// A file abstraction for reading sequentially through a file
class LEVELDB_EXPORT SequentialFile {
 public:
  SequentialFile() = default;

  SequentialFile(const SequentialFile&) = delete;
  SequentialFile& operator=(const SequentialFile&) = delete;

  virtual ~SequentialFile();

  // Read up to "n" bytes from the file.  "scratch[0..n-1]" may be
  // written by this routine.  Sets "*result" to the data that was
  // read (including if fewer than "n" bytes were successfully read).
  // May set "*result" to point at data in "scratch[0..n-1]", so
  // "scratch[0..n-1]" must be live when "*result" is used.
  // If an error was encountered, returns a non-OK status.
  //
  // REQUIRES: External synchronization
  virtual Status Read(size_t n, Slice* result, char* scratch) = 0;

  // Skip "n" bytes from the file. This is guaranteed to be no
  // slower that reading the same data, but may be faster.
  //
  // If end of file is reached, skipping will stop at the end of the
  // file, and Skip will return OK.
  //
  // REQUIRES: External synchronization
  virtual Status Skip(uint64_t n) = 0;
};

// Create an object that sequentially reads the file with the specified name.
// On success, stores a pointer to the new file in *result and returns OK.
// On failure stores nullptr in *result and returns non-OK.  If the file does
// not exist, returns a non-OK status.  Implementations should return a
// NotFound status when the file does not exist.
//
// The returned file will only be accessed by one thread at a time.
Status PosixEnv::NewSequentialFile(const std::string& filename,
                         SequentialFile** result) override {
  int fd = ::open(filename.c_str(), O_RDONLY | kOpenBaseFlags);
  if (fd < 0) {
    *result = nullptr;
    return PosixError(filename, errno);
  }

  *result = new PosixSequentialFile(filename, fd);
  return Status::OK();
}

// Implements sequential read access in a file using read().
//
// Instances of this class are thread-friendly but not thread-safe, as required
// by the SequentialFile API.
class PosixSequentialFile final : public SequentialFile {
 public:
  PosixSequentialFile(std::string filename, int fd)
      : fd_(fd), filename_(filename) {}
  ~PosixSequentialFile() override { close(fd_); }

  Status Read(size_t n, Slice* result, char* scratch) override {
    Status status;
    while (true) {
      ::ssize_t read_size = ::read(fd_, scratch, n);
      if (read_size < 0) {  // Read error.
        if (errno == EINTR) {
          continue;  // Retry
        }
        status = PosixError(filename_, errno);
        break;
      }
      *result = Slice(scratch, read_size);
      break;
    }
    return status;
  }

  Status Skip(uint64_t n) override {
    if (::lseek(fd_, n, SEEK_CUR) == static_cast<off_t>(-1)) {
      return PosixError(filename_, errno);
    }
    return Status::OK();
  }

 private:
  const int fd_;
  const std::string filename_;
};

// A file abstraction for randomly reading the contents of a file.
class LEVELDB_EXPORT RandomAccessFile {
 public:
  RandomAccessFile() = default;

  RandomAccessFile(const RandomAccessFile&) = delete;
  RandomAccessFile& operator=(const RandomAccessFile&) = delete;

  virtual ~RandomAccessFile();

  // Read up to "n" bytes from the file starting at "offset".
  // "scratch[0..n-1]" may be written by this routine.  Sets "*result"
  // to the data that was read (including if fewer than "n" bytes were
  // successfully read).  May set "*result" to point at data in
  // "scratch[0..n-1]", so "scratch[0..n-1]" must be live when
  // "*result" is used.  If an error was encountered, returns a non-OK
  // status.
  //
  // Safe for concurrent use by multiple threads.
  virtual Status Read(uint64_t offset, size_t n, Slice* result,
                      char* scratch) const = 0;
};

