subprocess.h

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// Based on the https://github.com/arun11299/cpp-subprocess project.
 
#ifndef BITCOIN_UTIL_SUBPROCESS_H
#define BITCOIN_UTIL_SUBPROCESS_H
 
#include <util/fs.h>
#include <util/strencodings.h>
#include <util/syserror.h>
 
#include <algorithm>
#include <cassert>
#include <csignal>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <exception>
#include <future>
#include <initializer_list>
#include <iostream>
#include <locale>
#include <map>
#include <memory>
#include <sstream>
#include <string>
#include <vector>
 
#if (defined _MSC_VER) || (defined __MINGW32__)
  #define __USING_WINDOWS__
#endif
 
#ifdef __USING_WINDOWS__
  #include <codecvt>
#endif
 
extern "C" {
#ifdef __USING_WINDOWS__
  #include <windows.h>
  #include <io.h>
  #include <cwchar>
#else
  #include <sys/wait.h>
  #include <unistd.h>
#endif
  #include <csignal>
  #include <fcntl.h>
  #include <sys/types.h>
}
 
// The Microsoft C++ compiler issues deprecation warnings
// for the standard POSIX function names.
// Its preferred implementations have a leading underscore.
// See: https://learn.microsoft.com/en-us/cpp/c-runtime-library/compatibility.
#if (defined _MSC_VER)
  #define subprocess_close _close
  #define subprocess_fileno _fileno
  #define subprocess_open _open
  #define subprocess_write _write
#else
  #define subprocess_close close
  #define subprocess_fileno fileno
  #define subprocess_open open
  #define subprocess_write write
#endif
 
namespace subprocess {
 
// Max buffer size allocated on stack for read error
// from pipe
static const size_t SP_MAX_ERR_BUF_SIZ = 1024;
 
// Default buffer capacity for OutBuffer and ErrBuffer.
// If the data exceeds this capacity, the buffer size is grown
// by 1.5 times its previous capacity
static const size_t DEFAULT_BUF_CAP_BYTES = 8192;
 
 
/*-----------------------------------------------
 *    EXCEPTION CLASSES
 *-----------------------------------------------
 */
 
class CalledProcessError: public std::runtime_error
{
public:
  int retcode;
  CalledProcessError(const std::string& error_msg, int retcode):
    std::runtime_error(error_msg), retcode(retcode)
  {}
};
 
 
class OSError: public std::runtime_error
{
public:
  OSError(const std::string& err_msg, int err_code):
    std::runtime_error(err_msg + ": " + SysErrorString(err_code))
  {}
};
 
//--------------------------------------------------------------------
namespace util
{
#ifdef __USING_WINDOWS__
  inline void quote_argument(const std::wstring &argument, std::wstring &command_line,
                      bool force)
  {
    //
    // Unless we're told otherwise, don't quote unless we actually
    // need to do so --- hopefully avoid problems if programs won't
    // parse quotes properly
    //
 
    if (force == false && argument.empty() == false &&
        argument.find_first_of(L" \t\n\v") == argument.npos) {
      command_line.append(argument);
    }
    else {
      command_line.push_back(L'"');
 
      for (auto it = argument.begin();; ++it) {
        unsigned number_backslashes = 0;
 
        while (it != argument.end() && *it == L'\\') {
          ++it;
          ++number_backslashes;
        }
 
        if (it == argument.end()) {
 
          //
          // Escape all backslashes, but let the terminating
          // double quotation mark we add below be interpreted
          // as a metacharacter.
          //
 
          command_line.append(number_backslashes * 2, L'\\');
          break;
        }
        else if (*it == L'"') {
 
          //
          // Escape all backslashes and the following
          // double quotation mark.
          //
 
          command_line.append(number_backslashes * 2 + 1, L'\\');
          command_line.push_back(*it);
        }
        else {
 
          //
          // Backslashes aren't special here.
          //
 
          command_line.append(number_backslashes, L'\\');
          command_line.push_back(*it);
        }
      }
 
      command_line.push_back(L'"');
    }
  }
 
  inline std::string get_last_error(DWORD errorMessageID)
  {
    if (errorMessageID == 0)
      return std::string();
 
    LPSTR messageBuffer = nullptr;
    size_t size = FormatMessageA(
        FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_FROM_SYSTEM |
            FORMAT_MESSAGE_IGNORE_INSERTS | FORMAT_MESSAGE_MAX_WIDTH_MASK,
        NULL, errorMessageID, MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),
        (LPSTR)&messageBuffer, 0, NULL);
 
    std::string message(messageBuffer, size);
 
    LocalFree(messageBuffer);
 
    return message;
  }
 
  inline FILE *file_from_handle(HANDLE h, const char *mode)
  {
    int md;
    if (!mode) {
      throw OSError("invalid_mode", 0);
    }
 
    if (mode[0] == 'w') {
      md = _O_WRONLY;
    }
    else if (mode[0] == 'r') {
      md = _O_RDONLY;
    }
    else {
      throw OSError("file_from_handle", 0);
    }
 
    int os_fhandle = _open_osfhandle((intptr_t)h, md);
    if (os_fhandle == -1) {
      CloseHandle(h);
      throw OSError("_open_osfhandle", 0);
    }
 
