runtime/bin/process_linux.cc

// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

#include "platform/globals.h"
#if defined(DART_HOST_OS_LINUX) || defined(DART_HOST_OS_ANDROID)

#include "bin/process.h"

#include <errno.h>         // NOLINT
#include <fcntl.h>         // NOLINT
#include <poll.h>          // NOLINT
#include <stdio.h>         // NOLINT
#include <stdlib.h>        // NOLINT
#include <string.h>        // NOLINT
#include <sys/resource.h>  // NOLINT
#include <sys/wait.h>      // NOLINT
#include <unistd.h>        // NOLINT

#include "bin/dartutils.h"
#include "bin/directory.h"
#include "bin/fdutils.h"
#include "bin/file.h"
#include "bin/lockers.h"
#include "bin/reference_counting.h"
#include "bin/thread.h"
#include "platform/syslog.h"

#include "platform/signal_blocker.h"
#include "platform/utils.h"

extern char** environ;

namespace dart {
namespace bin {

int Process::global_exit_code_ = 0;
Mutex* Process::global_exit_code_mutex_ = nullptr;
Process::ExitHook Process::exit_hook_ = nullptr;

// ProcessInfo is used to map a process id to the file descriptor for
// the pipe used to communicate the exit code of the process to Dart.
// ProcessInfo objects are kept in the static singly-linked
// ProcessInfoList.
class ProcessInfo {
 public:
  ProcessInfo(pid_t pid, intptr_t fd) : pid_(pid), fd_(fd) {}
  ~ProcessInfo() {
    int closed = close(fd_);
    if (closed != 0) {
      FATAL("Failed to close process exit code pipe");
    }
  }
  pid_t pid() { return pid_; }
  intptr_t fd() { return fd_; }
  ProcessInfo* next() { return next_; }
  void set_next(ProcessInfo* info) { next_ = info; }

 private:
  pid_t pid_;
  intptr_t fd_;
  ProcessInfo* next_;

  DISALLOW_COPY_AND_ASSIGN(ProcessInfo);
};

// Singly-linked list of ProcessInfo objects for all active processes
// started from Dart.
class ProcessInfoList {
 public:
  static void Init();
  static void Cleanup();

  static void AddProcess(pid_t pid, intptr_t fd) {
    MutexLocker locker(mutex_);
    ProcessInfo* info = new ProcessInfo(pid, fd);
    info->set_next(active_processes_);
    active_processes_ = info;
  }

  static intptr_t LookupProcessExitFd(pid_t pid) {
    MutexLocker locker(mutex_);
    ProcessInfo* current = active_processes_;
    while (current != nullptr) {
      if (current->pid() == pid) {
        return current->fd();
      }
      current = current->next();
    }
    return 0;
  }

  static void RemoveProcess(pid_t pid) {
    MutexLocker locker(mutex_);
    ProcessInfo* prev = nullptr;
    ProcessInfo* current = active_processes_;
    while (current != nullptr) {
      if (current->pid() == pid) {
        if (prev == nullptr) {
          active_processes_ = current->next();
        } else {
          prev->set_next(current->next());
        }
        delete current;
        return;
      }
      prev = current;
      current = current->next();
    }
  }

 private:
  // Linked list of ProcessInfo objects for all active processes
  // started from Dart code.
  static ProcessInfo* active_processes_;
  // Mutex protecting all accesses to the linked list of active
  // processes.
  static Mutex* mutex_;

  DISALLOW_ALLOCATION();
  DISALLOW_IMPLICIT_CONSTRUCTORS(ProcessInfoList);
};

ProcessInfo* ProcessInfoList::active_processes_ = nullptr;
Mutex* ProcessInfoList::mutex_ = nullptr;

// The exit code handler sets up a separate thread which waits for child
// processes to terminate. That separate thread can then get the exit code from
// processes that have exited and communicate it to Dart through the
// event loop.
class ExitCodeHandler {
 public:
  static void Init();
  static void Cleanup();

  static void EnsureStarted() {
    MonitorLocker locker(monitor_);
    if (running_) {
      return;
    }
    // Start thread that handles process exits when wait returns.
    Thread::Start("dart:io Process.start", ExitCodeHandlerEntry, 0);
    running_ = true;
  }

  // Notify the ExitCodeHandler that another process exists.
  static void ProcessStarted() {
    MonitorLocker locker(monitor_);
    ASSERT(running_);
    process_count_++;
    monitor_->Notify();
  }

  static void TerminateExitCodeThread() {
    MonitorLocker locker(monitor_);

    if (!running_) {
      return;
    }

    // Set terminate_done_ to false, so we can use it as a guard for our
    // monitor.
    running_ = false;

