// Based on the https://github.com/arun11299/cpp-subprocess project.

/*!

Documentation for C++ subprocessing library.

@copyright The code is licensed under the [MIT
  License](http://opensource.org/licenses/MIT):
  <br>
  Copyright &copy; 2016-2018 Arun Muralidharan.
  <br>
  Permission is hereby granted, free of charge, to any person obtaining a copy
  of this software and associated documentation files (the "Software"), to deal
  in the Software without restriction, including without limitation the rights
  to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
  copies of the Software, and to permit persons to whom the Software is
  furnished to do so, subject to the following conditions:
  <br>
  The above copyright notice and this permission notice shall be included in
  all copies or substantial portions of the Software.
  <br>
  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
  AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
  OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
  SOFTWARE.

@author [Arun Muralidharan]
@see https://github.com/arun11299/cpp-subprocess to download the source code

@version 1.0.0
*/

#ifndef BITCOIN_UTIL_SUBPROCESS_H
#define BITCOIN_UTIL_SUBPROCESS_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>

  #define close _close
  #define open _open
  #define fileno _fileno
#else
  #include <sys/wait.h>
  #include <unistd.h>
#endif
  #include <csignal>
  #include <fcntl.h>
  #include <sys/types.h>
}

/*!
 * Getting started with reading this source code.
 * The source is mainly divided into four parts:
 * 1. Exception Classes:
 *    These are very basic exception classes derived from
 *    runtime_error exception.
 *    There are two types of exception thrown from subprocess
 *    library: OSError and CalledProcessError
 *
 * 2. Popen Class
 *    This is the main class the users will deal with. It
 *    provides with all the API's to deal with processes.
 *
 * 3. Util namespace
 *    It includes some helper functions to split/join a string,
 *    reading from file descriptors, waiting on a process, fcntl
 *    options on file descriptors etc.
 *
 * 4. Detail namespace
 *    This includes some metaprogramming and helper classes.
 */


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
 * Thrown when there was error executing the command.
 * Check Popen class API's to know when this exception
 * can be thrown.
 *
 */
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
 * Thrown when some system call fails to execute or give result.
 * The exception message contains the name of the failed system call
 * with the stringisized errno code.
 * Check Popen class API's to know when this exception would be
 * thrown.
 * Its usual that the API exception specification would have
 * this exception together with CalledProcessError.
 */
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
{
  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'"');
    }
  }

#ifdef __USING_WINDOWS__
  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) {
      _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

  /*!
   * Function: split
   * Parameters:
   * [in] str : Input string which needs to be split based upon the
   *            delimiters provided.
   * [in] deleims : Delimiter characters based upon which the string needs
   *                to be split. Default constructed to ' '(space) and '\t'(tab)
   * [out] vector<string> : Vector of strings split at deleimiter.
   */
  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__
  /*!
   * Function: set_clo_on_exec
   * Sets/Resets the FD_CLOEXEC flag on the provided file descriptor
   * based upon the `set` parameter.
   * Parameters:
   * [in] fd : The descriptor on which FD_CLOEXEC needs to be set/reset.
   * [in] set : If 'true', set FD_CLOEXEC.
   *            If 'false' unset FD_CLOEXEC.
   */
  static inline
  void set_clo_on_exec(int fd, bool set = true)
  {
    int flags = fcntl(fd, F_GETFD, 0);
    if (set) flags |= FD_CLOEXEC;
    else flags &= ~FD_CLOEXEC;
    //TODO: should check for errors
    fcntl(fd, F_SETFD, flags);
  }


  /*!
   * Function: pipe_cloexec
   * Creates a pipe and sets FD_CLOEXEC flag on both
   * read and write descriptors of the pipe.
   * Parameters:
   * [out] : A pair of file descriptors.
   *         First element of pair is the read descriptor of pipe.
   *         Second element is the write descriptor of pipe.
   */
  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


