Update README.md

Fix Android/MIPS build.

In preparation for accepting GitHub pull requests this new README
section outlines the general criteria that the leveldb project owners
will use when accepting external (and internal) project contributions.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=111349899

readme: improved documentation link

The write operations in the table happens without holding the mutex
lock, but concurrent writes are avoided using "writers_" queue.
The Arena::MemoryUsage could access the blocks when write happens.
So, the memory usage is cached in atomic word and can be loaded
from any thread safely.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=107573379

Incorporated change by zmodem at https://github.com/google/leveldb/pull/310
to fix issue #310.

This change will only build TrimSace on linux to avoid unused function
warning/error.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=105323419

A recent CL (104348226) created the port_posix library, but omitted: port/atomic_pointer.h.

And when:

    [] test third_party/leveldb:all

was run this error was reported:

    //third_party/leveldb:port_posix does not depend on a
    module exporting 'third_party/leveldb/port/atomic_pointer.h'
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=105243399

This allows build agents to select from multiple Xcode installations.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=104859097

The approximate RAM usage of the database is calculated from the memory
allocated for write buffers and the block cache. This is to give an
estimate of memory usage to leveldb clients.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=104222307

Prune() drops on-memory read cache of the database, so that the client can
relief its memory shortage.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=101335710

The create function took |num_keys| as an int, but callers and implementers wanted it to function as a size_t (e.g. passing std::vector::size() in, passing it to vector constructors as a size arg, indexing containers by it, etc.).  This resulted in implicit conversions between the two types as well as warnings (found with Chromium's external copy of these sources, built with MSVC) about signed vs. unsigned comparisons.

The leveldb sources were already widely using size_t elsewhere, e.g. for key and filter lengths, so using size_t here is not inconsistent with the existing code.  However, it does change the public C API.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=101074871

All other Status::Code enum values have an Is**() method with the one
exception of InvalidArgument.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=97166441

Fix a spelling mistake.

Fixes reported public issue #280.

Prior implementation would always try to reuse the manifest, even if reuse_logs
was false (the default). This was missed because the stock
Env::NewAppendableFile implementation returns false forcing the creation of a
new log.

(Based on a suggestion by cmumford.)

"open" benchmark on my workstation speeds up significantly since we
can now avoid three fdatasync calls and a compaction per open:

  Before: ~80000 microseconds
  After:    ~130 microseconds

Details:

(1) Added Options::reuse_logs (currently defaults to false) to control
new behavior.  The intention is to change the default to true after some
baking.

(2) Added Env::NewAppendableFile() whose default implementation returns
a not-supported error.

(3) VersionSet::Recovery attempts to reuse the MANIFEST from which
it is recovering.

(4) DBImpl recovery attempts to reuse the last log file and memtable.

(5) db_test.cc now tests a new configuration that sets reuse_logs to true.

(6) fault_injection_test also tests a reuse_logs==true config.

(7) Added a new recovery_test.

This replaces htmlpreview with [rawgit](http://rawgit.com/).
rawgit is faster and doesnt use JS to render the page, making it SEO friendly.

port/atomic_pointer.h was missing an implementation for
MemoryBarrier() for this platform.

With these changes, this package should be properly cleaned up and not require
any further changes.

This test is intended to ensure leveldb properly detects and recovers from
faults - specifically unwritten file data lost as a result of a system reset.

README was superseded by README.md and should have been deleted in 803d692.

Changes are:

* Update version number to 1.18
* Replace the basic fprintf call with a call to fwrite in order to
  work around the apparent compiler optimization/rewrite failure that we are
  seeing with the new toolchain/iOS SDKs provided with Xcode6 and iOS8.
* Fix ALL the header guards.
* Createed a README.md with the LevelDB project description.
* A new CONTRIBUTING file.
* Don't implicitly convert uint64_t to size_t or int.  Either preserve it as
  uint64_t, or explicitly cast. This fixes MSVC warnings about possible value
  truncation when compiling this code in Chromium.
* Added a DumpFile() library function that encapsulates the guts of the
  "leveldbutil dump" command. This will allow clients to dump
  data to their log files instead of stdout. It will also allow clients to
  supply their own environment.
* leveldb: Remove unused function 'ConsumeChar'.
* leveldbutil: Remove unused member variables from WriteBatchItemPrinter.
* OpenBSD, NetBSD and DragonflyBSD have _LITTLE_ENDIAN, so define
  PLATFORM_IS_LITTLE_ENDIAN like on FreeBSD. This fixes:
   * issue #143
   * issue #198
   * issue #249
* Switch from <cstdatomic> to <atomic>. The former never made it into the
  standard and doesn't exist in modern gcc versions at all.  The later contains
  everything that leveldb was using from the former.
  This problem was noticed when porting to Portable Native Client where no memory
  barrier is defined.  The fact that <cstdatomic> is missing normally goes
  unnoticed since memory barriers are defined for most architectures.
* Make Hash() treat its input as unsigned.  Before this change LevelDB files
  from platforms with different signedness of char were not compatible. This
  change fixes: issue #243
* Verify checksums of index/meta/filter blocks when paranoid_checks set.
* Invoke all tools for iOS with xcrun. (This was causing problems with the new
  XCode 5.1.1 image on pulse.)
* include <sys/stat.h> only once, and fix the following linter warning:
  "Found C system header after C++ system header"
* When encountering a corrupted table file, return Status::Corruption instead of
  Status::InvalidArgument.
* Support cygwin as build platform, patch is from https://code.google.com/p/leveldb/issues/detail?id=188
* Fix typo, merge patch from https://code.google.com/p/leveldb/issues/detail?id=159
* Fix typos and comments, and address the following two issues:
  * issue #166
  * issue #241
* Add missing db synchronize after "fillseq" in the benchmark.
* Removed unused variable in SeekRandom: value (issue #201)

- Cleanup: delete unused IntSetToString

  It was added in http://cr/19491949 (and was referenced at the time).
  The last reference was removed in http://cr/19507363.