    // Wake up the [ExitCodeHandler] thread which is blocked on `wait()` (see
    // [ExitCodeHandlerEntry]).
    if (TEMP_FAILURE_RETRY(fork()) == 0) {
      // Avoid calling any atexit callbacks to prevent deadlocks.
      _exit(0);
    }

    monitor_->Notify();

    while (!terminate_done_) {
      monitor_->Wait(Monitor::kNoTimeout);
    }
  }

 private:
  // Entry point for the separate exit code handler thread started by
  // the ExitCodeHandler.
  static void ExitCodeHandlerEntry(uword param) {
    pid_t pid = 0;
    int status = 0;
    while (true) {
      {
        MonitorLocker locker(monitor_);
        while (running_ && process_count_ == 0) {
          monitor_->Wait(Monitor::kNoTimeout);
        }
        if (!running_) {
          terminate_done_ = true;
          monitor_->Notify();
          return;
        }
      }

      if ((pid = TEMP_FAILURE_RETRY(wait(&status))) > 0) {
        int exit_code = 0;
        int negative = 0;
        if (WIFEXITED(status)) {
          exit_code = WEXITSTATUS(status);
        }
        if (WIFSIGNALED(status)) {
          exit_code = WTERMSIG(status);
          negative = 1;
        }
        intptr_t exit_code_fd = ProcessInfoList::LookupProcessExitFd(pid);
        if (exit_code_fd != 0) {
          int message[2] = {exit_code, negative};
          ssize_t result =
              FDUtils::WriteToBlocking(exit_code_fd, &message, sizeof(message));
          // If the process has been closed, the read end of the exit
          // pipe has been closed. It is therefore not a problem that
          // write fails with a broken pipe error. Other errors should
          // not happen.
          if ((result != -1) && (result != sizeof(message))) {
            FATAL("Failed to write entire process exit message");
          } else if ((result == -1) && (errno != EPIPE)) {
            FATAL("Failed to write exit code: %d", errno);
          }
          ProcessInfoList::RemoveProcess(pid);
          {
            MonitorLocker locker(monitor_);
            process_count_--;
          }
        }
      } else if (pid < 0) {
        FATAL("Wait for process exit failed: %d", errno);
      }
    }
  }

  static bool terminate_done_;
  static int process_count_;
  static bool running_;
  static Monitor* monitor_;

  DISALLOW_ALLOCATION();
  DISALLOW_IMPLICIT_CONSTRUCTORS(ExitCodeHandler);
};

bool ExitCodeHandler::running_ = false;
int ExitCodeHandler::process_count_ = 0;
bool ExitCodeHandler::terminate_done_ = false;
Monitor* ExitCodeHandler::monitor_ = nullptr;

// Tries to find path relative to the current namespace unless it should be
// searched in the PATH environment variable.
// The path that should be passed to exec is returned in realpath.
// Returns true on success, and false if there was an error that should
// be reported to the parent.
static bool PathInNamespace(char* realpath,
                            intptr_t realpath_size,
                            Namespace* namespc,
                            const char* path) {
  // Perform a PATH search if there's no slash in the path.
  if (Namespace::IsDefault(namespc) || strchr(path, '/') == nullptr) {
    // TODO(zra): If there is a non-default namespace, the entries in PATH
    // should be treated as relative to the namespace.
    strncpy(realpath, path, realpath_size);
    realpath[realpath_size - 1] = '\0';
    return true;
  }
  NamespaceScope ns(namespc, path);
  const int fd =
      TEMP_FAILURE_RETRY(openat64(ns.fd(), ns.path(), O_RDONLY | O_CLOEXEC));
  if (fd == -1) {
    return false;
  }
  char procpath[PATH_MAX];
  snprintf(procpath, PATH_MAX, "/proc/self/fd/%d", fd);
  const intptr_t length =
      TEMP_FAILURE_RETRY(readlink(procpath, realpath, realpath_size));
  if (length < 0) {
    FDUtils::SaveErrorAndClose(fd);
    return false;
  }
  realpath[length] = '\0';
  FDUtils::SaveErrorAndClose(fd);
  return true;
}

// Linux 5.9, glibc 2.34.
extern "C" int close_range(unsigned int first, unsigned int last, int flags)
    __attribute__((weak));

void CloseAllButStdioAndExecControl(int exec_control_fd) {
#if defined(DART_HOST_OS_ANDROID)
  if (__builtin_available(android 34, *)) {
#elif defined(DART_HOST_OS_LINUX)
  if (&close_range != nullptr) {
#else
#error "DART_HOST_OS_LINUX or DART_HOST_OS_ANDROID must be defined"
#endif
    close_range(3, exec_control_fd - 1, 0);
    close_range(exec_control_fd + 1, ~0u, 0);
  } else {
    int max_fds = sysconf(_SC_OPEN_MAX);
    if (max_fds == -1) {
      max_fds = _POSIX_OPEN_MAX;
    }
    for (int fd = 3; fd < max_fds; fd++) {
      if (fd != exec_control_fd) {
        close(fd);
      }
    }
  }
}