  /*!
   * Function: write_n
   * Writes `length` bytes to the file descriptor `fd`
   * from the buffer `buf`.
   * Parameters:
   * [in] fd : The file descriptotr to write to.
   * [in] buf: Buffer from which data needs to be written to fd.
   * [in] length: The number of bytes that needs to be written from
   *              `buf` to `fd`.
   * [out] int : Number of bytes written or -1 in case of failure.
   */
  static inline
  int write_n(int fd, const char* buf, size_t length)
  {
    size_t nwritten = 0;
    while (nwritten < length) {
      int written = write(fd, buf + nwritten, length - nwritten);
      if (written == -1) return -1;
      nwritten += written;
    }
    return nwritten;
  }


  /*!
   * Function: read_atmost_n
   * Reads at the most `read_upto` bytes from the
   * file object `fp` before returning.
   * Parameters:
   * [in] fp : The file object from which it needs to read.
   * [in] buf : The buffer into which it needs to write the data.
   * [in] read_upto: Max number of bytes which must be read from `fd`.
   * [out] int : Number of bytes written to `buf` or read from `fd`
   *             OR -1 in case of error.
   *  NOTE: In case of EINTR while reading from socket, this API
   *  will retry to read from `fd`, but only till the EINTR counter
   *  reaches 50 after which it will return with whatever data it read.
   */
  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 = 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
  }


  /*!
   * Function: read_all
   * Reads all the available data from `fp` into
   * `buf`. Internally calls read_atmost_n.
   * Parameters:
   * [in] fp : The file object from which to read from.
   * [in] buf : The buffer of type `class Buffer` into which
   *            the read data is written to.
   * [out] int: Number of bytes read OR -1 in case of failure.
   *
   * NOTE: `class Buffer` is a exposed public class. See below.
   */

  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__
  /*!
   * Function: wait_for_child_exit
   * Waits for the process with pid `pid` to exit
   * and returns its status.
   * Parameters:
   * [in] pid : The pid of the process.
   * [out] pair<int, int>:
   *    pair.first : Return code of the waitpid call.
   *    pair.second : Exit status of the process.
   *
   *  NOTE: This is a blocking call as in, it will loop
   *  till the child is exited.
   */
  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
 * -------------------------------
 */

/*!
 * Base class for all arguments involving string value.
 */
struct string_arg
{
  string_arg(const char* arg): arg_value(arg) {}
  string_arg(std::string&& arg): arg_value(std::move(arg)) {}
  string_arg(std::string arg): arg_value(std::move(arg)) {}
  std::string arg_value;
};

/*!
 * Option to specify the executable name separately
 * from the args sequence.
 * In this case the cmd args must only contain the
 * options required for this executable.
 *
 * Eg: executable{"ls"}
 */
struct executable: string_arg
{
  template <typename T>
  executable(T&& arg): string_arg(std::forward<T>(arg)) {}
};

/*!
 * Used for redirecting input/output/error
 */
enum IOTYPE {
  STDOUT = 1,
  STDERR,
  PIPE,
};

//TODO: A common base/interface for below stream structures ??

/*!
 * Option to specify the input channel for the child
 * process. It can be:
 * 1. An already open file descriptor.
 * 2. A file name.
 * 3. IOTYPE. Usual a PIPE
 *
 * Eg: input{PIPE}
 * OR in case of redirection, output of another Popen
 * input{popen.output()}
 */
struct input
{
  // For an already existing file descriptor.
  explicit input(int fd): rd_ch_(fd) {}

  // FILE pointer.
  explicit input (FILE* fp):input(fileno(fp)) { assert(fp); }

  explicit input(const char* filename) {
    int fd = 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;
};


/*!
 * Option to specify the output channel for the child
 * process. It can be:
 * 1. An already open file descriptor.
 * 2. A file name.
 * 3. IOTYPE. Usually a PIPE.
 *
 * Eg: output{PIPE}
 * OR output{"output.txt"}
 */
struct output
{
  explicit output(int fd): wr_ch_(fd) {}

  explicit output (FILE* fp):output(fileno(fp)) { assert(fp); }

  explicit output(const char* filename) {
    int fd = 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;
};