// Create an object supporting random-access reads from the file with the
// specified name.  On success, stores a pointer to the new file in
// *result and returns OK.  On failure stores nullptr in *result and
// returns non-OK.  If the file does not exist, returns a non-OK
// status.  Implementations should return a NotFound status when the file does
// not exist.
//
// The returned file may be concurrently accessed by multiple threads.
Status PosixEnv::NewRandomAccessFile(const std::string& filename,
                             RandomAccessFile** result) override {
  *result = nullptr;
  int fd = ::open(filename.c_str(), O_RDONLY | kOpenBaseFlags);
  if (fd < 0) {
    return PosixError(filename, errno);
  }

  if (!mmap_limiter_.Acquire()) {
 // 打开的 mmap 文件数量超过阈值
    *result = new PosixRandomAccessFile(filename, fd, &fd_limiter_);
    return Status::OK();
  }

  uint64_t file_size;
  Status status = GetFileSize(filename, &file_size);
  if (status.ok()) {
    void* mmap_base =
        ::mmap(/*addr=*/nullptr, file_size, PROT_READ, MAP_SHARED, fd, 0);
    if (mmap_base != MAP_FAILED) {
      *result = new PosixMmapReadableFile(filename,
                                          reinterpret_cast<char*>(mmap_base),
                                          file_size, &mmap_limiter_);
    } else {
      status = PosixError(filename, errno);
    }
  }
  ::close(fd);
  if (!status.ok()) {
    mmap_limiter_.Release();
  }
  return status;
}

// Implements random read access in a file using pread().
// pread相当于先调用lseek接着调用read。但又不完全是这样：
//（1）pread是原子操作，定位和读操作在一个原子操作中完成，期间不可中断。但分开的lseek和read中间可能被其他程序中断。
//（2）pread不更改当前文件的指针，也就是说不改变当前文件偏移量。
//（3）pread中的offset是一个绝对量，相对于文件开始处的绝对量，与当前文件指针位置无关。
//
// Instances of this class are thread-safe, as required by the RandomAccessFile
// API. Instances are immutable and Read() only calls thread-safe library
// functions.
class PosixRandomAccessFile final : public RandomAccessFile {
 public:
  // The new instance takes ownership of |fd|. |fd_limiter| must outlive this
  // instance, and will be used to determine if .
  PosixRandomAccessFile(std::string filename, int fd, Limiter* fd_limiter)
      : has_permanent_fd_(fd_limiter->Acquire()),
        fd_(has_permanent_fd_ ? fd : -1),
        fd_limiter_(fd_limiter),
        filename_(std::move(filename)) {
    if (!has_permanent_fd_) {
      assert(fd_ == -1);
      ::close(fd);  // The file will be opened on every read.
    }
  }

  ~PosixRandomAccessFile() override {
    if (has_permanent_fd_) {
      assert(fd_ != -1);
      ::close(fd_);
      fd_limiter_->Release();
    }
  }

  Status Read(uint64_t offset, size_t n, Slice* result,
              char* scratch) const override {
    int fd = fd_;
    if (!has_permanent_fd_) {
      fd = ::open(filename_.c_str(), O_RDONLY | kOpenBaseFlags);
      if (fd < 0) {
        return PosixError(filename_, errno);
      }
    }

    assert(fd != -1);

    Status status;
    ssize_t read_size = ::pread(fd, scratch, n, static_cast<off_t>(offset));
    *result = Slice(scratch, (read_size < 0) ? 0 : read_size);
    if (read_size < 0) {
      // An error: return a non-ok status.
      status = PosixError(filename_, errno);
    }
    if (!has_permanent_fd_) {
      // Close the temporary file descriptor opened earlier.
      assert(fd != fd_);
      ::close(fd);
    }
    return status;
  }

 private:
  const bool has_permanent_fd_;  // If false, the file is opened on every read.
  const int fd_;                 // -1 if has_permanent_fd_ is false.
  Limiter* const fd_limiter_;
  const std::string filename_;
};

// Implements random read access in a file using mmap().
//
// Instances of this class are thread-safe, as required by the RandomAccessFile
// API. Instances are immutable and Read() only calls thread-safe library
// functions.
class PosixMmapReadableFile final : public RandomAccessFile {
 public:
  // mmap_base[0, length-1] points to the memory-mapped contents of the file. It
  // must be the result of a successful call to mmap(). This instances takes
  // over the ownership of the region.
  //
  // |mmap_limiter| must outlive this instance. The caller must have already
  // aquired the right to use one mmap region, which will be released when this
  // instance is destroyed.
  PosixMmapReadableFile(std::string filename, char* mmap_base, size_t length,
                        Limiter* mmap_limiter)
      : mmap_base_(mmap_base),
        length_(length),
        mmap_limiter_(mmap_limiter),
        filename_(std::move(filename)) {}