class ProcessStarter {
 public:
  ProcessStarter(Namespace* namespc,
                 const char* path,
                 const char* arguments[],
                 intptr_t arguments_length,
                 const char* working_directory,
                 char* environment[],
                 intptr_t environment_length,
                 ProcessStartMode mode,
                 intptr_t* in,
                 intptr_t* out,
                 intptr_t* err,
                 intptr_t* id,
                 intptr_t* exit_event,
                 char** os_error_message)
      : namespc_(namespc),
        path_(path),
        working_directory_(working_directory),
        mode_(mode),
        in_(in),
        out_(out),
        err_(err),
        id_(id),
        exit_event_(exit_event),
        os_error_message_(os_error_message) {
    stdout_pipe_[0] = -1;
    stdout_pipe_[1] = -1;
    stderr_pipe_[0] = -1;
    stderr_pipe_[1] = -1;
    stdin_pipe_[0] = -1;
    stdin_pipe_[1] = -1;
    exec_control_[0] = -1;
    exec_control_[1] = -1;
    exit_pipe_[0] = -1;
    exit_pipe_[1] = -1;

    program_arguments_ = reinterpret_cast<const char**>(Dart_ScopeAllocate(
        (arguments_length + 2) * sizeof(*program_arguments_)));
    program_arguments_[0] = const_cast<char*>(path_);
    for (int i = 0; i < arguments_length; i++) {
      program_arguments_[i + 1] = arguments[i];
    }
    program_arguments_[arguments_length + 1] = nullptr;

    program_environment_ = nullptr;
    if (environment != nullptr) {
      program_environment_ = reinterpret_cast<char**>(Dart_ScopeAllocate(
          (environment_length + 1) * sizeof(*program_environment_)));
      for (int i = 0; i < environment_length; i++) {
        program_environment_[i] = environment[i];
      }
      program_environment_[environment_length] = nullptr;
    }
  }

  int Start() {
    // Create pipes required.
    int err = CreatePipes();
    if (err != 0) {
      return err;
    }

    if (Process::ModeIsAttached(mode_)) {
      ExitCodeHandler::EnsureStarted();
    }

    // Fork to create the new process.
    pid_t pid = TEMP_FAILURE_RETRY(fork());
    if (pid < 0) {
      // Failed to fork.
      return CleanupAndReturnError();
    } else if (pid == 0) {
      // This runs in the new process.
      NewProcess();
    }

    // This runs in the original process.

    // If the child process is not started in detached mode, be sure to
    // listen for exit-codes, now that we have a non detached child process
    // and also Register this child process.
    if (Process::ModeIsAttached(mode_)) {
      RegisterProcess(pid);
      ExitCodeHandler::ProcessStarted();

      // Notify child process to start. This is done to delay the call to exec
      // until the process is registered above, and we are ready to receive the
      // exit code.
      char msg = '1';
      int bytes_written =
          FDUtils::WriteToBlocking(stdin_pipe_[1], &msg, sizeof(msg));
      if (bytes_written != sizeof(msg)) {
        return CleanupAndReturnError();
      }
    }

    // Read the result of executing the child process.
    CloseWriteEndOfPipe(exec_control_);
    if (Process::ModeIsAttached(mode_)) {
      err = ReadExecResult();
    } else {
      err = ReadDetachedExecResult(&pid);
    }
    CloseReadEndOfPipe(exec_control_);

    // Return error code if any failures.
    if (err != 0) {
      if (Process::ModeIsAttached(mode_)) {
        // Since exec() failed, we're not interested in the exit code.
        // We close the reading side of the exit code pipe here.
        // GetProcessExitCodes will get a broken pipe error when it
        // tries to write to the writing side of the pipe and it will
        // ignore the error.
        close(*exit_event_);
        *exit_event_ = -1;
      }
      CloseAllPipes();
      return err;
    }

    if (Process::ModeHasStdio(mode_)) {
      // Connect stdio, stdout and stderr.
      FDUtils::SetNonBlocking(stdout_pipe_[0]);
      *in_ = stdout_pipe_[0];
      stdout_pipe_[0] = -1;
      CloseWriteEndOfPipe(stdout_pipe_);
      FDUtils::SetNonBlocking(stdin_pipe_[1]);
      *out_ = stdin_pipe_[1];
      stdin_pipe_[1] = -1;
      CloseReadEndOfPipe(stdin_pipe_);
      FDUtils::SetNonBlocking(stderr_pipe_[0]);
      *err_ = stderr_pipe_[0];
      stderr_pipe_[0] = -1;
      CloseWriteEndOfPipe(stderr_pipe_);
    } else {
      // Close all fds.
      ClosePipe(stdin_pipe_);
      ASSERT(stdout_pipe_[0] == -1);
      ASSERT(stdout_pipe_[1] == -1);
      ASSERT(stderr_pipe_[0] == -1);
      ASSERT(stderr_pipe_[1] == -1);
    }
    ASSERT(exec_control_[0] == -1);
    ASSERT(exec_control_[1] == -1);
    ASSERT(exit_pipe_[0] == -1);
    ASSERT(exit_pipe_[1] == -1);