    FILE *fp = _fdopen(os_fhandle, mode);
    if (fp == 0) {
      subprocess_close(os_fhandle);
      throw OSError("_fdopen", 0);
    }
 
    return fp;
  }
 
  inline void configure_pipe(HANDLE* read_handle, HANDLE* write_handle, HANDLE* child_handle)
  {
    SECURITY_ATTRIBUTES saAttr;
 
    // Set the bInheritHandle flag so pipe handles are inherited.
    saAttr.nLength = sizeof(SECURITY_ATTRIBUTES);
    saAttr.bInheritHandle = TRUE;
    saAttr.lpSecurityDescriptor = NULL;
 
    // Create a pipe for the child process's STDIN.
    if (!CreatePipe(read_handle, write_handle, &saAttr,0))
      throw OSError("CreatePipe", 0);
 
    // Ensure the write handle to the pipe for STDIN is not inherited.
    if (!SetHandleInformation(*child_handle, HANDLE_FLAG_INHERIT, 0))
      throw OSError("SetHandleInformation", 0);
  }
#endif
 
  static inline std::vector<std::string>
  split(const std::string& str, const std::string& delims=" \t")
  {
    std::vector<std::string> res;
    size_t init = 0;
 
    while (true) {
      auto pos = str.find_first_of(delims, init);
      if (pos == std::string::npos) {
        res.emplace_back(str.substr(init, str.length()));
        break;
      }
      res.emplace_back(str.substr(init, pos - init));
      pos++;
      init = pos;
    }
 
    return res;
  }
 
 
#ifndef __USING_WINDOWS__
  static inline
  void set_clo_on_exec(int fd, bool set = true)
  {
    int flags = fcntl(fd, F_GETFD, 0);
    if (flags == -1) {
        throw OSError("fcntl F_GETFD failed", errno);
    }
    if (set) flags |= FD_CLOEXEC;
    else flags &= ~FD_CLOEXEC;
    if (fcntl(fd, F_SETFD, flags) == -1) {
        throw OSError("fcntl F_SETFD failed", errno);
    }
  }
 
 
  static inline
  std::pair<int, int> pipe_cloexec() noexcept(false)
  {
    int pipe_fds[2];
    int res = pipe(pipe_fds);
    if (res) {
      throw OSError("pipe failure", errno);
    }
 
    set_clo_on_exec(pipe_fds[0]);
    set_clo_on_exec(pipe_fds[1]);
 
    return std::make_pair(pipe_fds[0], pipe_fds[1]);
  }
#endif
 
 
  static inline
  int write_n(int fd, const char* buf, size_t length)
  {
    size_t nwritten = 0;
    while (nwritten < length) {
      int written = subprocess_write(fd, buf + nwritten, length - nwritten);
      if (written == -1) return -1;
      nwritten += written;
    }
    return nwritten;
  }
 
 
  static inline
  int read_atmost_n(FILE* fp, char* buf, size_t read_upto)
  {
#ifdef __USING_WINDOWS__
    return (int)fread(buf, 1, read_upto, fp);
#else
    int fd = subprocess_fileno(fp);
    int rbytes = 0;
    int eintr_cnter = 0;
 
    while (1) {
      int read_bytes = read(fd, buf + rbytes, read_upto - rbytes);
      if (read_bytes == -1) {
        if (errno == EINTR) {
          if (eintr_cnter >= 50) return -1;
          eintr_cnter++;
          continue;
        }
        return -1;
      }
      if (read_bytes == 0) return rbytes;
 
      rbytes += read_bytes;
    }
    return rbytes;
#endif
  }
 
 
  static inline int read_all(FILE* fp, std::vector<char>& buf)
  {
    auto buffer = buf.data();
    int total_bytes_read = 0;
    int fill_sz = buf.size();
 
    while (1) {
      const int rd_bytes = read_atmost_n(fp, buffer, fill_sz);
 
      if (rd_bytes == -1) { // Read finished
        if (total_bytes_read == 0) return -1;
        break;
 
      } else if (rd_bytes == fill_sz) { // Buffer full
        const auto orig_sz = buf.size();
        const auto new_sz = orig_sz * 2;
        buf.resize(new_sz);
        fill_sz = new_sz - orig_sz;
 
        //update the buffer pointer
        buffer = buf.data();
        total_bytes_read += rd_bytes;
        buffer += total_bytes_read;
 
      } else { // Partial data ? Continue reading
        total_bytes_read += rd_bytes;
        fill_sz -= rd_bytes;
        break;
      }
    }
    buf.erase(buf.begin()+total_bytes_read, buf.end()); // remove extra nulls
    return total_bytes_read;
  }
 