  ~PosixMmapReadableFile() override {
    ::munmap(static_cast<void*>(mmap_base_), length_);
    mmap_limiter_->Release();
  }

  Status Read(uint64_t offset, size_t n, Slice* result,
              char* scratch) const override {
    if (offset + n > length_) {
      *result = Slice();
      return PosixError(filename_, EINVAL);
    }

    *result = Slice(mmap_base_ + offset, n);
    return Status::OK();
  }

 private:
  char* const mmap_base_;
  const size_t length_;
  Limiter* const mmap_limiter_;
  const std::string filename_;
};

// A file abstraction for sequential writing.  The implementation
// must provide buffering since callers may append small fragments
// at a time to the file.
class LEVELDB_EXPORT WritableFile {
 public:
  WritableFile() = default;

  WritableFile(const WritableFile&) = delete;
  WritableFile& operator=(const WritableFile&) = delete;

  virtual ~WritableFile();

  virtual Status Append(const Slice& data) = 0;
  virtual Status Close() = 0;
  virtual Status Flush() = 0;
  virtual Status Sync() = 0;
};

// Create an object that writes to a new file with the specified
// name.  Deletes any existing file with the same name and creates a
// new file.  On success, stores a pointer to the new file in
// *result and returns OK.  On failure stores nullptr in *result and
// returns non-OK.
//
// The returned file will only be accessed by one thread at a time.
Status PosixEnv::NewWritableFile(const std::string& filename,
                       WritableFile** result) override {
  int fd = ::open(filename.c_str(),
                  O_TRUNC | O_WRONLY | O_CREAT | kOpenBaseFlags, 0644);
  if (fd < 0) {
    *result = nullptr;
    return PosixError(filename, errno);
  }

  *result = new PosixWritableFile(filename, fd);
  return Status::OK();
}

// Create an object that either appends to an existing file, or
// writes to a new file (if the file does not exist to begin with).
// On success, stores a pointer to the new file in *result and
// returns OK.  On failure stores nullptr in *result and returns
// non-OK.
//
// The returned file will only be accessed by one thread at a time.
Status PosixEnv::NewAppendableFile(const std::string& filename,
                         WritableFile** result) override {
  int fd = ::open(filename.c_str(),
                  O_APPEND | O_WRONLY | O_CREAT | kOpenBaseFlags, 0644);
  if (fd < 0) {
    *result = nullptr;
    return PosixError(filename, errno);
  }

  *result = new PosixWritableFile(filename, fd);
  return Status::OK();
}

// A file abstraction for sequential writing.  The implementation
// must provide buffering since callers may append small fragments
// at a time to the file.
// 注意 flush 和 sync 的区别
class PosixWritableFile final : public WritableFile {
 public:
  PosixWritableFile(std::string filename, int fd)
      : pos_(0),
        fd_(fd),
        is_manifest_(IsManifest(filename)),
        filename_(std::move(filename)),
        dirname_(Dirname(filename_)) {}

  ~PosixWritableFile() override {
    if (fd_ >= 0) {
      // Ignoring any potential errors
      Close();
    }
  }

  Status Append(const Slice& data) override {
    size_t write_size = data.size();
    const char* write_data = data.data();

    // Fit as much as possible into buffer.
    size_t copy_size = std::min(write_size, kWritableFileBufferSize - pos_);
    std::memcpy(buf_ + pos_, write_data, copy_size);
    write_data += copy_size;
    write_size -= copy_size;
    pos_ += copy_size;
    if (write_size == 0) {
      return Status::OK();
    }

    // Can't fit in buffer, so need to do at least one write.
    Status status = FlushBuffer();
    if (!status.ok()) {
      return status;
    }

    // Small writes go to buffer, large writes are written directly.
    if (write_size < kWritableFileBufferSize) {
      std::memcpy(buf_, write_data, write_size);
      pos_ = write_size;
      return Status::OK();
    }
    return WriteUnbuffered(write_data, write_size);
  }

  Status Close() override {
    Status status = FlushBuffer();
    const int close_result = ::close(fd_);
    if (close_result < 0 && status.ok()) {
      status = PosixError(filename_, errno);
    }
    fd_ = -1;
    return status;
  }