    *id_ = pid;
    return 0;
  }

 private:
  static constexpr int kErrorBufferSize = 1024;

  int CreatePipes() {
    int result;
    result = TEMP_FAILURE_RETRY(pipe2(exec_control_, O_CLOEXEC));
    if (result < 0) {
      return CleanupAndReturnError();
    }

    if (Process::ModeHasStdio(mode_)) {
      result = TEMP_FAILURE_RETRY(pipe2(stdout_pipe_, O_CLOEXEC));
      if (result < 0) {
        return CleanupAndReturnError();
      }

      result = TEMP_FAILURE_RETRY(pipe2(stderr_pipe_, O_CLOEXEC));
      if (result < 0) {
        return CleanupAndReturnError();
      }
    }

    // The stdin_pipe_ pipe is used for connecting stdin when ModeHasStdio is
    // true (kNormal, kDetachedWithStdio), and also for the 1-byte
    // synchronization message when ModeIsAttached is true (kNormal,
    // kInheritStdio). Only in kDetached mode is stdin_pipe_ not needed at all.
    if (Process::ModeHasStdio(mode_) || Process::ModeIsAttached(mode_)) {
      result = TEMP_FAILURE_RETRY(pipe2(stdin_pipe_, O_CLOEXEC));
      if (result < 0) {
        return CleanupAndReturnError();
      }
    }

    if (Process::ModeIsAttached(mode_)) {
      result = TEMP_FAILURE_RETRY(pipe2(exit_pipe_, O_CLOEXEC));
      if (result < 0) {
        return CleanupAndReturnError();
      }
    }

    return 0;
  }

  void NewProcess() {
    CloseUnusedPipeEndsInChild();
    if (Process::ModeIsAttached(mode_)) {
      // Wait for parent process before setting up the child process.
      char msg;
      int bytes_read =
          FDUtils::ReadFromBlocking(stdin_pipe_[0], &msg, sizeof(msg));
      if (bytes_read != sizeof(msg)) {
        perror("Failed receiving notification message");
        _exit(1);
      }
      if (!Process::ModeHasStdio(mode_)) {
        CloseReadEndOfPipe(stdin_pipe_);
      }
      ExecProcess();
    } else {
      ExecDetachedProcess();
    }
  }

  void CloseUnusedPipeEndsInChild() {
    ClosePipe(exit_pipe_);
    CloseReadEndOfPipe(exec_control_);
    CloseReadEndOfPipe(stderr_pipe_);
    CloseReadEndOfPipe(stdout_pipe_);
    CloseWriteEndOfPipe(stdin_pipe_);
  }

  // Tries to find path_ relative to the current namespace unless it should be
  // searched in the PATH.
  // The path that should be passed to exec is returned in realpath.
  // Returns true on success, and false if there was an error that should
  // be reported to the parent.
  bool FindPathInNamespace(char* realpath, intptr_t realpath_size) {
    return PathInNamespace(realpath, realpath_size, namespc_, path_);
  }

  void ExecProcess() {
    if (mode_ == kNormal) {
      if (TEMP_FAILURE_RETRY(dup2(stdin_pipe_[0], STDIN_FILENO)) == -1) {
        ReportChildError();
      }
      ClosePipe(stdin_pipe_);

      if (TEMP_FAILURE_RETRY(dup2(stdout_pipe_[1], STDOUT_FILENO)) == -1) {
        ReportChildError();
      }
      ClosePipe(stdout_pipe_);

      if (TEMP_FAILURE_RETRY(dup2(stderr_pipe_[1], STDERR_FILENO)) == -1) {
        ReportChildError();
      }
      ClosePipe(stderr_pipe_);
    } else {
      ASSERT(mode_ == kInheritStdio);
    }

    if (working_directory_ != nullptr &&
        !Directory::SetCurrent(namespc_, working_directory_)) {
      ReportChildError();
    }

    if (program_environment_ != nullptr) {
      environ = program_environment_;
    }

    char realpath[PATH_MAX];
    if (!FindPathInNamespace(realpath, PATH_MAX)) {
      ReportChildError();
    }
    // TODO(dart:io) Test for the existence of execveat, and use it instead.
    execvp(realpath, const_cast<char* const*>(program_arguments_));
    ReportChildError();
  }

  void ExecDetachedProcess() {
    if (mode_ == kDetached) {
      ASSERT(stdin_pipe_[0] == -1);
      ASSERT(stdin_pipe_[1] == -1);
      ASSERT(stderr_pipe_[0] == -1);
      ASSERT(stderr_pipe_[1] == -1);
      ASSERT(stdout_pipe_[0] == -1);
      ASSERT(stdout_pipe_[1] == -1);
    } else {
      // Don't close any fds if keeping stdio open to the detached process.
      ASSERT(mode_ == kDetachedWithStdio);
    }
    // Fork once more to start a new session.
    pid_t pid = TEMP_FAILURE_RETRY(fork());
    if (pid < 0) {
      ReportChildError();
    } else if (pid == 0) {
      // Start a new session.
      if (TEMP_FAILURE_RETRY(setsid()) == -1) {
        ReportChildError();
      } else {
        // Do a final fork to not be the session leader.
        pid = TEMP_FAILURE_RETRY(fork());
        if (pid < 0) {
          ReportChildError();
        } else if (pid == 0) {
          if (mode_ == kDetached) {
            SetupDetached();
          } else {
            SetupDetachedWithStdio();
          }