#ifndef __USING_WINDOWS__
  static inline
  std::pair<int, int> wait_for_child_exit(int pid)
  {
    int status = 0;
    int ret = -1;
    while (1) {
      ret = waitpid(pid, &status, 0);
      if (ret == -1) break;
      if (ret == 0) continue;
      return std::make_pair(ret, status);
    }
 
    return std::make_pair(ret, status);
  }
#endif
 
} // end namespace util
 
 
 
/* -------------------------------
 *     Popen Arguments
 * -------------------------------
 */
 
struct close_fds {
  explicit close_fds(bool c): close_all(c) {}
  bool close_all = false;
};
 
struct string_arg
{
  string_arg(const char* arg): arg_value(arg) {}
  string_arg(std::string&& arg): arg_value(std::move(arg)) {}
  string_arg(const std::string& arg): arg_value(arg) {}
  std::string arg_value;
};
 
struct executable: string_arg
{
  template <typename T>
  executable(T&& arg): string_arg(std::forward<T>(arg)) {}
};
 
enum IOTYPE {
  STDOUT = 1,
  STDERR,
  PIPE,
};
 
//TODO: A common base/interface for below stream structures ??
 
struct input
{
  // For an already existing file descriptor.
  explicit input(int fd): rd_ch_(fd) {}
 
  // FILE pointer.
  explicit input (FILE* fp):input(subprocess_fileno(fp)) { assert(fp); }
 
  explicit input(const char* filename) {
    int fd = subprocess_open(filename, O_RDONLY);
    if (fd == -1) throw OSError("File not found: ", errno);
    rd_ch_ = fd;
  }
  explicit input(IOTYPE typ) {
    assert (typ == PIPE && "STDOUT/STDERR not allowed");
#ifndef __USING_WINDOWS__
    std::tie(rd_ch_, wr_ch_) = util::pipe_cloexec();
#endif
  }
 
  int rd_ch_ = -1;
  int wr_ch_ = -1;
};
 
 
struct output
{
  explicit output(int fd): wr_ch_(fd) {}
 
  explicit output (FILE* fp):output(subprocess_fileno(fp)) { assert(fp); }
 
  explicit output(const char* filename) {
    int fd = subprocess_open(filename, O_APPEND | O_CREAT | O_RDWR, 0640);
    if (fd == -1) throw OSError("File not found: ", errno);
    wr_ch_ = fd;
  }
  explicit output(IOTYPE typ) {
    assert (typ == PIPE && "STDOUT/STDERR not allowed");
#ifndef __USING_WINDOWS__
    std::tie(rd_ch_, wr_ch_) = util::pipe_cloexec();
#endif
  }
 
  int rd_ch_ = -1;
  int wr_ch_ = -1;
};
 
 
struct error
{
  explicit error(int fd): wr_ch_(fd) {}
 
  explicit error(FILE* fp):error(subprocess_fileno(fp)) { assert(fp); }
 
  explicit error(const char* filename) {
    int fd = subprocess_open(filename, O_APPEND | O_CREAT | O_RDWR, 0640);
    if (fd == -1) throw OSError("File not found: ", errno);
    wr_ch_ = fd;
  }
  explicit error(IOTYPE typ) {
    assert ((typ == PIPE || typ == STDOUT) && "STDERR not allowed");
    if (typ == PIPE) {
#ifndef __USING_WINDOWS__
      std::tie(rd_ch_, wr_ch_) = util::pipe_cloexec();
#endif
    } else {
      // Need to defer it till we have checked all arguments
      deferred_ = true;
    }
  }
 
  bool deferred_ = false;
  int rd_ch_ = -1;
  int wr_ch_ = -1;
};
 
// ~~~~ End Popen Args ~~~~
 
 
class Buffer
{
public:
  Buffer() = default;
  explicit Buffer(size_t cap) { buf.resize(cap); }
  void add_cap(size_t cap) { buf.resize(cap); }
 
public:
  std::vector<char> buf;
  size_t length = 0;
};
 
// Buffer for storing output written to output fd
using OutBuffer = Buffer;
// Buffer for storing output written to error fd
using ErrBuffer = Buffer;
 
 
// Fwd Decl.
class Popen;
 
/*---------------------------------------------------
 *      DETAIL NAMESPACE
 *---------------------------------------------------
 */
 
namespace detail {
struct ArgumentDeducer
{
  ArgumentDeducer(Popen* p): popen_(p) {}
 
  void set_option(executable&& exe);
  void set_option(input&& inp);
  void set_option(output&& out);
  void set_option(error&& err);
  void set_option(close_fds&& cfds);
 
private:
  Popen* popen_ = nullptr;
};
 
class Child
{
public:
  Child(Popen* p, int err_wr_pipe):
    parent_(p),
    err_wr_pipe_(err_wr_pipe)
  {}
 
  void execute_child();
 
private:
  // Lets call it parent even though
  // technically a bit incorrect
  Popen* parent_ = nullptr;
  int err_wr_pipe_ = -1;
};
 
