text: // Copyright (c) 2018 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.
  
  // Prevent Windows headers from defining min/max macros and instead
  // use STL.
  #define NOMINMAX
  #include <windows.h>
  
  #include <algorithm>
  #include <atomic>
  #include <chrono>
  #include <condition_variable>
  #include <deque>
  #include <memory>
  #include <mutex>
  #include <sstream>
  #include <string>
  #include <vector>
  
  #include "leveldb/env.h"
  #include "leveldb/slice.h"
  #include "port/port.h"
  #include "port/thread_annotations.h"
  #include "util/env_windows_test_helper.h"
  #include "util/logging.h"
  #include "util/mutexlock.h"
  #include "util/windows_logger.h"
  
  #if defined(DeleteFile)
  #undef DeleteFile
  #endif  // defined(DeleteFile)
  
  namespace leveldb {
  
  namespace {
  
  constexpr const size_t kWritableFileBufferSize = 65536;
  
  // Up to 1000 mmaps for 64-bit binaries; none for 32-bit.
  constexpr int kDefaultMmapLimit = sizeof(void*) >= 8 ? 1000 : 0;
  
  // Modified by EnvWindowsTestHelper::SetReadOnlyMMapLimit().
  int g_mmap_limit = kDefaultMmapLimit;
  
  std::string GetWindowsErrorMessage(DWORD error_code) {
    std::string message;
    char* error_text = nullptr;
    // Use MBCS version of FormatMessage to match return value.
    size_t error_text_size = ::FormatMessageA(
        FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_ALLOCATE_BUFFER |
            FORMAT_MESSAGE_IGNORE_INSERTS,
        nullptr, error_code, MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),
        reinterpret_cast<char*>(&error_text), 0, nullptr);
    if (!error_text) {
      return message;
    }
    message.assign(error_text, error_text_size);
    ::LocalFree(error_text);
    return message;
  }
  
  Status WindowsError(const std::string& context, DWORD error_code) {
    if (error_code == ERROR_FILE_NOT_FOUND || error_code == ERROR_PATH_NOT_FOUND)
      return Status::NotFound(context, GetWindowsErrorMessage(error_code));
    return Status::IOError(context, GetWindowsErrorMessage(error_code));
  }
  
  class ScopedHandle {
   public:
    ScopedHandle(HANDLE handle) : handle_(handle) {}
    ScopedHandle(ScopedHandle&& other) noexcept : handle_(other.Release()) {}
    ~ScopedHandle() { Close(); }
  
    ScopedHandle& operator=(ScopedHandle&& rhs) noexcept {
      if (this != &rhs) handle_ = rhs.Release();
      return *this;
    }
  
    bool Close() {
      if (!is_valid()) {
        return true;
      }
      HANDLE h = handle_;
      handle_ = INVALID_HANDLE_VALUE;
      return ::CloseHandle(h);
    }
  
    bool is_valid() const {
      return handle_ != INVALID_HANDLE_VALUE && handle_ != nullptr;
    }
  
    HANDLE get() const { return handle_; }
  
    HANDLE Release() {
      HANDLE h = handle_;
      handle_ = INVALID_HANDLE_VALUE;
      return h;
    }
  
   private:
    HANDLE handle_;
  };
  
  // 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 WindowsSequentialFile : public SequentialFile {
   public:
    WindowsSequentialFile(std::string fname, ScopedHandle file)
        : filename_(fname), file_(std::move(file)) {}
    ~WindowsSequentialFile() override {}
  
    Status Read(size_t n, Slice* result, char* scratch) override {
      Status s;
      DWORD bytes_read;
      // DWORD is 32-bit, but size_t could technically be larger. However leveldb
      // files are limited to leveldb::Options::max_file_size which is clamped to
      // 1<<30 or 1 GiB.
      assert(n <= std::numeric_limits<DWORD>::max());
      if (!::ReadFile(file_.get(), scratch, static_cast<DWORD>(n), &bytes_read,
                      nullptr)) {
        s = WindowsError(filename_, ::GetLastError());
      } else {
        *result = Slice(scratch, bytes_read);
      }
      return s;
    }
  
    Status Skip(uint64_t n) override {
      LARGE_INTEGER distance;
      distance.QuadPart = n;
      if (!::SetFilePointerEx(file_.get(), distance, nullptr, FILE_CURRENT)) {
        return WindowsError(filename_, ::GetLastError());
      }
      return Status::OK();
    }
  
   private:
    std::string filename_;
    ScopedHandle file_;
  };
  
  class WindowsRandomAccessFile : public RandomAccessFile {
   public:
    WindowsRandomAccessFile(std::string fname, ScopedHandle handle)
        : filename_(fname), handle_(std::move(handle)) {}
  