          if ((working_directory_ != nullptr) &&
              !Directory::SetCurrent(namespc_, working_directory_)) {
            ReportChildError();
          }
          if (program_environment_ != nullptr) {
            environ = program_environment_;
          }

          char realpath[PATH_MAX];
          if (!FindPathInNamespace(realpath, PATH_MAX)) {
            ReportChildError();
          }

          // Report the final PID and do the exec.
          ReportPid(getpid());  // getpid cannot fail.
          // TODO(dart:io) Test for the existence of execveat, and use it
          // instead.
          execvp(realpath, const_cast<char* const*>(program_arguments_));
          ReportChildError();
        } else {
          // Exit the intermediate process. Avoid calling any atexit callbacks
          // to avoid potential issues (e.g. deadlocks).
          _exit(0);
        }
      }
    } else {
      // Exit the intermediate process. Avoid calling any atexit callbacks
      // to avoid potential issues (e.g. deadlocks).
      _exit(0);
    }
  }

  void RegisterProcess(pid_t pid) {
    ASSERT(exit_pipe_[0] != -1);
    ASSERT(exit_pipe_[1] != -1);
    ProcessInfoList::AddProcess(pid, exit_pipe_[1]);
    exit_pipe_[1] = -1;
    *exit_event_ = exit_pipe_[0];
    FDUtils::SetNonBlocking(exit_pipe_[0]);
    exit_pipe_[0] = -1;
  }

  int ReadExecResult() {
    int child_errno;
    int bytes_read = -1;
    // Read exec result from child. If no data is returned the exec was
    // successful and the exec call closed the pipe. Otherwise the errno
    // is written to the pipe.
    bytes_read = FDUtils::ReadFromBlocking(exec_control_[0], &child_errno,
                                           sizeof(child_errno));
    if (bytes_read == sizeof(child_errno)) {
      ReadChildError();
      return child_errno;
    } else if (bytes_read == -1) {
      return errno;
    }
    return 0;
  }

  int ReadDetachedExecResult(pid_t* pid) {
    int child_errno;
    int bytes_read = -1;
    // Read exec result from child. If only pid data is returned the exec was
    // successful and the exec call closed the pipe. Otherwise the errno
    // is written to the pipe as well.
    int result[2];
    bytes_read =
        FDUtils::ReadFromBlocking(exec_control_[0], result, sizeof(result));
    if (bytes_read == sizeof(int)) {
      *pid = result[0];
    } else if (bytes_read == 2 * sizeof(int)) {
      *pid = result[0];
      child_errno = result[1];
      ReadChildError();
      return child_errno;
    } else if (bytes_read == -1) {
      return errno;
    }
    return 0;
  }

  void SetupDetached() {
    ASSERT(mode_ == kDetached);

    // Connect stdin, stdout and stderr to /dev/null.
    int fd = TEMP_FAILURE_RETRY(open("/dev/null", O_RDWR));
    if (TEMP_FAILURE_RETRY(dup2(fd, STDIN_FILENO)) != STDIN_FILENO) {
      ReportChildError();
    }
    if (TEMP_FAILURE_RETRY(dup2(fd, STDOUT_FILENO)) != STDOUT_FILENO) {
      ReportChildError();
    }
    if (TEMP_FAILURE_RETRY(dup2(fd, STDERR_FILENO)) != STDERR_FILENO) {
      ReportChildError();
    }

    CloseAllButStdioAndExecControl(exec_control_[1]);
  }

  void SetupDetachedWithStdio() {
    if (TEMP_FAILURE_RETRY(dup2(stdin_pipe_[0], STDIN_FILENO)) == -1) {
      ReportChildError();
    }
    if (TEMP_FAILURE_RETRY(dup2(stdout_pipe_[1], STDOUT_FILENO)) == -1) {
      ReportChildError();
    }
    if (TEMP_FAILURE_RETRY(dup2(stderr_pipe_[1], STDERR_FILENO)) == -1) {
      ReportChildError();
    }