// Fwd Decl.
class Streams;
 
class Communication
{
public:
  Communication(Streams* stream): stream_(stream)
  {}
  Communication(const Communication&) = delete;
  Communication& operator=(const Communication&) = delete;
  Communication(Communication&&) = default;
  Communication& operator=(Communication&&) = default;
public:
  int send(const char* msg, size_t length);
  int send(const std::vector<char>& msg);
 
  std::pair<OutBuffer, ErrBuffer> communicate(const char* msg, size_t length);
  std::pair<OutBuffer, ErrBuffer> communicate(const std::vector<char>& msg)
  { return communicate(msg.data(), msg.size()); }
 
  void set_out_buf_cap(size_t cap) { out_buf_cap_ = cap; }
  void set_err_buf_cap(size_t cap) { err_buf_cap_ = cap; }
 
private:
  std::pair<OutBuffer, ErrBuffer> communicate_threaded(
      const char* msg, size_t length);
 
private:
  Streams* stream_;
  size_t out_buf_cap_ = DEFAULT_BUF_CAP_BYTES;
  size_t err_buf_cap_ = DEFAULT_BUF_CAP_BYTES;
};
 
 
 
class Streams
{
public:
  Streams():comm_(this) {}
  Streams(const Streams&) = delete;
  Streams& operator=(const Streams&) = delete;
  Streams(Streams&&) = default;
  Streams& operator=(Streams&&) = default;
 
public:
  void setup_comm_channels();
 
  void cleanup_fds()
  {
    if (write_to_child_ != -1 && read_from_parent_ != -1) {
      subprocess_close(write_to_child_);
    }
    if (write_to_parent_ != -1 && read_from_child_ != -1) {
      subprocess_close(read_from_child_);
    }
    if (err_write_ != -1 && err_read_ != -1) {
      subprocess_close(err_read_);
    }
  }
 
  void close_parent_fds()
  {
    if (write_to_child_ != -1)  subprocess_close(write_to_child_);
    if (read_from_child_ != -1) subprocess_close(read_from_child_);
    if (err_read_ != -1)        subprocess_close(err_read_);
  }
 
  void close_child_fds()
  {
    if (write_to_parent_ != -1)  subprocess_close(write_to_parent_);
    if (read_from_parent_ != -1) subprocess_close(read_from_parent_);
    if (err_write_ != -1)        subprocess_close(err_write_);
  }
 
  FILE* input()  { return input_.get(); }
  FILE* output() { return output_.get(); }
  FILE* error()  { return error_.get(); }
 
  void input(FILE* fp)  { input_.reset(fp, fclose); }
  void output(FILE* fp) { output_.reset(fp, fclose); }
  void error(FILE* fp)  { error_.reset(fp, fclose); }
 
  void set_out_buf_cap(size_t cap) { comm_.set_out_buf_cap(cap); }
  void set_err_buf_cap(size_t cap) { comm_.set_err_buf_cap(cap); }
 
public: /* Communication forwarding API's */
  int send(const char* msg, size_t length)
  { return comm_.send(msg, length); }
 
  int send(const std::vector<char>& msg)
  { return comm_.send(msg); }
 
  std::pair<OutBuffer, ErrBuffer> communicate(const char* msg, size_t length)
  { return comm_.communicate(msg, length); }
 
  std::pair<OutBuffer, ErrBuffer> communicate(const std::vector<char>& msg)
  { return comm_.communicate(msg); }
 
 
public:// Yes they are public
 
  std::shared_ptr<FILE> input_  = nullptr;
  std::shared_ptr<FILE> output_ = nullptr;
  std::shared_ptr<FILE> error_  = nullptr;
 
#ifdef __USING_WINDOWS__
  HANDLE g_hChildStd_IN_Rd = nullptr;
  HANDLE g_hChildStd_IN_Wr = nullptr;
  HANDLE g_hChildStd_OUT_Rd = nullptr;
  HANDLE g_hChildStd_OUT_Wr = nullptr;
  HANDLE g_hChildStd_ERR_Rd = nullptr;
  HANDLE g_hChildStd_ERR_Wr = nullptr;
#endif
 
  // Pipes for communicating with child
 
  // Emulates stdin
  int write_to_child_   = -1; // Parent owned descriptor
  int read_from_parent_ = -1; // Child owned descriptor
 
  // Emulates stdout
  int write_to_parent_ = -1; // Child owned descriptor
  int read_from_child_ = -1; // Parent owned descriptor
 
  // Emulates stderr
  int err_write_ = -1; // Write error to parent (Child owned)
  int err_read_  = -1; // Read error from child (Parent owned)
 
private:
  Communication comm_;
};
 
} // end namespace detail
 
 
 
class Popen
{
public:
  friend struct detail::ArgumentDeducer;
  friend class detail::Child;
 
  template <typename... Args>
  Popen(const std::string& cmd_args, Args&& ...args):
    args_(cmd_args)
  {
    vargs_ = util::split(cmd_args);
    init_args(std::forward<Args>(args)...);
 