    ~WindowsRandomAccessFile() override = default;
  
    Status Read(uint64_t offset, size_t n, Slice* result,
                char* scratch) const override {
      DWORD bytes_read = 0;
      OVERLAPPED overlapped = {0};
  
      overlapped.OffsetHigh = static_cast<DWORD>(offset >> 32);
      overlapped.Offset = static_cast<DWORD>(offset);
      if (!::ReadFile(handle_.get(), scratch, static_cast<DWORD>(n), &bytes_read,
                      &overlapped)) {
        DWORD error_code = ::GetLastError();
        if (error_code != ERROR_HANDLE_EOF) {
          *result = Slice(scratch, 0);
          return Status::IOError(filename_, GetWindowsErrorMessage(error_code));
        }
      }
  
      *result = Slice(scratch, bytes_read);
      return Status::OK();
    }
  
   private:
    std::string filename_;
    ScopedHandle handle_;
  };
  
  class WindowsMmapReadableFile : public RandomAccessFile {
   public:
    // base[0,length-1] contains the mmapped contents of the file.
    WindowsMmapReadableFile(std::string fname, void* base, size_t length,
                            Limiter* limiter)
        : filename_(std::move(fname)),
          mmapped_region_(base),
          length_(length),
          limiter_(limiter) {}
  
    ~WindowsMmapReadableFile() override {
      ::UnmapViewOfFile(mmapped_region_);
      limiter_->Release();
    }
  
    Status Read(uint64_t offset, size_t n, Slice* result,
                char* scratch) const override {
      Status s;
      if (offset + n > length_) {
        *result = Slice();
        s = WindowsError(filename_, ERROR_INVALID_PARAMETER);
      } else {
        *result = Slice(reinterpret_cast<char*>(mmapped_region_) + offset, n);
      }
      return s;
    }
  
   private:
    std::string filename_;
    void* mmapped_region_;
    size_t length_;
    Limiter* limiter_;
  };
  
  class WindowsWritableFile : public WritableFile {
   public:
    WindowsWritableFile(std::string fname, ScopedHandle handle)
        : filename_(std::move(fname)), handle_(std::move(handle)), pos_(0) {}
  
    ~WindowsWritableFile() override = default;
  
    Status Append(const Slice& data) override {
      size_t n = data.size();
      const char* p = data.data();
  
      // Fit as much as possible into buffer.
      size_t copy = std::min(n, kWritableFileBufferSize - pos_);
      memcpy(buf_ + pos_, p, copy);
      p += copy;
      n -= copy;
      pos_ += copy;
      if (n == 0) {
        return Status::OK();
      }
  
      // Can't fit in buffer, so need to do at least one write.
      Status s = FlushBuffered();
      if (!s.ok()) {
        return s;
      }
  
      // Small writes go to buffer, large writes are written directly.
      if (n < kWritableFileBufferSize) {
        memcpy(buf_, p, n);
        pos_ = n;
        return Status::OK();
      }
      return WriteRaw(p, n);
    }
  
    Status Close() override {
      Status result = FlushBuffered();
      if (!handle_.Close() && result.ok()) {
        result = WindowsError(filename_, ::GetLastError());
      }
      return result;
    }
  
    Status Flush() override { return FlushBuffered(); }
  
    Status Sync() override {
      // On Windows no need to sync parent directory. It's metadata will be
      // updated via the creation of the new file, without an explicit sync.
      return FlushBuffered();
    }
  
   private:
    Status FlushBuffered() {
      Status s = WriteRaw(buf_, pos_);
      pos_ = 0;
      return s;
    }
  
    Status WriteRaw(const char* p, size_t n) {
      DWORD bytes_written;
      if (!::WriteFile(handle_.get(), p, static_cast<DWORD>(n), &bytes_written,
                       nullptr)) {
        return Status::IOError(filename_,
                               GetWindowsErrorMessage(::GetLastError()));
      }
      return Status::OK();
    }
  
    // buf_[0, pos_-1] contains data to be written to handle_.
    const std::string filename_;
    ScopedHandle handle_;
    char buf_[kWritableFileBufferSize];
    size_t pos_;
  };
  