    CloseAllButStdioAndExecControl(exec_control_[1]);
  }

  int CleanupAndReturnError() {
    int actual_errno = errno;
    // If CleanupAndReturnError is called without an actual errno make
    // sure to return an error anyway.
    if (actual_errno == 0) {
      actual_errno = EPERM;
    }
    SetChildOsErrorMessage();
    CloseAllPipes();
    return actual_errno;
  }

  void SetChildOsErrorMessage() {
    char* error_message = DartUtils::ScopedCString(kErrorBufferSize);
    *os_error_message_ =
        Utils::StrError(errno, error_message, kErrorBufferSize);
  }

  void ReportChildError() {
    // In the case of failure in the child process write the errno and
    // the OS error message to the exec control pipe and exit.
    int child_errno = errno;
    char error_buf[kErrorBufferSize];
    char* os_error_message =
        Utils::StrError(errno, error_buf, kErrorBufferSize);
    int bytes_written = FDUtils::WriteToBlocking(exec_control_[1], &child_errno,
                                                 sizeof(child_errno));
    if (bytes_written == sizeof(child_errno)) {
      FDUtils::WriteToBlocking(exec_control_[1], os_error_message,
                               strlen(os_error_message) + 1);
    }
    close(exec_control_[1]);

    // We avoid running through registered atexit() handlers because that is
    // unnecessary work. It can also cause deadlocks on exit in the forked
    // process.
    _exit(1);
  }

  void ReportPid(int pid) {
    // In the case of starting a detached process the actual pid of that process
    // is communicated using the exec control pipe.
    int bytes_written =
        FDUtils::WriteToBlocking(exec_control_[1], &pid, sizeof(pid));
    ASSERT(bytes_written == sizeof(int));
    USE(bytes_written);
  }

  void ReadChildError() {
    char* message = DartUtils::ScopedCString(kErrorBufferSize);
    if (message != nullptr) {
      FDUtils::ReadFromBlocking(exec_control_[0], message, kErrorBufferSize);
      message[kErrorBufferSize - 1] = '\0';
      *os_error_message_ = message;
    } else {
      // Could not get error message. It will be nullptr.
      ASSERT(*os_error_message_ == nullptr);
    }
  }

  void CloseReadEndOfPipe(int* fds) {
    if (fds[0] != -1) {
      close(fds[0]);
      fds[0] = -1;
    }
  }

  void CloseWriteEndOfPipe(int* fds) {
    if (fds[1] != -1) {
      close(fds[1]);
      fds[1] = -1;
    }
  }

  void ClosePipe(int* fds) {
    CloseReadEndOfPipe(fds);
    CloseWriteEndOfPipe(fds);
  }

  void CloseAllPipes() {
    ClosePipe(exec_control_);
    ClosePipe(stdout_pipe_);
    ClosePipe(stderr_pipe_);
    ClosePipe(stdin_pipe_);
    ClosePipe(exit_pipe_);
  }

  int stdout_pipe_[2];   // Pipe for stdout to child process.
  int stderr_pipe_[2];   // Pipe for stderr to child process.
  int stdin_pipe_[2];    // Pipe for stdin to child process.
  int exec_control_[2];  // Pipe to get the result from exec.
  int exit_pipe_[2];     // Pipe for exit event.

  const char** program_arguments_;
  char** program_environment_;

  Namespace* namespc_;
  const char* path_;
  const char* working_directory_;
  ProcessStartMode mode_;
  intptr_t* in_;
  intptr_t* out_;
  intptr_t* err_;
  intptr_t* id_;
  intptr_t* exit_event_;
  char** os_error_message_;

  DISALLOW_ALLOCATION();
  DISALLOW_IMPLICIT_CONSTRUCTORS(ProcessStarter);
};

int Process::Start(Namespace* namespc,
                   const char* path,
                   const char* arguments[],
                   intptr_t arguments_length,
                   const char* working_directory,
                   char* environment[],
                   intptr_t environment_length,
                   ProcessStartMode mode,
                   intptr_t* in,
                   intptr_t* out,
                   intptr_t* err,
                   intptr_t* id,
                   intptr_t* exit_event,
                   char** os_error_message) {
  ProcessStarter starter(namespc, path, arguments, arguments_length,
                         working_directory, environment, environment_length,
                         mode, in, out, err, id, exit_event, os_error_message);
  return starter.Start();
}

static bool CloseProcessBuffers(struct pollfd* fds, int alive) {
  int e = errno;
  for (int i = 0; i < alive; i++) {
    close(fds[i].fd);
  }
  errno = e;
  return false;
}

bool Process::Wait(intptr_t pid,
                   intptr_t in,
                   intptr_t out,
                   intptr_t err,
                   intptr_t exit_event,
                   ProcessResult* result) {
  // Close input to the process right away.
  close(in);

  // There is no return from this function using Dart_PropagateError
  // as memory used by the buffer lists is freed through their
  // destructors.
  BufferList out_data;
  BufferList err_data;
  union {
    uint8_t bytes[8];
    int32_t ints[2];
  } exit_code_data = {};

  struct pollfd fds[3];
  fds[0].fd = out;
  fds[1].fd = err;
  fds[2].fd = exit_event;

  for (int i = 0; i < 3; i++) {
    fds[i].events = POLLIN;
  }

  int alive = 3;
  while (alive > 0) {
    // Blocking call waiting for events from the child process.
    if (TEMP_FAILURE_RETRY(poll(fds, alive, -1)) <= 0) {
      return CloseProcessBuffers(fds, alive);
    }