/*!
 * Option to specify the error channel for the child
 * process. It can be:
 * 1. An already open file descriptor.
 * 2. A file name.
 * 3. IOTYPE. Usually a PIPE or STDOUT
 *
 */
struct error
{
  explicit error(int fd): wr_ch_(fd) {}

  explicit error(FILE* fp):error(fileno(fp)) { assert(fp); }

  explicit error(const char* filename) {
    int fd = 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
 * This class is a very thin wrapper around std::vector<char>
 * This is basically used to determine the length of the actual
 * data stored inside the dynamically resized vector.
 *
 * This is what is returned as the output to the communicate
 * function, so, users must know about this class.
 *
 * OutBuffer and ErrBuffer are just different typedefs to this class.
 */
class Buffer
{
public:
  Buffer() {}
  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 {
/*!
 * A helper class to Popen class for setting
 * options as provided in the Popen constructor.
 * This design allows us to _not_ have any fixed position
 * to any arguments and specify them in a way similar to what
 * can be done in python.
 */
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);

private:
  Popen* popen_ = nullptr;
};

/*!
 * A helper class to Popen.
 * This takes care of all the fork-exec logic
 * in the execute_child API.
 */
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;

/*!
 * A helper class to Streams.
 * This takes care of management of communicating
 * with the child process with the means of the correct
 * file descriptor.
 */
class Communication
{
public:
  Communication(Streams* stream): stream_(stream)
  {}
  void operator=(const Communication&) = delete;
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;
};



/*!
 * This is a helper class to Popen.
 * It takes care of management of all the file descriptors
 * and file pointers.
 * It dispatches of the communication aspects to the
 * Communication class.
 * Read through the data members to understand about the
 * various file descriptors used.
 */
class Streams
{
public:
  Streams():comm_(this) {}
  void operator=(const Streams&) = delete;

public:
  void setup_comm_channels();

  void cleanup_fds()
  {
    if (write_to_child_ != -1 && read_from_parent_ != -1) {
      close(write_to_child_);
    }
    if (write_to_parent_ != -1 && read_from_child_ != -1) {
      close(read_from_child_);
    }
    if (err_write_ != -1 && err_read_ != -1) {
      close(err_read_);
    }
  }

  void close_parent_fds()
  {
    if (write_to_child_ != -1)  close(write_to_child_);
    if (read_from_child_ != -1) close(read_from_child_);
    if (err_read_ != -1)        close(err_read_);
  }

  void close_child_fds()
  {
    if (write_to_parent_ != -1)  close(write_to_parent_);
    if (read_from_parent_ != -1) close(read_from_parent_);
    if (err_write_ != -1)        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
 * This is the single most important class in the whole library
 * and glues together all the helper classes to provide a common
 * interface to the client.
 *
 * API's provided by the class:
 * Popen({"cmd"}, output{..}, error{..}, ....)
 *    Command provided as a sequence.
 * Popen("cmd arg1", output{..}, error{..}, ....)
 *    Command provided in a single string.
 * wait()             - Wait for the child to exit.
 * retcode()          - The return code of the exited child.
 * send(...)          - Send input to the input channel of the child.
 * communicate(...)   - Get the output/error from the child and close the channels
 *                      from the parent side.
 */
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

  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);

  return 0;
#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;

  for (auto arg : this->vargs_) {
    argument = converter.from_bytes(arg);
    util::quote_argument(argument, command_line, false);
    command_line += L" ";
  }

  // 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) {
    close(err_rd_pipe);
    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
    close(err_rd_pipe);

    detail::Child chld(this, err_wr_pipe);
    chld.execute_child();
  }
  else
  {
    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,};

      int read_bytes = util::read_atmost_n(
                                  fdopen(err_rd_pipe, "r"),
                                  err_buf,
                                  SP_MAX_ERR_BUF_SIZ);
      close(err_rd_pipe);

      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 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)
        close(stream.read_from_parent_);

      if (stream.write_to_parent_ != -1 && stream.write_to_parent_ > 2)
        close(stream.write_to_parent_);

      if (stream.err_write_ != -1 && stream.err_write_ > 2)
        close(stream.err_write_);

      // 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_ = _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_ = _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_ = _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