  Status Flush() override { return FlushBuffer(); }

  Status Sync() override {
    // Ensure new files referred to by the manifest are in the filesystem.
    //
    // This needs to happen before the manifest file is flushed to disk, to
    // avoid crashing in a state where the manifest refers to files that are not
    // yet on disk.
    Status status = SyncDirIfManifest();
    if (!status.ok()) {
      return status;
    }

    status = FlushBuffer();
    if (!status.ok()) {
      return status;
    }

    return SyncFd(fd_, filename_);
  }

 private:
  Status FlushBuffer() {
    Status status = WriteUnbuffered(buf_, pos_);
    pos_ = 0;
    return status;
  }

  Status WriteUnbuffered(const char* data, size_t size) {
    while (size > 0) {
      ssize_t write_result = ::write(fd_, data, size);
      if (write_result < 0) {
        if (errno == EINTR) {
          continue;  // Retry
        }
        return PosixError(filename_, errno);
      }
      data += write_result;
      size -= write_result;
    }
    return Status::OK();
  }

  Status SyncDirIfManifest() {
    Status status;
    if (!is_manifest_) {
      return status;
    }

    int fd = ::open(dirname_.c_str(), O_RDONLY | kOpenBaseFlags);
    if (fd < 0) {
      status = PosixError(dirname_, errno);
    } else {
      status = SyncFd(fd, dirname_);
      ::close(fd);
    }
    return status;
  }

  // Ensures that all the caches associated with the given file descriptor's
  // data are flushed all the way to durable media, and can withstand power
  // failures.
  //
  // The path argument is only used to populate the description string in the
  // returned Status if an error occurs.
  static Status SyncFd(int fd, const std::string& fd_path) {
#if HAVE_FULLFSYNC
    // On macOS and iOS, fsync() doesn't guarantee durability past power
    // failures. fcntl(F_FULLFSYNC) is required for that purpose. Some
    // filesystems don't support fcntl(F_FULLFSYNC), and require a fallback to
    // fsync().
    if (::fcntl(fd, F_FULLFSYNC) == 0) {
      return Status::OK();
    }
#endif  // HAVE_FULLFSYNC

#if HAVE_FDATASYNC
    bool sync_success = ::fdatasync(fd) == 0;
#else
    bool sync_success = ::fsync(fd) == 0;
#endif  // HAVE_FDATASYNC

    if (sync_success) {
      return Status::OK();
    }
    return PosixError(fd_path, errno);
  }

  // Returns the directory name in a path pointing to a file.
  //
  // Returns "." if the path does not contain any directory separator.
  static std::string Dirname(const std::string& filename);

  // Extracts the file name from a path pointing to a file.
  //
  // The returned Slice points to |filename|'s data buffer, so it is only valid
  // while |filename| is alive and unchanged.
  static Slice Basename(const std::string& filename);

  // True if the given file is a manifest file.
  static bool IsManifest(const std::string& filename) {
    return Basename(filename).starts_with("MANIFEST");
  }

  // buf_[0, pos_ - 1] contains data to be written to fd_.
  char buf_[kWritableFileBufferSize];
  size_t pos_;
  int fd_;

  const bool is_manifest_;  // True if the file's name starts with MANIFEST.
  const std::string filename_;
  const std::string dirname_;  // The directory of filename_.
};

class LEVELDB_EXPORT Env {
 public:
  Env();

  Env(const Env&) = delete;
  Env& operator=(const Env&) = delete;

  virtual ~Env();

  // Return a default environment suitable for the current operating
  // system.  Sophisticated users may wish to provide their own Env
  // implementation instead of relying on this default environment.
  //
  // The result of Default() belongs to leveldb and must never be deleted.
  static Env* Default();

  virtual Status NewSequentialFile(const std::string& fname,
                                   SequentialFile** result) = 0;
  virtual Status NewRandomAccessFile(const std::string& fname,
                                     RandomAccessFile** result) = 0;
  virtual Status NewWritableFile(const std::string& fname,
                                 WritableFile** result) = 0;
  virtual Status NewAppendableFile(const std::string& fname,
                                   WritableFile** result);

  // Returns true iff the named file exists.
  virtual bool FileExists(const std::string& fname) = 0;

  // Store in *result the names of the children of the specified directory.
  // The names are relative to "dir".
  // Original contents of *results are dropped.
  virtual Status GetChildren(const std::string& dir,
                             std::vector<std::string>* result) = 0;
  // Delete the named file.
  //
  // The default implementation calls DeleteFile, to support legacy Env
  // implementations. Updated Env implementations must override RemoveFile and
  // ignore the existence of DeleteFile. Updated code calling into the Env API
  // must call RemoveFile instead of DeleteFile.
  //
  // A future release will remove DeleteDir and the default implementation of
  // RemoveDir.
  virtual Status RemoveFile(const std::string& fname);