    // Process incoming data.
    for (int i = 0; i < alive; i++) {
      if ((fds[i].revents & (POLLNVAL | POLLERR)) != 0) {
        return CloseProcessBuffers(fds, alive);
      }
      if ((fds[i].revents & (POLLIN | POLLHUP)) != 0) {
        intptr_t avail = FDUtils::AvailableBytes(fds[i].fd);
        if (fds[i].fd == out) {
          if (!out_data.Read(out, avail)) {
            return CloseProcessBuffers(fds, alive);
          }
        } else if (fds[i].fd == err) {
          if (!err_data.Read(err, avail)) {
            return CloseProcessBuffers(fds, alive);
          }
        } else if (fds[i].fd == exit_event) {
          if (avail == 8) {
            intptr_t b =
                TEMP_FAILURE_RETRY(read(exit_event, exit_code_data.bytes, 8));
            if (b != 8) {
              return CloseProcessBuffers(fds, alive);
            }
          }
        } else {
          UNREACHABLE();
        }
      }
      if ((fds[i].revents & POLLHUP) != 0) {
        // Remove the pollfd from the list of pollfds.
        close(fds[i].fd);
        alive--;
        if (i < alive) {
          fds[i] = fds[alive];
        }
        // Process the same index again.
        i--;
        continue;
      }
    }
  }

  // All handles closed and all data read.
  result->set_stdout_data(out_data.GetData());
  result->set_stderr_data(err_data.GetData());
  DEBUG_ASSERT(out_data.IsEmpty());
  DEBUG_ASSERT(err_data.IsEmpty());

  // Calculate the exit code.
  intptr_t exit_code = exit_code_data.ints[0];
  intptr_t negative = exit_code_data.ints[1];
  if (negative != 0) {
    exit_code = -exit_code;
  }
  result->set_exit_code(exit_code);

  return true;
}

int Process::Exec(Namespace* namespc,
                  const char* path,
                  const char* arguments[],
                  intptr_t arguments_length,
                  const char* working_directory,
                  char* errmsg,
                  intptr_t errmsg_len) {
  if (working_directory != nullptr &&
      !Directory::SetCurrent(namespc, working_directory)) {
    Utils::StrError(errno, errmsg, errmsg_len);
    return -1;
  }

  char realpath[PATH_MAX];
  if (!PathInNamespace(realpath, PATH_MAX, namespc, path)) {
    Utils::StrError(errno, errmsg, errmsg_len);
    return -1;
  }

  // TODO(dart:io) Test for the existence of execveat, and use it instead.
  execvp(const_cast<const char*>(realpath),
         const_cast<char* const*>(arguments));
  Utils::StrError(errno, errmsg, errmsg_len);
  return -1;
}

bool Process::Kill(intptr_t id, int signal) {
  return (TEMP_FAILURE_RETRY(kill(id, signal)) != -1);
}

void Process::TerminateExitCodeHandler() {
  ExitCodeHandler::TerminateExitCodeThread();
}

intptr_t Process::CurrentProcessId() {
  return static_cast<intptr_t>(getpid());
}

static void SaveErrorAndClose(FILE* file) {
  int actual_errno = errno;
  fclose(file);
  errno = actual_errno;
}

int64_t Process::CurrentRSS() {
  // The second value in /proc/self/statm is the current RSS in pages.
  // It is not possible to use getrusage() because the interested fields are not
  // implemented by the linux kernel.
  FILE* statm = fopen("/proc/self/statm", "r");
  if (statm == nullptr) {
    return -1;
  }
  int64_t current_rss_pages = 0;
  int matches = fscanf(statm, "%*s%" Pd64 "", &current_rss_pages);
  if (matches != 1) {
    SaveErrorAndClose(statm);
    return -1;
  }
  fclose(statm);
  return current_rss_pages * getpagesize();
}

int64_t Process::MaxRSS() {
  struct rusage usage;
  usage.ru_maxrss = 0;
  int r = getrusage(RUSAGE_SELF, &usage);
  if (r < 0) {
    return -1;
  }
  return usage.ru_maxrss * KB;
}

static Mutex* signal_mutex = nullptr;
static SignalInfo* signal_handlers = nullptr;
static constexpr int kSignalsCount = 7;
static const int kSignals[kSignalsCount] = {
    SIGHUP, SIGINT, SIGTERM, SIGUSR1, SIGUSR2, SIGWINCH,
    SIGQUIT  // Allow VMService to listen on SIGQUIT.
};

