Improve Backup Engine.

Summary:
Improve the backup engine by not deleting the corrupted
backup when it is detected; instead leaving it to the client
to delete the corrupted backup.

Also add a BackupEngine::Open() call.

Test Plan:
Add check to CorruptionTest inside backupable_db_test
to check that the corrupt backups are not deleted. The previous
version of the code failed this test as backups were deleted,
but after the changes in this commit, this test passes.

Run make check to ensure that no other tests fail.

Reviewers: sdong, benj, sanketh, sumeet, igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D28521
Fix bug of reading from empty DB.

Summary: I found that db_stress sometimes segfault on my machine. Fix the bug.

Test Plan: make all check. Run db_stress

Reviewers: ljin, yhchiang, rven, igor

Reviewed By: igor

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D28803
Fix iOS compile with -Wshorten-64-to-32

Summary: So iOS size_t is 32-bit, so we need to static_cast<size_t> any uint64_t :(

Test Plan: TARGET_OS=IOS make static_lib

Reviewers: dhruba, ljin, yhchiang, rven, sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D28743
Fix include
Add concurrency to compacting SpatialDB

Summary: This will speed up our import times

Test Plan: Added simple unit test just to get code coverage

Reviewers: sdong, ljin, yhchiang, rven, mohaps

Reviewed By: mohaps

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D28869
Move NeedsCompaction() from VersionStorageInfo to CompactionPicker

Summary:
Move NeedsCompaction() from VersionStorageInfo to CompactionPicker
to allow different compaction strategy to have their own way to
determine whether doing compaction is necessary.

When compaction style is set to kCompactionStyleNone, then
NeedsCompaction() will always return false.

Test Plan:
export ROCKSDB_TESTS=Compact
./db_test

Reviewers: ljin, sdong, igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D28719
No CompactFiles in ROCKSDB_LITE

Summary: It adds lots of code.

Test Plan: compile for iOS, compile for mac. works.

Reviewers: rven, sdong, ljin, yhchiang

Reviewed By: yhchiang

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D28857
Fix broken test in 31b02d.

Summary:
CorruptionTest for backupable_db_test did not call
GarbageCollect() after deleting a corrupt backup,
which sometimes lead to test failures as the newly created backup
would reuse the same backup ID and files and fail the consistency
check.

Moved around some of the test logic to ensure that GarbageCollect()
is called at the right time.

Test Plan:
Run backupable_db_test eight times and make sure
it passes repeatedly. Also run make check to make sure other
tests don't fail.

Reviewers: igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D28863
Fix SIGSEGV

Summary: As a short-term fix, let's go back to previous way of calculating NeedsCompaction(). SIGSEGV happens because NeedsCompaction() can happen before super_version (and thus MutableCFOptions) is initialized.

Test Plan: make check

Reviewers: ljin, sdong, rven, yhchiang

Reviewed By: yhchiang

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D28875
Add a unit test for behavior when merge operator and compaction filter co-exist.

Summary: Add a unit test in db_test to verify the behavior when both of merge operator and compaction filter apply to a key when merging.

Test Plan: Run the new test

Reviewers: ljin, yhchiang, rven, igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D28455
Relax the block count check on deallocation in env_test

It seems that on some FS we get more blocks than we ask for. This is
already handled when checking the allocated number of blocks, but
after the file is closed it checks for an exact number of blocks,
which fails on my machine.

I changed the test to add one full page to the size, then calculate
the expected number of blocks and check if the actual number of blocks
is less or equal to that.
Make db_stress built for ROCKSDB_LITE

Summary:
Make db_stress built for ROCKSDB_LITE.
The test doesn't pass tough. It seg fault quickly. But I took a look and it doesn't seem to be related to lite version. Likely to be a bug inside RocksDB.

Test Plan: make db_stress

Reviewers: yhchiang, rven, ljin, igor

Reviewed By: igor

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D28797
Update HISTROY.md for 3.8 release
Merge pull request #395 from lalinsky/fix-env-test

Relax the block count check on deallocation in env_test
Update HISTORY.md for RocksJava
CompactionJobTest

Summary:
This is just a simple test that passes two files though a compaction. It shows the framework so that people can continue building new compaction *unit* tests.
In the future we might want to move some Compaction* tests from DBTest here. For example, CompactBetweenSnapshot seems a good candidate.

Hopefully this test can be simpler when we mock out VersionSet.

Test Plan: this is a test

Reviewers: ljin, rven, yhchiang, sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D28449
Provide openable snapshots

Summary: Store links to live files in directory on same disk

Test Plan:
Take snapshot and open it. Added a test GetSnapshotLink in
db_test.

Reviewers: sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D28713
Fix build