  // Create the specified directory.
  virtual Status CreateDir(const std::string& dirname) = 0;

  // Delete the specified directory.
  //
  // The default implementation calls DeleteDir, to support legacy Env
  // implementations. Updated Env implementations must override RemoveDir and
  // ignore the existence of DeleteDir. Modern code calling into the Env API
  // must call RemoveDir instead of DeleteDir.
  //
  // A future release will remove DeleteDir and the default implementation of
  // RemoveDir.
  virtual Status RemoveDir(const std::string& dirname);

  // Store the size of fname in *file_size.
  virtual Status GetFileSize(const std::string& fname, uint64_t* file_size) = 0;

  // Rename file src to target.
  virtual Status RenameFile(const std::string& src,
                            const std::string& target) = 0;

  // Lock the specified file.  Used to prevent concurrent access to
  // the same db by multiple processes.  On failure, stores nullptr in
  // *lock and returns non-OK.
  //
  // On success, stores a pointer to the object that represents the
  // acquired lock in *lock and returns OK.  The caller should call
  // UnlockFile(*lock) to release the lock.  If the process exits,
  // the lock will be automatically released.
  //
  // If somebody else already holds the lock, finishes immediately
  // with a failure.  I.e., this call does not wait for existing locks
  // to go away.
  //
  // May create the named file if it does not already exist.
  virtual Status LockFile(const std::string& fname, FileLock** lock) = 0;

  // Release the lock acquired by a previous successful call to LockFile.
  // REQUIRES: lock was returned by a successful LockFile() call
  // REQUIRES: lock has not already been unlocked.
  virtual Status UnlockFile(FileLock* lock) = 0;

  // Arrange to run "(*function)(arg)" once in a background thread.
  //
  // "function" may run in an unspecified thread.  Multiple functions
  // added to the same Env may run concurrently in different threads.
  // I.e., the caller may not assume that background work items are
  // serialized.
  virtual void Schedule(void (*function)(void* arg), void* arg) = 0;

  // Start a new thread, invoking "function(arg)" within the new thread.
  // When "function(arg)" returns, the thread will be destroyed.
  virtual void StartThread(void (*function)(void* arg), void* arg) = 0;

  // *path is set to a temporary directory that can be used for testing. It may
  // or may not have just been created. The directory may or may not differ
  // between runs of the same process, but subsequent calls will return the
  // same directory.
  virtual Status GetTestDirectory(std::string* path) = 0;

  // Create and return a log file for storing informational messages.
  virtual Status NewLogger(const std::string& fname, Logger** result) = 0;

  // Returns the number of micro-seconds since some fixed point in time. Only
  // useful for computing deltas of time.
  virtual uint64_t NowMicros() = 0;

  // Sleep/delay the thread for the prescribed number of micro-seconds.
  virtual void SleepForMicroseconds(int micros) = 0;
};

namespace leveldb {

namespace {

// Wraps an Env instance whose destructor is never created.
//
// Intended usage:
//   using PlatformSingletonEnv = SingletonEnv<PlatformEnv>;
//   void ConfigurePosixEnv(int param) {
//     PlatformSingletonEnv::AssertEnvNotInitialized();
//     // set global configuration flags.
//   }
//   Env* Env::Default() {
//     static PlatformSingletonEnv default_env;
//     return default_env.env();
//   }
template <typename EnvType>
class SingletonEnv {
 public:
  SingletonEnv() {
    static_assert(sizeof(env_storage_) >= sizeof(EnvType),
                  "env_storage_ will not fit the Env");
    static_assert(alignof(decltype(env_storage_)) >= alignof(EnvType),
                  "env_storage_ does not meet the Env's alignment needs");
    new (&env_storage_) EnvType();
  }
  ~SingletonEnv() = default;

  SingletonEnv(const SingletonEnv&) = delete;
  SingletonEnv& operator=(const SingletonEnv&) = delete;

  Env* env() { return reinterpret_cast<Env*>(&env_storage_); }

 private:
  typename std::aligned_storage<sizeof(EnvType), alignof(EnvType)>::type
      env_storage_;
};

using PosixDefaultEnv = SingletonEnv<PosixEnv>;