    // Setup the communication channels of the Popen class
    stream_.setup_comm_channels();
 
    execute_process();
  }
 
  template <typename... Args>
  Popen(std::initializer_list<const char*> cmd_args, Args&& ...args)
  {
    vargs_.insert(vargs_.end(), cmd_args.begin(), cmd_args.end());
    init_args(std::forward<Args>(args)...);
 
    // Setup the communication channels of the Popen class
    stream_.setup_comm_channels();
 
    execute_process();
  }
 
  template <typename... Args>
  Popen(std::vector<std::string> vargs_, Args &&... args) : vargs_(vargs_)
  {
    init_args(std::forward<Args>(args)...);
 
    // Setup the communication channels of the Popen class
    stream_.setup_comm_channels();
 
    execute_process();
  }
 
  int retcode() const noexcept { return retcode_; }
 
  int wait() noexcept(false);
 
  void set_out_buf_cap(size_t cap) { stream_.set_out_buf_cap(cap); }
 
  void set_err_buf_cap(size_t cap) { stream_.set_err_buf_cap(cap); }
 
  int send(const char* msg, size_t length)
  { return stream_.send(msg, length); }
 
  int send(const std::string& msg)
  { return send(msg.c_str(), msg.size()); }
 
  int send(const std::vector<char>& msg)
  { return stream_.send(msg); }
 
  std::pair<OutBuffer, ErrBuffer> communicate(const char* msg, size_t length)
  {
    auto res = stream_.communicate(msg, length);
    retcode_ = wait();
    return res;
  }
 
  std::pair<OutBuffer, ErrBuffer> communicate(const std::string& msg)
  {
    return communicate(msg.c_str(), msg.size());
  }
 
  std::pair<OutBuffer, ErrBuffer> communicate(const std::vector<char>& msg)
  {
    auto res = stream_.communicate(msg);
    retcode_ = wait();
    return res;
  }
 
  std::pair<OutBuffer, ErrBuffer> communicate()
  {
    return communicate(nullptr, 0);
  }
 
private:
  template <typename F, typename... Args>
  void init_args(F&& farg, Args&&... args);
  void init_args();
  void populate_c_argv();
  void execute_process() noexcept(false);
 
private:
  detail::Streams stream_;
 
#ifdef __USING_WINDOWS__
  HANDLE process_handle_;
  std::future<void> cleanup_future_;
#endif
 
  bool close_fds_ = false;
 
  std::string exe_name_;
 
  // Command in string format
  std::string args_;
  // Command provided as sequence
  std::vector<std::string> vargs_;
  std::vector<char*> cargv_;
 
  // Pid of the child process
  int child_pid_ = -1;
 
  int retcode_ = -1;
};
 
inline void Popen::init_args() {
  populate_c_argv();
}
 
template <typename F, typename... Args>
inline void Popen::init_args(F&& farg, Args&&... args)
{
  detail::ArgumentDeducer argd(this);
  argd.set_option(std::forward<F>(farg));
  init_args(std::forward<Args>(args)...);
}
 
inline void Popen::populate_c_argv()
{
  cargv_.clear();
  cargv_.reserve(vargs_.size() + 1);
  for (auto& arg : vargs_) cargv_.push_back(&arg[0]);
  cargv_.push_back(nullptr);
}
 
inline int Popen::wait() noexcept(false)
{
#ifdef __USING_WINDOWS__
  int ret = WaitForSingleObject(process_handle_, INFINITE);
 
  // WaitForSingleObject with INFINITE should only return when process has signaled
  if (ret != WAIT_OBJECT_0) {
    throw OSError("Unexpected return code from WaitForSingleObject", 0);
  }
 
  DWORD dretcode_;
 
  if (FALSE == GetExitCodeProcess(process_handle_, &dretcode_))
      throw OSError("Failed during call to GetExitCodeProcess", 0);
 
  CloseHandle(process_handle_);
 
  return (int)dretcode_;
#else
  int ret, status;
  std::tie(ret, status) = util::wait_for_child_exit(child_pid_);
  if (ret == -1) {
    if (errno != ECHILD) throw OSError("waitpid failed", errno);
    return 0;
  }
  if (WIFEXITED(status)) return WEXITSTATUS(status);
  if (WIFSIGNALED(status)) return WTERMSIG(status);
  else return 255;
 
  return 0;
#endif
}
 
inline void Popen::execute_process() noexcept(false)
{
#ifdef __USING_WINDOWS__
  if (exe_name_.length()) {
    this->vargs_.insert(this->vargs_.begin(), this->exe_name_);
    this->populate_c_argv();
  }
  this->exe_name_ = vargs_[0];
 
  std::wstring_convert<std::codecvt_utf8_utf16<wchar_t>> converter;
  std::wstring argument;
  std::wstring command_line;
  bool first_arg = true;
 
  for (auto arg : this->vargs_) {
    if (!first_arg) {
      command_line += L" ";
    } else {
      first_arg = false;
    }
    argument = converter.from_bytes(arg);
    util::quote_argument(argument, command_line, false);
  }
 
  // CreateProcessW can modify szCmdLine so we allocate needed memory
  wchar_t *szCmdline = new wchar_t[command_line.size() + 1];
  wcscpy_s(szCmdline, command_line.size() + 1, command_line.c_str());
  PROCESS_INFORMATION piProcInfo;
  STARTUPINFOW siStartInfo;
  BOOL bSuccess = FALSE;
  DWORD creation_flags = CREATE_UNICODE_ENVIRONMENT | CREATE_NO_WINDOW;
 
  // Set up members of the PROCESS_INFORMATION structure.
  ZeroMemory(&piProcInfo, sizeof(PROCESS_INFORMATION));
 
  // Set up members of the STARTUPINFOW structure.
  // This structure specifies the STDIN and STDOUT handles for redirection.
 