  // Lock or unlock the entire file as specified by |lock|. Returns true
  // when successful, false upon failure. Caller should call ::GetLastError()
  // to determine cause of failure
  bool LockOrUnlock(HANDLE handle, bool lock) {
    if (lock) {
      return ::LockFile(handle,
                        /*dwFileOffsetLow=*/0, /*dwFileOffsetHigh=*/0,
                        /*nNumberOfBytesToLockLow=*/MAXDWORD,
                        /*nNumberOfBytesToLockHigh=*/MAXDWORD);
    } else {
      return ::UnlockFile(handle,
                          /*dwFileOffsetLow=*/0, /*dwFileOffsetHigh=*/0,
                          /*nNumberOfBytesToLockLow=*/MAXDWORD,
                          /*nNumberOfBytesToLockHigh=*/MAXDWORD);
    }
  }
  
  class WindowsFileLock : public FileLock {
   public:
    WindowsFileLock(ScopedHandle handle, std::string name)
        : handle_(std::move(handle)), name_(std::move(name)) {}
  
    ScopedHandle& handle() { return handle_; }
    const std::string& name() const { return name_; }
  
   private:
    ScopedHandle handle_;
    std::string name_;
  };
  
  class WindowsEnv : public Env {
   public:
    WindowsEnv();
    ~WindowsEnv() override {
      static char msg[] = "Destroying Env::Default()\n";
      fwrite(msg, 1, sizeof(msg), stderr);
      abort();
    }
  
    Status NewSequentialFile(const std::string& fname,
                             SequentialFile** result) override {
      *result = nullptr;
      DWORD desired_access = GENERIC_READ;
      DWORD share_mode = FILE_SHARE_READ;
      ScopedHandle handle =
          ::CreateFileA(fname.c_str(), desired_access, share_mode, nullptr,
                        OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, nullptr);
      if (!handle.is_valid()) {
        return WindowsError(fname, ::GetLastError());
      }
      *result = new WindowsSequentialFile(fname, std::move(handle));
      return Status::OK();
    }
  
    Status NewRandomAccessFile(const std::string& fname,
                               RandomAccessFile** result) override {
      *result = nullptr;
      DWORD desired_access = GENERIC_READ;
      DWORD share_mode = FILE_SHARE_READ;
      DWORD file_flags = FILE_ATTRIBUTE_READONLY;
  
      ScopedHandle handle =
          ::CreateFileA(fname.c_str(), desired_access, share_mode, nullptr,
                        OPEN_EXISTING, file_flags, nullptr);
      if (!handle.is_valid()) {
        return WindowsError(fname, ::GetLastError());
      }
      if (!mmap_limiter_.Acquire()) {
        *result = new WindowsRandomAccessFile(fname, std::move(handle));
        return Status::OK();
      }
  
      LARGE_INTEGER file_size;
      if (!::GetFileSizeEx(handle.get(), &file_size)) {
        return WindowsError(fname, ::GetLastError());
      }
  
      ScopedHandle mapping =
          ::CreateFileMappingA(handle.get(),
                               /*security attributes=*/nullptr, PAGE_READONLY,
                               /*dwMaximumSizeHigh=*/0,
                               /*dwMaximumSizeLow=*/0, nullptr);
      if (mapping.is_valid()) {
        void* base = MapViewOfFile(mapping.get(), FILE_MAP_READ, 0, 0, 0);
        if (base) {
          *result = new WindowsMmapReadableFile(
              fname, base, static_cast<size_t>(file_size.QuadPart),
              &mmap_limiter_);
          return Status::OK();
        }
      }
      Status s = WindowsError(fname, ::GetLastError());
  
      if (!s.ok()) {
        mmap_limiter_.Release();
      }
      return s;
    }
  
    Status NewWritableFile(const std::string& fname,
                           WritableFile** result) override {
      DWORD desired_access = GENERIC_WRITE;
      DWORD share_mode = 0;
  
      ScopedHandle handle =
          ::CreateFileA(fname.c_str(), desired_access, share_mode, nullptr,
                        CREATE_ALWAYS, FILE_ATTRIBUTE_NORMAL, nullptr);
      if (!handle.is_valid()) {
        *result = nullptr;
        return WindowsError(fname, ::GetLastError());
      }
  
      *result = new WindowsWritableFile(fname, std::move(handle));
      return Status::OK();
    }
  
    Status NewAppendableFile(const std::string& fname,
                             WritableFile** result) override {
      ScopedHandle handle =
          ::CreateFileA(fname.c_str(), FILE_APPEND_DATA, 0, nullptr, OPEN_ALWAYS,
                        FILE_ATTRIBUTE_NORMAL, nullptr);
      if (!handle.is_valid()) {
        *result = nullptr;
        return WindowsError(fname, ::GetLastError());
      }
  
      *result = new WindowsWritableFile(fname, std::move(handle));
      return Status::OK();
    }
  
    bool FileExists(const std::string& fname) override {
      return GetFileAttributesA(fname.c_str()) != INVALID_FILE_ATTRIBUTES;
    }
  