  This fixes warning/error with pre-release crosstoolv18:
    'std::string leveldb::{anonymous}::IntSetToString(const std::set<long unsigned int>&)' defined but not used [-Werror=unused-function]

- Added arm64 and and armv7s to IOS build as suggested on leveldb mailing list.

- Changed local variable type from int to size_t

  This eliminates compiler warning/error and resolves
  https://code.google.com/p/leveldb/issues/detail?id=140

- Make Log::Reader not report a corruption when the last record in a
  log file is truncated.
- Fix issue 224: variable created but not utilized.
- Remove comment that referenced a removed feature.

- switched from mmap based writing to simpler stdio based writing. Has a
  minor impact (0.5 microseconds) on microbenchmarks for asynchronous
  writes. Synchronous writes speed up from 30ms to 10ms on linux/ext4.
  Should be much more reliable on diverse platforms.
- compaction errors now immediately put the database into a read-only
  mode (until it is re-opened). As a downside, a disk going out of
  space and then space being created will require a re-open to recover
  from, whereas previously that would happen automatically. On the
  plus side, many corruption possibilities go away.
- force the DB to enter an error-state so that all future writes fail
  when a synchronous log write succeeds but the sync fails.
- repair now regenerates sstables that exhibit problems
- fix issue 218 - Use native memory barriers on OSX
- fix issue 212 - QNX build is broken
- fix build on iOS with xcode 5
- make tests compile and pass on windows

Fix issues 200, 201

Also,
* Fix link to bigtable paper in docs.
* New sstables will have the file extension .ldb. .sst files will
continue to be recognized.
* When building for iOS, use xcrun to execute the compiler. This may
affect issue 177.

Fix issues 77, 87, 182, 190.

Additionally, fix the bug described in
https://groups.google.com/d/msg/leveldb/yL6h1mAOc20/vLU64RylIdMJ
where a large contiguous keyspace of deleted data was not getting
compacted.

Also fix a bug where options.max_open_files was not getting clamped
properly.

Non-functional changes only:
1) Add Kevin Regan to the AUTHORS file
2) Add standard license header to issues/issue178_test.cc

Fixes issues
161
174
178

As well as the issue reported by edouarda14@gmail.com about
MissingSSTFile unit test failing on windows.

Fixes issues
147 - thanks feniksgordonfreeman
153
156
166

Additionally,
* Remove calls to exit(1).
* Fix unused-variable warnings from clang.
* Fix possible overflow error related to num_restart value >= (2^32/4).
* Add leveldbutil to .gitignore.
* Add better log messages when Write is stalled on a compaction.

Also, cleanup for Clang's -Wimplicit-fallthrough warning.

https://groups.google.com/d/msg/leveldb/Kc9JxuIUu5A/9P0N9RL4ar8J

Details:
* Fix shared library building.
* Reorganize linking commands so flags like --as-needed can be passed.
* C binding exports version numbers.
* Fix small typos in documention.

Highlights
----------
Mmap at most 1000 files on Posix to improve performance for large databases.
Support for more architectures (thanks to Alexander K.)

Building and porting
--------------------
HP/UX support (issue 126)
AtomicPointer for ia64 (issue 123)
Sparc v9 support (issue 124)
Atomic ops for powerpc
Use -fno-builtin-memcmp only when using g++
Simplify IOS build rules (issue 114)
Use CXXFLAGS instead of CFLAGS when invoking C++ compiler (issue 118)
Fix snappy shared library problem (issue 94)
Fix shared library installation path regression
Endian-ness detection tweak for FreeBSD

Bug fixes
---------
Stop ignoring FLAGS_open_files in db_bench
Make bloom test behavior agnostic to endian-ness

Performance
-----------
Limit number of mmapped files to 1000 to improve perf for large dbs
Do not delay for 1 second on shutdown path (issue 125)

Misc
----
Make InMemoryEnv return a no-op logger
C binding now has a wrapper for free (issue 117)
Add thread-safety annotations
Added an in-process lock table (issue 120)
Make RandomAccessFile and SequentialFile non-copyable

various platforms; improve android port speed.

Avoid static initializer by using a new portability interface for
thread-safe lazy initialization.  Custom ports will need to be
extended to implement InitOnce/OnceType/LEVELDB_ONCE_INIT.

Fix endian-ness detection (fixes Powerpc builds).

Build related fixes:
- Support platforms that have unversioned shared libraries.
- Fix IOS build rules.

Android improvements
- Speed up atomic pointers
- Share more code with port_posix.

Do not spin in a tight loop attempting compactions if the file system
is inaccessible (e.g., if kerberos tickets have expired or if it is out
of space).

In particular, we add a new FilterPolicy class.  An instance
of this class can be supplied in Options when opening a
database.  If supplied, the instance is used to generate
summaries of keys (e.g., a bloom filter) which are placed in
sstables.  These summaries are consulted by DB::Get() so we
can avoid reading sstable blocks that are guaranteed to not
contain the key we are looking for.

This change provides one implementation of FilterPolicy
based on bloom filters.

Other changes:
- Updated version number to 1.4.
- Some build tweaks.
- C binding for CompactRange.
- A few more benchmarks: deleteseq, deleterandom, readmissing, seekrandom.
- Minor .gitignore update.

(1) Separate out C++ and CC flags (fixes c_test compilation)
(2) Move snappy/perftools detection to script
(3) Fix db_bench_sqlite3 and db_bench_tree_db build rules