulled By: pdillinger

fbshipit-source-id: 73bbc7ca32c1adaf619134c0c830f12894880b8a

Summary:
The level args should be output level instead of input levels.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7346

Test Plan: make check

Reviewed By: ajkr

Differential Revision: D23506373

Pulled By: cheng-chang

fbshipit-source-id: b2f701d44c13581c5c10c4dbebded4fcd354d641

Summary:
Although added to db_stress, testing of backup/restore
was never integrated into the crash test, originally concerned about
performance. I've enabled it now and to address the peformance concern,
testing backup/restore is always skipped once the db exceeds a certain
size threshold, default 100MB. This should provide sufficient
opportunity for testing BackupEngine without bogging down everything
else with heavier and heavier operations.

Also fixed backup/restore in db_stress by making sure PurgeOldBackups
can remove manifest files, which are normally kept around for db_stress.

Added more coverage of backup options, and up to three backups being
saved in one backup directory (in some cases).

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7348

Test Plan:
ran 'make blackbox_crash_test' for a while, with heightened
probabilitly of taking backups (1/10k). Also confirmed with some debug
output that the code is being covered, TestBackupRestore only takes
a few seconds to complete when triggered, and even at 1/10k and ~50MB
database, there's <,~ 1 thread testing backups at any time.

Reviewed By: ajkr

Differential Revision: D23510835

Pulled By: pdillinger

fbshipit-source-id: b6b8735591808141f81f10773ac31634cf03b6c0

Summary:
This is adapted from https://github.com/facebook/rocksdb/issues/6678 but takes a different approach, avoiding opening a read-write DB and avoiding the `DeleteFile()` API.

First, this PR refactors how options variables are initialized in `ldb` so it can be reused in a subcommand that doesn't open a DB:

- Separated remaining option initialization logic out of `OpenDB()`. The new `PrepareOptions()` function initializes the full options state.
- Fixed an old TODO about applying the subcommand CF option overrides to the proper `ColumnFamilyOptions` object.

Second, this PR adds the `ldb unsafe_remove_sst_file` subcommand. It uses the `VersionSet`-level APIs to remove the file with the specified number.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7335

Test Plan: played with interactive python and this file removal command. Verified openability/correct results in case of multiple column families, multiple levels, etc.

Reviewed By: pdillinger

Differential Revision: D23454575

Pulled By: ajkr

fbshipit-source-id: 039b7a8cbfc42fd123dcb25821eef51d61148afe

Summary:
Since we can't land https://github.com/facebook/rocksdb/issues/7336 until the next major release, added a strong warning against the `DeleteFile()` API in the meantime.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7337

Reviewed By: pdillinger

Differential Revision: D23459728

Pulled By: ajkr

fbshipit-source-id: 326cb9b18190386080c35c761a8736d8a877dafb

Summary:
In block-based table builder, the cut-over from buffered to unbuffered
mode involves sampling the buffered blocks and generating a dictionary.
There was a bug where `SstFileWriter` passed zero as the `target_file_size`
causing the cutover to happen immediately, so there were no samples
available for generating the dictionary.

This PR changes the meaning of `target_file_size == 0` to mean buffer
the whole file before cutting over. It also adds dictionary compression
support to `sst_dump --command=recompress` for easy evaluation.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7323

Reviewed By: cheng-chang

Differential Revision: D23412158

Pulled By: ajkr

fbshipit-source-id: 3b232050e70ef3c2ee85a4b5f6fadb139c569873

Summary:
This PR creates `rocksdb_open_column_families_with_ttl` which allows C API users to open a DBWithTLL with column families.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7314

Reviewed By: cheng-chang

Differential Revision: D23430287

Pulled By: ajkr

fbshipit-source-id: 307aa21d170d1402653263a91f6f832ef76afba0

Summary:
- Closes https://github.com/facebook/rocksdb/issues/6490
- Currently MERGEs are converted to PUTs at bottom or compaction has reached the beginning of the key, this can wrongly cover a PUT future base case.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7166

Test Plan:
- Automated: `make all check`
- Manual: With `allow_ingest_behind = true`, add Merge operations to a key then run compaction. Then run ingesting external files to make sure the base case is probably compacted with existing Merges.

Reviewed By: cheng-chang

Differential Revision: D23325425

Pulled By: ajkr

fbshipit-source-id: 3eb415eb7b381b5453e45245393566153b1abb68

Summary:
Delete database instances to make sure there are no loose threads
running before exit(). This fixes segfaults seen when running
workloads through CompositeEnvs with custom file systems.

For further background on the issues arising when using CompositeEnvs, see the discussion in:
https://github.com/facebook/rocksdb/pull/6878

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7327

Reviewed By: cheng-chang

Differential Revision: D23433244

Pulled By: ajkr

fbshipit-source-id: 4e19cf2067e3fe68c2a3fe1823f24b4091336bbe