    Status GetChildren(const std::string& dir,
                       std::vector<std::string>* result) override {
      const std::string find_pattern = dir + "\\*";
      WIN32_FIND_DATAA find_data;
      HANDLE dir_handle = ::FindFirstFileA(find_pattern.c_str(), &find_data);
      if (dir_handle == INVALID_HANDLE_VALUE) {
        DWORD last_error = ::GetLastError();
        if (last_error == ERROR_FILE_NOT_FOUND) {
          return Status::OK();
        }
        return WindowsError(dir, last_error);
      }
      do {
        char base_name[_MAX_FNAME];
        char ext[_MAX_EXT];
  
        if (!_splitpath_s(find_data.cFileName, nullptr, 0, nullptr, 0, base_name,
                          ARRAYSIZE(base_name), ext, ARRAYSIZE(ext))) {
          result->emplace_back(std::string(base_name) + ext);
        }
      } while (::FindNextFileA(dir_handle, &find_data));
      DWORD last_error = ::GetLastError();
      ::FindClose(dir_handle);
      if (last_error != ERROR_NO_MORE_FILES) {
        return WindowsError(dir, last_error);
      }
      return Status::OK();
    }
  
    Status DeleteFile(const std::string& fname) override {
      if (!::DeleteFileA(fname.c_str())) {
        return WindowsError(fname, ::GetLastError());
      }
      return Status::OK();
    }
  
    Status CreateDir(const std::string& name) override {
      if (!::CreateDirectoryA(name.c_str(), nullptr)) {
        return WindowsError(name, ::GetLastError());
      }
      return Status::OK();
    }
  
    Status DeleteDir(const std::string& name) override {
      if (!::RemoveDirectoryA(name.c_str())) {
        return WindowsError(name, ::GetLastError());
      }
      return Status::OK();
    }
  
    Status GetFileSize(const std::string& fname, uint64_t* size) override {
      WIN32_FILE_ATTRIBUTE_DATA attrs;
      if (!::GetFileAttributesExA(fname.c_str(), GetFileExInfoStandard, &attrs)) {
        return WindowsError(fname, ::GetLastError());
      }
      ULARGE_INTEGER file_size;
      file_size.HighPart = attrs.nFileSizeHigh;
      file_size.LowPart = attrs.nFileSizeLow;
      *size = file_size.QuadPart;
      return Status::OK();
    }
  
    Status RenameFile(const std::string& src,
                      const std::string& target) override {
      // Try a simple move first.  It will only succeed when |to_path| doesn't
      // already exist.
      if (::MoveFileA(src.c_str(), target.c_str())) {
        return Status::OK();
      }
      DWORD move_error = ::GetLastError();
  
      // Try the full-blown replace if the move fails, as ReplaceFile will only
      // succeed when |to_path| does exist. When writing to a network share, we
      // may not be able to change the ACLs. Ignore ACL errors then
      // (REPLACEFILE_IGNORE_MERGE_ERRORS).
      if (::ReplaceFileA(target.c_str(), src.c_str(), nullptr,
                         REPLACEFILE_IGNORE_MERGE_ERRORS, nullptr, nullptr)) {
        return Status::OK();
      }
      DWORD replace_error = ::GetLastError();
      // In the case of FILE_ERROR_NOT_FOUND from ReplaceFile, it is likely
      // that |to_path| does not exist. In this case, the more relevant error
      // comes from the call to MoveFile.
      if (replace_error == ERROR_FILE_NOT_FOUND ||
          replace_error == ERROR_PATH_NOT_FOUND) {
        return WindowsError(src, move_error);
      } else {
        return WindowsError(src, replace_error);
      }
    }
  
    Status LockFile(const std::string& fname, FileLock** lock) override {
      *lock = nullptr;
      Status result;
      ScopedHandle handle = ::CreateFileA(
          fname.c_str(), GENERIC_READ | GENERIC_WRITE, FILE_SHARE_READ,
          /*lpSecurityAttributes=*/nullptr, OPEN_ALWAYS, FILE_ATTRIBUTE_NORMAL,
          nullptr);
      if (!handle.is_valid()) {
        result = WindowsError(fname, ::GetLastError());
      } else if (!LockOrUnlock(handle.get(), true)) {
        result = WindowsError("lock " + fname, ::GetLastError());
      } else {
        *lock = new WindowsFileLock(std::move(handle), std::move(fname));
      }
      return result;
    }
  
