text: // Copyright (c) 2011 The LevelDB Authors. All rights reserved.
  // Use of this source code is governed by a BSD-style license that can be
  // found in the LICENSE file. See the AUTHORS file for names of contributors.
  //
  // An Env is an interface used by the leveldb implementation to access
  // operating system functionality like the filesystem etc.  Callers
  // may wish to provide a custom Env object when opening a database to
  // get fine gain control; e.g., to rate limit file system operations.
  //
  // All Env implementations are safe for concurrent access from
  // multiple threads without any external synchronization.
  
  #ifndef STORAGE_LEVELDB_INCLUDE_ENV_H_
  #define STORAGE_LEVELDB_INCLUDE_ENV_H_
  
  #include <string>
  #include <vector>
  #include <stdarg.h>
  #include <stdint.h>
  #include "leveldb/status.h"
  
  namespace leveldb {
  class FileLock;
  class Logger;
  class RandomAccessFile;
  class SequentialFile;
  class Slice;
  class WritableFile;
  
  class Env {
   public:
    Env() { }
    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();
  
    // Create a brand new sequentially-readable file with the specified name.
    // On success, stores a pointer to the new file in *result and returns OK.
    // On failure stores NULL in *result and returns non-OK.  If the file does
    // not exist, returns a non-OK status.
    //
    // The returned file will only be accessed by one thread at a time.
    virtual Status NewSequentialFile(const std::string& fname,
                                     SequentialFile** result) = 0;
  
    // Create a brand new random access read-only file with the
    // specified name.  On success, stores a pointer to the new file in
    // *result and returns OK.  On failure stores NULL in *result and
    // returns non-OK.  If the file does not exist, returns a non-OK
    // status.
    //
    // The returned file may be concurrently accessed by multiple threads.
    virtual Status NewRandomAccessFile(const std::string &fname, RandomAccessFile **result) = 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 NULL in *result and
    // returns non-OK.
    //
    // The returned file will only be accessed by one thread at a time.
    virtual Status NewWritableFile(const std::string& fname,
                                   WritableFile** result) = 0;
  
    // 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.
    virtual Status DeleteFile(const std::string& fname) = 0;
  
    // Create the specified directory.
    virtual Status CreateDir(const std::string& dirname) = 0;
  
    // Delete the specified directory.
    virtual Status DeleteDir(const std::string& dirname) = 0;
  
    // 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 NULL 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 many 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;
  
   private:
    // No copying allowed
    Env(const Env&);
    void operator=(const Env&);
  };
  
  // A file abstraction for reading sequentially through a file
  class SequentialFile {
   public:
    SequentialFile() { }
    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;
  
   private:
    // No copying allowed
    SequentialFile(const SequentialFile&);
    void operator=(const SequentialFile&);
  };
  
  // A file abstraction for randomly reading the contents of a file.
  class RandomAccessFile {
   public:
    RandomAccessFile() { }
    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;
  
   private:
    // No copying allowed
    RandomAccessFile(const RandomAccessFile&);
    void operator=(const RandomAccessFile&);
  };
  
  // A file abstraction for sequential writing.  The implementation
  // must provide buffering since callers may append small fragments
  // at a time to the file.
  class WritableFile {
   public:
    WritableFile() { }
    virtual ~WritableFile();
  
    virtual Status Append(const Slice& data) = 0;
    virtual Status Close() = 0;
    virtual Status Flush() = 0;
    virtual Status Sync() = 0;
  
   private:
    // No copying allowed
    WritableFile(const WritableFile&);
    void operator=(const WritableFile&);
  };
  
  // An interface for writing log messages.
  class Logger {
   public:
    Logger() { }
    virtual ~Logger();
  
    // Write an entry to the log file with the specified format.
    virtual void Logv(const char* format, va_list ap) = 0;
  
   private:
    // No copying allowed
    Logger(const Logger&);
    void operator=(const Logger&);
  };
  
  
  // Identifies a locked file.
  class FileLock {
   public:
    FileLock() { }
    virtual ~FileLock();
   private:
    // No copying allowed
    FileLock(const FileLock&);
    void operator=(const FileLock&);
  };
  
  // Log the specified data to *info_log if info_log is non-NULL.
  extern void Log(Logger* info_log, const char* format, ...)
  #   if defined(__GNUC__) || defined(__clang__)
      __attribute__((__format__ (__printf__, 2, 3)))
  #   endif
      ;
  
  // A utility routine: write "data" to the named file.
  extern Status WriteStringToFile(Env* env, const Slice& data,
                                  const std::string& fname);
  
  // A utility routine: read contents of named file into *data
  extern Status ReadFileToString(Env* env, const std::string& fname,
                                 std::string* data);
  
  // An implementation of Env that forwards all calls to another Env.
  // May be useful to clients who wish to override just part of the
  // functionality of another Env.
  class EnvWrapper : public Env {
   public:
    // Initialize an EnvWrapper that delegates all calls to *t
    explicit EnvWrapper(Env* t) : target_(t) { }
    virtual ~EnvWrapper();
  
    // Return the target to which this Env forwards all calls
    Env* target() const { return target_; }
  
    // The following text is boilerplate that forwards all methods to target()
    Status NewSequentialFile(const std::string& f, SequentialFile** r) {
      return target_->NewSequentialFile(f, r);
    }
    Status NewRandomAccessFile(const std::string& f, RandomAccessFile** r) {
      return target_->NewRandomAccessFile(f, r);
    }
    Status NewWritableFile(const std::string& f, WritableFile** r) {
      return target_->NewWritableFile(f, r);
    }
    bool FileExists(const std::string& f) { return target_->FileExists(f); }
    Status GetChildren(const std::string& dir, std::vector<std::string>* r) {
      return target_->GetChildren(dir, r);
    }
    Status DeleteFile(const std::string& f) { return target_->DeleteFile(f); }
    Status CreateDir(const std::string& d) { return target_->CreateDir(d); }
    Status DeleteDir(const std::string& d) { return target_->DeleteDir(d); }
    Status GetFileSize(const std::string& f, uint64_t* s) {
      return target_->GetFileSize(f, s);
    }
    Status RenameFile(const std::string& s, const std::string& t) {
      return target_->RenameFile(s, t);
    }
    Status LockFile(const std::string& f, FileLock** l) {
      return target_->LockFile(f, l);
    }
    Status UnlockFile(FileLock* l) { return target_->UnlockFile(l); }
    void Schedule(void (*f)(void*), void* a) {
      return target_->Schedule(f, a);
    }
    void StartThread(void (*f)(void*), void* a) {
      return target_->StartThread(f, a);
    }
    virtual Status GetTestDirectory(std::string* path) {
      return target_->GetTestDirectory(path);
    }
    virtual Status NewLogger(const std::string& fname, Logger** result) {
      return target_->NewLogger(fname, result);
    }
    uint64_t NowMicros() {
      return target_->NowMicros();
    }
    void SleepForMicroseconds(int micros) {
      target_->SleepForMicroseconds(micros);
    }
   private:
    Env *target_;
  };
  }  // namespace leveldb
  #endif  // STORAGE_LEVELDB_INCLUDE_ENV_H_
  