}  // namespace

Env* Env::Default() {
  static PosixDefaultEnv env_container;
  return env_container.env();
}
}  // namespace leveldb

namespace leveldb {
namespace port {

class CondVar;

// Thinly wraps std::mutex.
class LOCKABLE Mutex {
 public:
  Mutex() = default;
  ~Mutex() = default;

  Mutex(const Mutex&) = delete;
  Mutex& operator=(const Mutex&) = delete;

  void Lock() EXCLUSIVE_LOCK_FUNCTION() { mu_.lock(); }
  void Unlock() UNLOCK_FUNCTION() { mu_.unlock(); }
  void AssertHeld() ASSERT_EXCLUSIVE_LOCK() {}

 private:
  friend class CondVar;
  std::mutex mu_;
};

// Thinly wraps std::condition_variable.
class CondVar {
 public:
  explicit CondVar(Mutex* mu) : mu_(mu) { assert(mu != nullptr); }
  ~CondVar() = default;

  CondVar(const CondVar&) = delete;
  CondVar& operator=(const CondVar&) = delete;

  void Wait() {
 // 调用 Wait() 的线程必须已经调用了 mu->Lock()
    std::unique_lock<std::mutex> lock(mu_->mu_, std::adopt_lock);
    cv_.wait(lock);
 // lock 不再管理 mu_ (lock 析构的时候不会 unlock mu_)，需要本线程在后面自己调用 mu->UnLock()
    lock.release();
  }
  void Signal() { cv_.notify_one(); }
  void SignalAll() { cv_.notify_all(); }

 private:
  std::condition_variable cv_;
  Mutex* const mu_;
};

}  // namespace port
}  // namespace leveldb

int LockOrUnlock(int fd, bool lock) {
  errno = 0;
  struct ::flock file_lock_info;
  std::memset(&file_lock_info, 0, sizeof(file_lock_info));
  file_lock_info.l_type = (lock ? F_WRLCK : F_UNLCK);
  file_lock_info.l_whence = SEEK_SET;
  file_lock_info.l_start = 0;
  file_lock_info.l_len = 0;  // Lock/unlock entire file.
  return ::fcntl(fd, F_SETLK, &file_lock_info);
}


// Lock the specified file.  Used to prevent concurrent access to
// the same db by multiple processes.  On failure, stores nullptr in
// *lock and returns non-OK.
//
// On success, stores a pointer to the object that represents the
// acquired lock in *lock and returns OK.  The caller should call
// UnlockFile(*lock) to release the lock.  If the process exits,
// the lock will be automatically released.
//
// If somebody else already holds the lock, finishes immediately
// with a failure.  I.e., this call does not wait for existing locks
// to go away.
//
// May create the named file if it does not already exist.
Status PosixEnv::LockFile(const std::string& filename, FileLock** lock) override {
  *lock = nullptr;

  int fd = ::open(filename.c_str(), O_RDWR | O_CREAT | kOpenBaseFlags, 0644);
  if (fd < 0) {
    return PosixError(filename, errno);
  }
// 如果已经有其他线程 open 了此文件，那么失败
  if (!locks_.Insert(filename)) {
    ::close(fd);
    return Status::IOError("lock " + filename, "already held by process");
  }
// 如果已经有其他进程 open 了此文件，那么失败
  if (LockOrUnlock(fd, true) == -1) {
    int lock_errno = errno;
    ::close(fd);
    locks_.Remove(filename);
    return PosixError("lock " + filename, lock_errno);
  }

  *lock = new PosixFileLock(fd, filename);
  return Status::OK();
}

// Release the lock acquired by a previous successful call to LockFile.
// REQUIRES: lock was returned by a successful LockFile() call
// REQUIRES: lock has not already been unlocked.
Status PosixEnv::UnlockFile(FileLock* lock) override {
  PosixFileLock* posix_file_lock = static_cast<PosixFileLock*>(lock);
  if (LockOrUnlock(posix_file_lock->fd(), false) == -1) {
    return PosixError("unlock " + posix_file_lock->filename(), errno);
  }
  locks_.Remove(posix_file_lock->filename());
  ::close(posix_file_lock->fd());
  delete posix_file_lock;
  return Status::OK();
}

class PosixEnv : public Env {
...
  PosixLockTable locks_;  // Thread-safe.
...
}