  ZeroMemory(&siStartInfo, sizeof(STARTUPINFOW));
  siStartInfo.cb = sizeof(STARTUPINFOW);
 
  siStartInfo.hStdError = this->stream_.g_hChildStd_ERR_Wr;
  siStartInfo.hStdOutput = this->stream_.g_hChildStd_OUT_Wr;
  siStartInfo.hStdInput = this->stream_.g_hChildStd_IN_Rd;
 
  siStartInfo.dwFlags |= STARTF_USESTDHANDLES;
 
  // Create the child process.
  bSuccess = CreateProcessW(NULL,
                            szCmdline,    // command line
                            NULL,         // process security attributes
                            NULL,         // primary thread security attributes
                            TRUE,         // handles are inherited
                            creation_flags,     // creation flags
                            NULL,         // use parent's environment
                            NULL,         // use parent's current directory
                            &siStartInfo, // STARTUPINFOW pointer
                            &piProcInfo); // receives PROCESS_INFORMATION
 
  // If an error occurs, exit the application.
  if (!bSuccess) {
    DWORD errorMessageID = ::GetLastError();
    throw CalledProcessError("CreateProcess failed: " + util::get_last_error(errorMessageID), errorMessageID);
  }
 
  CloseHandle(piProcInfo.hThread);
 
  /*
    TODO: use common apis to close linux handles
  */
 
  this->process_handle_ = piProcInfo.hProcess;
 
  this->cleanup_future_ = std::async(std::launch::async, [this] {
    WaitForSingleObject(this->process_handle_, INFINITE);
 
    CloseHandle(this->stream_.g_hChildStd_ERR_Wr);
    CloseHandle(this->stream_.g_hChildStd_OUT_Wr);
    CloseHandle(this->stream_.g_hChildStd_IN_Rd);
  });
 
/*
  NOTE: In the linux version, there is a check to make sure that the process
        has been started. Here, we do nothing because CreateProcess will throw
        if we fail to create the process.
*/
 
 
#else
 
  int err_rd_pipe, err_wr_pipe;
  std::tie(err_rd_pipe, err_wr_pipe) = util::pipe_cloexec();
 
  if (exe_name_.length()) {
    vargs_.insert(vargs_.begin(), exe_name_);
    populate_c_argv();
  }
  exe_name_ = vargs_[0];
 
  child_pid_ = fork();
 
  if (child_pid_ < 0) {
    subprocess_close(err_rd_pipe);
    subprocess_close(err_wr_pipe);
    throw OSError("fork failed", errno);
  }
 
  if (child_pid_ == 0)
  {
    // Close descriptors belonging to parent
    stream_.close_parent_fds();
 
    //Close the read end of the error pipe
    subprocess_close(err_rd_pipe);
 
    detail::Child chld(this, err_wr_pipe);
    chld.execute_child();
  }
  else
  {
    subprocess_close(err_wr_pipe);// close child side of pipe, else get stuck in read below
 
    stream_.close_child_fds();
 
    try {
      char err_buf[SP_MAX_ERR_BUF_SIZ] = {0,};
 
      FILE* err_fp = fdopen(err_rd_pipe, "r");
      if (!err_fp) {
          subprocess_close(err_rd_pipe);
          throw OSError("fdopen failed", errno);
      }
      int read_bytes = util::read_atmost_n(err_fp, err_buf, SP_MAX_ERR_BUF_SIZ);
      fclose(err_fp);
 
      if (read_bytes || strlen(err_buf)) {
        // Call waitpid to reap the child process
        // waitpid suspends the calling process until the
        // child terminates.
        int retcode = wait();
 
        // Throw whatever information we have about child failure
        throw CalledProcessError(err_buf, retcode);
      }
    } catch (std::exception& exp) {
      stream_.cleanup_fds();
      throw;
    }
 