    Status UnlockFile(FileLock* lock) override {
      std::unique_ptr<WindowsFileLock> my_lock(
          reinterpret_cast<WindowsFileLock*>(lock));
      Status result;
      if (!LockOrUnlock(my_lock->handle().get(), false)) {
        result = WindowsError("unlock", ::GetLastError());
      }
      return result;
    }
  
    void Schedule(void (*function)(void*), void* arg) override;
  
    void StartThread(void (*function)(void* arg), void* arg) override {
      std::thread t(function, arg);
      t.detach();
    }
  
    Status GetTestDirectory(std::string* result) override {
      const char* env = getenv("TEST_TMPDIR");
      if (env && env[0] != '\0') {
        *result = env;
        return Status::OK();
      }
  
      char tmp_path[MAX_PATH];
      if (!GetTempPathA(ARRAYSIZE(tmp_path), tmp_path)) {
        return WindowsError("GetTempPath", ::GetLastError());
      }
      std::stringstream ss;
      ss << tmp_path << "leveldbtest-" << std::this_thread::get_id();
      *result = ss.str();
  
      // Directory may already exist
      CreateDir(*result);
      return Status::OK();
    }
  
    Status NewLogger(const std::string& filename, Logger** result) override {
      std::FILE* fp = std::fopen(filename.c_str(), "w");
      if (fp == nullptr) {
        *result = nullptr;
        return WindowsError("NewLogger", ::GetLastError());
      } else {
        *result = new WindowsLogger(fp);
        return Status::OK();
      }
    }
  
    uint64_t NowMicros() override {
      // GetSystemTimeAsFileTime typically has a resolution of 10-20 msec.
      // TODO(cmumford): Switch to GetSystemTimePreciseAsFileTime which is
      // available in Windows 8 and later.
      FILETIME ft;
      ::GetSystemTimeAsFileTime(&ft);
      // Each tick represents a 100-nanosecond intervals since January 1, 1601
      // (UTC).
      uint64_t num_ticks =
          (static_cast<uint64_t>(ft.dwHighDateTime) << 32) + ft.dwLowDateTime;
      return num_ticks / 10;
    }
  
    void SleepForMicroseconds(int micros) override {
      std::this_thread::sleep_for(std::chrono::microseconds(micros));
    }
  
   private:
    // BGThread() is the body of the background thread
    void BGThread();
  
    std::mutex mu_;
    std::condition_variable bgsignal_;
    bool started_bgthread_;
  
    // Entry per Schedule() call
    struct BGItem {
      void* arg;
      void (*function)(void*);
    };
    typedef std::deque<BGItem> BGQueue;
    BGQueue queue_;
  
    Limiter mmap_limiter_;
  };
  
  // Return the maximum number of concurrent mmaps.
  int MaxMmaps() {
    if (g_mmap_limit >= 0) {
      return g_mmap_limit;
    }
    // Up to 1000 mmaps for 64-bit binaries; none for smaller pointer sizes.
    g_mmap_limit = sizeof(void*) >= 8 ? 1000 : 0;
    return g_mmap_limit;
  }
  
  WindowsEnv::WindowsEnv()
      : started_bgthread_(false), mmap_limiter_(MaxMmaps()) {}
  
  void WindowsEnv::Schedule(void (*function)(void*), void* arg) {
    std::lock_guard<std::mutex> guard(mu_);
  
    // Start background thread if necessary
    if (!started_bgthread_) {
      started_bgthread_ = true;
      std::thread t(&WindowsEnv::BGThread, this);
      t.detach();
    }
  
    // If the queue is currently empty, the background thread may currently be
    // waiting.
    if (queue_.empty()) {
      bgsignal_.notify_one();
    }
  
    // Add to priority queue
    queue_.push_back(BGItem());
    queue_.back().function = function;
    queue_.back().arg = arg;
  }
  
  void WindowsEnv::BGThread() {
    while (true) {
      // Wait until there is an item that is ready to run
      std::unique_lock<std::mutex> lk(mu_);
      bgsignal_.wait(lk, [this] { return !queue_.empty(); });
  
      void (*function)(void*) = queue_.front().function;
      void* arg = queue_.front().arg;
      queue_.pop_front();
  
      lk.unlock();
      (*function)(arg);
    }
  }
  
  }  // namespace
  
  static std::once_flag once;
  static Env* default_env;
  static void InitDefaultEnv() { default_env = new WindowsEnv(); }
  
  void EnvWindowsTestHelper::SetReadOnlyMMapLimit(int limit) {
    assert(default_env == nullptr);
    g_mmap_limit = limit;
  }
  
  Env* Env::Default() {
    std::call_once(once, InitDefaultEnv);
    return default_env;
  }
  
  }  // namespace leveldb
  