This prevents file descriptors from leaking to child processes.

When compiled for older (pre-2.6.23) kernels which lack support for
O_CLOEXEC there is no change in behavior.  With newer kernels, child
processes will no longer inherit leveldb's file handles, which
reduces the changes of accidentally corrupting the database.

Fixes https://github.com/google/leveldb/issues/623

Apple doesn't follow POSIX specifications for fsync(). Instead, fsync() guarantees to flush the buffer cache to the device, which means the data will survive kernel panics, but may not survive power outages. Applications that need stronger guarantees (like databases) need to use fcntl(F_FULLFSYNC).

This CL switches PosixWritableFile::Sync() to get the stronger guarantees on Apple systems. The improved implementation follows the same principles as SQLite [1] and node.js [2].

Research for the fcntl() to fsync() fallback strategy:

Apple's released source code at https://opensource.apple.com/ shows at least three different error codes being returned when a filesystem does not support F_FULLFSYNC.

fcntl() is implemented in xnu-4903.221.2 in bsd/kern/kern_descrip.c, where it delegates to fcntl_nocancel(). The documentation for fcntl_nocancel() mentions error codes for some operations, but does not include F_FULLFSYNC. The F_FULLSYNC branch in fcntl_nocancel() calls VNOP_IOCTL(_, F_FULLSYNC, NULL, 0, _), whose return value sets the error
code.

VNOP_IOCTL() is implemented in bsd/vfs/kpi_vfs.c and calls the ioctl function in the vnode's operation vector. The per-filesystem function names follow the pattern _vnop_ioctl() for all the instances in opensource code: {hfs,msdosfs,nfs,ntfs,smbfs,webdav,zfs}_vnop_ioctl().

hfs-407.30.1, msdosfs-229.200.3, and nfs in xnu-4903.221.2 handle F_FULLFSYNC. ntfs-94.200.1 and smb-759.40.1 do not handle F_FULLFSYNC, and the default branch returns ENOSUP. webdav-380.200.1 also does not handle F_FULLFSYNC, but the default branch returns EINVAL. zfs-59 also does not handle F_FULLSYNC, and its default branch returns ENOTTY.

From a different angle, Apple's ntfs-94.200.1 includes utility code that uses fcntl(F_FULLFSYNC) and falls back to fsync() just like we do, supporting the hypothesis that there is no good way to detect lack of F_FULLFSYNC support. Also, Apple's fcntl() man page [3] does not mention a way to detect lack of F_FULLFSYNC support.

[1] https://www.sqlite.org/src/doc/trunk/src/os_unix.c
[2] https://github.com/libuv/libuv/blob/master/src/unix/fs.c
[3] https://developer.apple.com/library/archive/documentatiVon/System/Conceptual/ManPages_iPhoneOS/man2/fcntl.2.html
Tested:
    https://travis-ci.org/pwnall/leveldb/builds/477318498
    TAP global presubmit

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=228593729

General cleanup principles:
* Use override when applicable.
* Remove static when redundant (methods and  globals in anonymous
  namespaces).
* Use const on class members where possible.
* Standardize on "status" for Status local variables.
* Renames where clarity can be improved.
* Qualify standard library names with std:: when possible, to
  distinguish from POSIX names.
* Qualify POSIX names with the global namespace (::) when possible, to
  distinguish from standard library names.

This also refactors the background thread synchronization logic so that
it's statically analyzable.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=219212089

C++11 guarantees thread-safe initialization of static variables inside
functions. This is a more restricted form of std::call_once or
pthread_once_t (e.g., single call site), so the compiler might be able
to generate better code [1]. Equally important, having less
platform-dependent code in env_posix.cc makes it easier to port to other
platforms.

Due to the change above, this CL introduced a new approach for storing
the singleton PosixEnv instance returned by Env::Default(). The new
approach avoids a dynamic memory allocation, which eliminates the false
positive from LeakSanitizer reported in
https://github.com/google/leveldb/issues/539 and
https://github.com/google/leveldb/issues/113

[1] https://stackoverflow.com/a/27206650/

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=214293129

This commit replaces the use of pthreads in the POSIX port with std::thread
and port::Mutex + port::CondVar. This is intended to simplify porting
the env to a different platform.

The indirect use of pthreads in PosixLogger is replaced with
std:🧵:id(), based on an approach prototyped by @cmumfordx@.

The pthreads dependency in CMakeFiles is not removed, because some C++
standard library implementations must be linked against pthreads for
std::thread use. Figuring out this dependency is left for future work.

Switching away from pthreads also fixes
https://github.com/google/leveldb/issues/381

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=212478311

This is separated from the general cleanup because of the logic changes
in SyncDirIfManifest().

General cleanup principles:
* Use override when applicable.
* Remove static when redundant (methods and  globals in anonymous
  namespaces).
* Use const on class members where possible.
* Standardize on "status" for Status local variables.
* Renames where clarity can be improved.
* Qualify standard library names with std:: when possible, to
  distinguish from POSIX names.
* Qualify POSIX names with the global namespace (::) when possible, to
  distinguish from standard library names.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=211709673

Now that we require C++11, we can use std::atomic<int>, which has
primitives for most of the logic we need. As a bonus, the happy path for
Limiter::Acquire() and Limiter::Release() only performs one atomic
operation.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=211469518

The porting layer implements threading primitives: atomic pointers,
condition variables, mutexes, thread-safe initialization. These are all
specified in C++11, so the reference open source port implementation can
become platform-independent.