// Helper class to limit resource usage to avoid exhaustion.
// Currently used to limit read-only file descriptors and mmap file usage
// so that we do not run out of file descriptors or virtual memory, or run into
// kernel performance problems for very large databases.
class Limiter {
 public:
  // Limit maximum number of resources to |max_acquires|.
  Limiter(int max_acquires) : acquires_allowed_(max_acquires) {}

  Limiter(const Limiter&) = delete;
  Limiter operator=(const Limiter&) = delete;

  // If another resource is available, acquire it and return true.
  // Else return false.
  bool Acquire() {
    int old_acquires_allowed =
        acquires_allowed_.fetch_sub(1, std::memory_order_relaxed);

    if (old_acquires_allowed > 0) return true;

    acquires_allowed_.fetch_add(1, std::memory_order_relaxed);
    return false;
  }

  // Release a resource acquired by a previous call to Acquire() that returned
  // true.
  void Release() { acquires_allowed_.fetch_add(1, std::memory_order_relaxed); }

 private:
  // The number of available resources.
  //
  // This is a counter and is not tied to the invariants of any other class, so
  // it can be operated on safely using std::memory_order_relaxed.
  std::atomic<int> acquires_allowed_;
};

class PosixEnv : public Env {
...
  Limiter mmap_limiter_;  // Thread-safe.
  Limiter fd_limiter_;    // Thread-safe.
}

class PosixEnv : public Env {
 public:
  PosixEnv();
  ...

  // Arrange to run "(*function)(arg)" once in a background thread.
  //
  // "function" may run in an unspecified thread.  Multiple functions
  // added to the same Env may run concurrently in different threads.
  // I.e., the caller may not assume that background work items are
  // serialized.
  void Schedule(void (*background_work_function)(void* background_work_arg),
                void* background_work_arg) override;
  ...
  
  void SleepForMicroseconds(int micros) override {
    std::this_thread::sleep_for(std::chrono::microseconds(micros));
  }

 private:
  void BackgroundThreadMain();

  static void BackgroundThreadEntryPoint(PosixEnv* env) {
    env->BackgroundThreadMain();
  }

  // Stores the work item data in a Schedule() call.
  //
  // Instances are constructed on the thread calling Schedule() and used on the
  // background thread.
  //
  // This structure is thread-safe beacuse it is immutable.
  struct BackgroundWorkItem {
    explicit BackgroundWorkItem(void (*function)(void* arg), void* arg)
        : function(function), arg(arg) {}

    void (*const function)(void*);
    void* const arg;
  };

  port::Mutex background_work_mutex_;
  port::CondVar background_work_cv_ GUARDED_BY(background_work_mutex_);
  bool started_background_thread_ GUARDED_BY(background_work_mutex_);

  std::queue<BackgroundWorkItem> background_work_queue_
      GUARDED_BY(background_work_mutex_);
  ...
};

void PosixEnv::Schedule(
    void (*background_work_function)(void* background_work_arg),
    void* background_work_arg) {
  background_work_mutex_.Lock();

  // Start the background thread, if we haven't done so already.
  if (!started_background_thread_) {
    started_background_thread_ = true;
    std::thread background_thread(PosixEnv::BackgroundThreadEntryPoint, this);
    background_thread.detach();
  }

  // If the queue is empty, the background thread may be waiting for work.
  if (background_work_queue_.empty()) {
    background_work_cv_.Signal();
  }

  background_work_queue_.emplace(background_work_function, background_work_arg);
  background_work_mutex_.Unlock();
}

void PosixEnv::BackgroundThreadMain() {
  while (true) {
    background_work_mutex_.Lock();

    // Wait until there is work to be done.
    while (background_work_queue_.empty()) {
      background_work_cv_.Wait();
    }

    assert(!background_work_queue_.empty());
    auto background_work_function = background_work_queue_.front().function;
    void* background_work_arg = background_work_queue_.front().arg;
    background_work_queue_.pop();

    background_work_mutex_.Unlock();
    background_work_function(background_work_arg);
  }
}

PosixEnv::PosixEnv()
    : background_work_cv_(&background_work_mutex_),
      started_background_thread_(false),
      mmap_limiter_(MaxMmaps()),
      fd_limiter_(MaxOpenFiles()) {}
}