  }
#endif
}
 
namespace detail {
 
  inline void ArgumentDeducer::set_option(executable&& exe) {
    popen_->exe_name_ = std::move(exe.arg_value);
  }
 
  inline void ArgumentDeducer::set_option(input&& inp) {
    if (inp.rd_ch_ != -1) popen_->stream_.read_from_parent_ = inp.rd_ch_;
    if (inp.wr_ch_ != -1) popen_->stream_.write_to_child_ = inp.wr_ch_;
  }
 
  inline void ArgumentDeducer::set_option(output&& out) {
    if (out.wr_ch_ != -1) popen_->stream_.write_to_parent_ = out.wr_ch_;
    if (out.rd_ch_ != -1) popen_->stream_.read_from_child_ = out.rd_ch_;
  }
 
  inline void ArgumentDeducer::set_option(error&& err) {
    if (err.deferred_) {
      if (popen_->stream_.write_to_parent_) {
        popen_->stream_.err_write_ = popen_->stream_.write_to_parent_;
      } else {
        throw std::runtime_error("Set output before redirecting error to output");
      }
    }
    if (err.wr_ch_ != -1) popen_->stream_.err_write_ = err.wr_ch_;
    if (err.rd_ch_ != -1) popen_->stream_.err_read_ = err.rd_ch_;
  }
 
  inline void ArgumentDeducer::set_option(close_fds&& cfds) {
    popen_->close_fds_ = cfds.close_all;
  }
 
 
  inline void Child::execute_child() {
#ifndef __USING_WINDOWS__
    int sys_ret = -1;
    auto& stream = parent_->stream_;
 
    try {
      if (stream.write_to_parent_ == 0)
        stream.write_to_parent_ = dup(stream.write_to_parent_);
 
      if (stream.err_write_ == 0 || stream.err_write_ == 1)
        stream.err_write_ = dup(stream.err_write_);
 
      // Make the child owned descriptors as the
      // stdin, stdout and stderr for the child process
      auto _dup2_ = [](int fd, int to_fd) {
        if (fd == to_fd) {
          // dup2 syscall does not reset the
          // CLOEXEC flag if the descriptors
          // provided to it are same.
          // But, we need to reset the CLOEXEC
          // flag as the provided descriptors
          // are now going to be the standard
          // input, output and error
          util::set_clo_on_exec(fd, false);
        } else if(fd != -1) {
          int res = dup2(fd, to_fd);
          if (res == -1) throw OSError("dup2 failed", errno);
        }
      };
 
      // Create the standard streams
      _dup2_(stream.read_from_parent_, 0); // Input stream
      _dup2_(stream.write_to_parent_,  1); // Output stream
      _dup2_(stream.err_write_,        2); // Error stream
 
      // Close the duped descriptors
      if (stream.read_from_parent_ != -1 && stream.read_from_parent_ > 2)
        subprocess_close(stream.read_from_parent_);
 
      if (stream.write_to_parent_ != -1 && stream.write_to_parent_ > 2)
        subprocess_close(stream.write_to_parent_);
 
      if (stream.err_write_ != -1 && stream.err_write_ > 2)
        subprocess_close(stream.err_write_);
 
      // Close all the inherited fd's except the error write pipe
      if (parent_->close_fds_) {
        // If possible, try to get the list of open file descriptors from the
        // operating system. This is more efficient, but not guaranteed to be
        // available.
#ifdef __linux__
        // For Linux, enumerate /proc/<pid>/fd.
        try {
          std::vector<int> fds_to_close;
          for (const auto& it : fs::directory_iterator(strprintf("/proc/%d/fd", getpid()))) {
            auto fd{ToIntegral<uint64_t>(it.path().filename().native())};
            if (!fd || *fd > std::numeric_limits<int>::max()) continue;
            if (*fd <= 2) continue;  // leave std{in,out,err} alone
            if (*fd == static_cast<uint64_t>(err_wr_pipe_)) continue;
            fds_to_close.push_back(*fd);
          }
          for (const int fd : fds_to_close) {
            close(fd);
          }
        } catch (const fs::filesystem_error &e) {
          throw OSError("/proc/<pid>/fd iteration failed", e.code().value());
        }
#else
        // On other operating systems, iterate over all file descriptor slots
        // and try to close them all.
        int max_fd = sysconf(_SC_OPEN_MAX);
        if (max_fd == -1) throw OSError("sysconf failed", errno);
 
        for (int i = 3; i < max_fd; i++) {
          if (i == err_wr_pipe_) continue;
          close(i);
        }
#endif
      }
 
      // Replace the current image with the executable
      sys_ret = execvp(parent_->exe_name_.c_str(), parent_->cargv_.data());
 
      if (sys_ret == -1) throw OSError("execve failed", errno);
 
    } catch (const OSError& exp) {
      // Just write the exception message
      // TODO: Give back stack trace ?
      std::string err_msg(exp.what());
      //ATTN: Can we do something on error here ?
      util::write_n(err_wr_pipe_, err_msg.c_str(), err_msg.length());
    }
 