The porting layer will remain in place to allow the use of other
implementations with more features, such as the built-in deadlock
detection in abseil's Mutex.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=193245934

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=192365747

After this CL, all classes with Mutex members should be covered by annotations. Exceptions are atomic members, which shouldn't need locking, and DBImpl members that cause errors when annotated, which will be tackled separately.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=190260865

Deleting a PosixWritableFile without calling Close() leaks the file
descriptor. While the API description in include/leveldb/env.h does not
specify whether the caller is responsible for Close()ing the file before
deleting it, all other Env file implementations do release underlying
resources when destroyed, even if Close() is not called.

The leak shows up when running db_tests on Mac Travis, or on a vanilla
MacOS install.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=170906843

If the file already existed, we should have truncated it. This was not
detected by leveldb tests since leveldb code avoids reusing same files,
but there was code elsewhere that was directly using leveldb files and
relying on this behavior.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=170769101

env_posix.cc and concurrent application calls to fflush(NULL).

The fix is to avoid using stdio in env_posix.cc but add our own
buffering where we need it.

Added a test to reproduce the bug.

Added a test for Env reads/writes.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=170738066

BTRFS reorders rename and write operations, so it is possible that a filesystem crash and recovery results in a situation where the file pointed to by CURRENT does not exist. DB::Open currently reports an I/O error in this case. Reporting database corruption is a better hint to the caller, which can attempt to recover the database or erase it and start over.

This issue is not merely theoretical. It was reported as having showed up in the wild at https://github.com/google/leveldb/issues/195 and at https://crbug.com/738961. Also, asides from the BTRFS case described above, incorrect data in CURRENT seems like a possible corruption case that should be handled gracefully.

The Env API changes here can be considered backwards compatible, because an implementation that returns Status::IOError instead of Status::NotFound will still get the same functionality as before.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=161432630

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=146841327

Background compaction can create an unbounded number of
leveldb::RandomAccessFile instances. On 64-bit systems mmap is used and
file descriptors are only used beyond a certain number of mmap's.
32-bit systems to not use mmap at all. leveldb::RandomAccessFile does not
observe Options.max_open_files so compaction could exhaust the file
descriptor limit.

This change uses getrlimit to determine the maximum number of open
files and limits RandomAccessFile to approximately 20% of that value.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=143505556

(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.

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)

- 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 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.

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.

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

- Replace raw slice comparison with a call to user comparator.
  Added test for custom comparators.

- Fix end of namespace comments.

- Fixed bug in picking inputs for a level-0 compaction.

  When finding overlapping files, the covered range may expand
  as files are added to the input set.  We now correctly expand
  the range when this happens instead of continuing to use the
  old range.  For example, suppose L0 contains files with the
  following ranges:

      F1: a .. d
      F2:    c .. g
      F3:       f .. j

  and the initial compaction target is F3.  We used to search
  for range f..j which yielded {F2,F3}.  However we now expand
  the range as soon as another file is added.  In this case,
  when F2 is added, we expand the range to c..j and restart the
  search.  That picks up file F1 as well.

  This change fixes a bug related to deleted keys showing up
  incorrectly after a compaction as described in Issue 44.

(Sync with upstream @25072954)

- Removed one copy of an uncompressed block contents changing
  the signature of Snappy_Uncompress() so it uncompresses into a
  flat array instead of a std::string.
        
  Speeds up readrandom ~10%.

- Instead of a combination of Env/WritableFile, we now have a
  Logger interface that can be easily overridden applications
  that want to supply their own logging.

- Separated out the gcc and Sun Studio parts of atomic_pointer.h
  so we can use 'asm', 'volatile' keywords for Sun Studio.




git-svn-id: https://leveldb.googlecode.com/svn/trunk@39 62dab493-f737-651d-591e-8d6aee1b9529

Slight tweak to the no-overlap optimization: only push to
level 2 to reduce the amount of wasted space when the same
small key range is being repeatedly overwritten.

Fix for Issue 18: Avoid failure on Windows by avoiding
deletion of lock file until the end of DestroyDB().

Fix for Issue 19: Disregard sequence numbers when checking for 
overlap in sstable ranges. This fixes issue 19: when writing 
the same key over and over again, we would generate a sequence 
of sstables that were never merged together since their sequence
numbers were disjoint.

Don't ignore map/unmap error checks.

Miscellaneous fixes for small problems Sanjay found while diagnosing
issue/9 and issue/16 (corruption_testr failures).
- log::Reader reports the record type when it finds an unexpected type.
- log::Reader no longer reports an error when it encounters an expected
  zero record regardless of the setting of the "checksum" flag.
- Added a missing forward declaration.
- Documented a side-effects of larger write buffer sizes
  (longer recovery time).



git-svn-id: https://leveldb.googlecode.com/svn/trunk@37 62dab493-f737-651d-591e-8d6aee1b9529

Check the NEWS file for details of what changed.

git-svn-id: https://leveldb.googlecode.com/svn/trunk@28 62dab493-f737-651d-591e-8d6aee1b9529

git-svn-id: https://leveldb.googlecode.com/svn/trunk@23 62dab493-f737-651d-591e-8d6aee1b9529

git-svn-id: https://leveldb.googlecode.com/svn/trunk@21 62dab493-f737-651d-591e-8d6aee1b9529

git-svn-id: https://leveldb.googlecode.com/svn/trunk@20 62dab493-f737-651d-591e-8d6aee1b9529

git-svn-id: https://leveldb.googlecode.com/svn/trunk@18 62dab493-f737-651d-591e-8d6aee1b9529

git-svn-id: https://leveldb.googlecode.com/svn/trunk@16 62dab493-f737-651d-591e-8d6aee1b9529

git-svn-id: https://leveldb.googlecode.com/svn/trunk@9 62dab493-f737-651d-591e-8d6aee1b9529

git-svn-id: https://leveldb.googlecode.com/svn/trunk@2 62dab493-f737-651d-591e-8d6aee1b9529