    // Calling application would not get this
    // exit failure
    _exit (EXIT_FAILURE);
#endif
  }
 
 
  inline void Streams::setup_comm_channels()
  {
#ifdef __USING_WINDOWS__
    util::configure_pipe(&this->g_hChildStd_IN_Rd, &this->g_hChildStd_IN_Wr, &this->g_hChildStd_IN_Wr);
    this->input(util::file_from_handle(this->g_hChildStd_IN_Wr, "w"));
    this->write_to_child_ = subprocess_fileno(this->input());
 
    util::configure_pipe(&this->g_hChildStd_OUT_Rd, &this->g_hChildStd_OUT_Wr, &this->g_hChildStd_OUT_Rd);
    this->output(util::file_from_handle(this->g_hChildStd_OUT_Rd, "r"));
    this->read_from_child_ = subprocess_fileno(this->output());
 
    util::configure_pipe(&this->g_hChildStd_ERR_Rd, &this->g_hChildStd_ERR_Wr, &this->g_hChildStd_ERR_Rd);
    this->error(util::file_from_handle(this->g_hChildStd_ERR_Rd, "r"));
    this->err_read_ = subprocess_fileno(this->error());
#else
 
    if (write_to_child_ != -1)  input(fdopen(write_to_child_, "wb"));
    if (read_from_child_ != -1) output(fdopen(read_from_child_, "rb"));
    if (err_read_ != -1)        error(fdopen(err_read_, "rb"));
 
    auto handles = {input(), output(), error()};
 
    for (auto& h : handles) {
      if (h == nullptr) continue;
      setvbuf(h, nullptr, _IONBF, BUFSIZ);
    }
  #endif
  }
 
  inline int Communication::send(const char* msg, size_t length)
  {
    if (stream_->input() == nullptr) return -1;
    return std::fwrite(msg, sizeof(char), length, stream_->input());
  }
 
  inline int Communication::send(const std::vector<char>& msg)
  {
    return send(msg.data(), msg.size());
  }
 
  inline std::pair<OutBuffer, ErrBuffer>
  Communication::communicate(const char* msg, size_t length)
  {
    // Optimization from subprocess.py
    // If we are using one pipe, or no pipe
    // at all, using select() or threads is unnecessary.
    auto hndls = {stream_->input(), stream_->output(), stream_->error()};
    int count = std::count(std::begin(hndls), std::end(hndls), nullptr);
    const int len_conv = length;
 
    if (count >= 2) {
      OutBuffer obuf;
      ErrBuffer ebuf;
      if (stream_->input()) {
        if (msg) {
          int wbytes = std::fwrite(msg, sizeof(char), length, stream_->input());
          if (wbytes < len_conv) {
            if (errno != EPIPE && errno != EINVAL) {
              throw OSError("fwrite error", errno);
            }
          }
        }
        // Close the input stream
        stream_->input_.reset();
      } else if (stream_->output()) {
        // Read till EOF
        // ATTN: This could be blocking, if the process
        // at the other end screws up, we get screwed as well
        obuf.add_cap(out_buf_cap_);
 
        int rbytes = util::read_all(
                            stream_->output(),
                            obuf.buf);
 
        if (rbytes == -1) {
          throw OSError("read to obuf failed", errno);
        }
 
        obuf.length = rbytes;
        // Close the output stream
        stream_->output_.reset();
 
      } else if (stream_->error()) {
        // Same screwness applies here as well
        ebuf.add_cap(err_buf_cap_);
 
        int rbytes = util::read_atmost_n(
                                  stream_->error(),
                                  ebuf.buf.data(),
                                  ebuf.buf.size());
 
        if (rbytes == -1) {
          throw OSError("read to ebuf failed", errno);
        }
 
        ebuf.length = rbytes;
        // Close the error stream
        stream_->error_.reset();
      }
      return std::make_pair(std::move(obuf), std::move(ebuf));
    }
 
    return communicate_threaded(msg, length);
  }
 
 
  inline std::pair<OutBuffer, ErrBuffer>
  Communication::communicate_threaded(const char* msg, size_t length)
  {
    OutBuffer obuf;
    ErrBuffer ebuf;
    std::future<int> out_fut, err_fut;
    const int length_conv = length;
 
    if (stream_->output()) {
      obuf.add_cap(out_buf_cap_);
 
      out_fut = std::async(std::launch::async,
                          [&obuf, this] {
                            return util::read_all(this->stream_->output(), obuf.buf);
                          });
    }
    if (stream_->error()) {
      ebuf.add_cap(err_buf_cap_);
 
      err_fut = std::async(std::launch::async,
                          [&ebuf, this] {
                            return util::read_all(this->stream_->error(), ebuf.buf);
                          });
    }
    if (stream_->input()) {
      if (msg) {
        int wbytes = std::fwrite(msg, sizeof(char), length, stream_->input());
        if (wbytes < length_conv) {
          if (errno != EPIPE && errno != EINVAL) {
            throw OSError("fwrite error", errno);
          }
        }
      }
      stream_->input_.reset();
    }
 
    if (out_fut.valid()) {
      int res = out_fut.get();
      if (res != -1) obuf.length = res;
      else obuf.length = 0;
    }
    if (err_fut.valid()) {
      int res = err_fut.get();
      if (res != -1) ebuf.length = res;
      else ebuf.length = 0;
    }
 
    return std::make_pair(std::move(obuf), std::move(ebuf));
  }
 
} // end namespace detail
 
}
 
#endif // BITCOIN_UTIL_SUBPROCESS_H