SignalInfo::~SignalInfo() {
  close(fd_);
}

static void SignalHandler(int signal) {
  MutexLocker lock(signal_mutex);
  const SignalInfo* handler = signal_handlers;
  while (handler != nullptr) {
    if (handler->signal() == signal) {
      int value = 0;
      VOID_TEMP_FAILURE_RETRY(write(handler->fd(), &value, 1));
    }
    handler = handler->next();
  }
}

intptr_t Process::SetSignalHandler(intptr_t signal) {
  bool found = false;
  for (int i = 0; i < kSignalsCount; i++) {
    if (kSignals[i] == signal) {
      found = true;
      break;
    }
  }
  if (!found) {
    return -1;
  }
  int fds[2];
  if (NO_RETRY_EXPECTED(pipe2(fds, O_CLOEXEC)) != 0) {
    return -1;
  }
  ThreadSignalBlocker blocker(kSignalsCount, kSignals);
  MutexLocker lock(signal_mutex);
  SignalInfo* handler = signal_handlers;
  bool listen = true;
  sa_handler_t oldact_handler = nullptr;
  while (handler != nullptr) {
    if (handler->signal() == signal) {
      oldact_handler = handler->oldact();
      listen = false;
      break;
    }
    handler = handler->next();
  }
  if (listen) {
    struct sigaction act = {};
    act.sa_handler = SignalHandler;
    sigemptyset(&act.sa_mask);
    for (int i = 0; i < kSignalsCount; i++) {
      sigaddset(&act.sa_mask, kSignals[i]);
    }
    struct sigaction oldact = {};
    int status = NO_RETRY_EXPECTED(sigaction(signal, &act, &oldact));
    if (status < 0) {
      int err = errno;
      close(fds[0]);
      close(fds[1]);
      errno = err;
      return -1;
    }
    oldact_handler = oldact.sa_handler;
  }
  signal_handlers =
      new SignalInfo(fds[1], signal, oldact_handler, signal_handlers);
  return fds[0];
}

void Process::ClearSignalHandler(intptr_t signal, Dart_Port port) {
  ThreadSignalBlocker blocker(kSignalsCount, kSignals);
  MutexLocker lock(signal_mutex);
  SignalInfo* handler = signal_handlers;
  sa_handler_t oldact_handler = SIG_DFL;
  bool any_removed = false;
  bool any_remaining = false;
  while (handler != nullptr) {
    bool remove = false;
    if (handler->signal() == signal) {
      if ((port == ILLEGAL_PORT) || (handler->port() == port)) {
        if (signal_handlers == handler) {
          signal_handlers = handler->next();
        }
        handler->Unlink();
        remove = true;
        oldact_handler = handler->oldact();
        any_removed = true;
      } else {
        any_remaining = true;
      }
    }
    SignalInfo* next = handler->next();
    if (remove) {
      delete handler;
    }
    handler = next;
  }
  if (any_removed && !any_remaining) {
    struct sigaction act = {};
    act.sa_handler = oldact_handler;
    VOID_NO_RETRY_EXPECTED(sigaction(signal, &act, nullptr));
  }
}

void Process::ClearSignalHandlerByFd(intptr_t fd, Dart_Port port) {
  ThreadSignalBlocker blocker(kSignalsCount, kSignals);
  MutexLocker lock(signal_mutex);
  SignalInfo* handler = signal_handlers;
  sa_handler_t oldact_handler = SIG_DFL;
  bool any_remaining = false;
  intptr_t signal = -1;
  while (handler != nullptr) {
    bool remove = false;
    if (handler->fd() == fd) {
      if ((port == ILLEGAL_PORT) || (handler->port() == port)) {
        if (signal_handlers == handler) {
          signal_handlers = handler->next();
        }
        handler->Unlink();
        remove = true;
        signal = handler->signal();
      } else {
        any_remaining = true;
      }
    }
    SignalInfo* next = handler->next();
    if (remove) {
      delete handler;
    }
    handler = next;
  }
  if ((signal != -1) && !any_remaining) {
    struct sigaction act = {};
    act.sa_handler = oldact_handler;
    VOID_NO_RETRY_EXPECTED(sigaction(signal, &act, nullptr));
  }
}

void ProcessInfoList::Init() {
  active_processes_ = nullptr;
  ASSERT(ProcessInfoList::mutex_ == nullptr);
  ProcessInfoList::mutex_ = new Mutex();
}

void ProcessInfoList::Cleanup() {
  ASSERT(ProcessInfoList::mutex_ != nullptr);
  delete ProcessInfoList::mutex_;
  ProcessInfoList::mutex_ = nullptr;
}

void ExitCodeHandler::Init() {
  running_ = false;
  process_count_ = 0;
  terminate_done_ = false;
  ASSERT(ExitCodeHandler::monitor_ == nullptr);
  ExitCodeHandler::monitor_ = new Monitor();
}

void ExitCodeHandler::Cleanup() {
  ASSERT(ExitCodeHandler::monitor_ != nullptr);
  delete ExitCodeHandler::monitor_;
  ExitCodeHandler::monitor_ = nullptr;
}

void Process::Init() {
  ExitCodeHandler::Init();
  ProcessInfoList::Init();

  ASSERT(signal_mutex == nullptr);
  signal_mutex = new Mutex();
  signal_handlers = nullptr;

  ASSERT(Process::global_exit_code_mutex_ == nullptr);
  Process::global_exit_code_mutex_ = new Mutex();
}

void Process::Cleanup() {
  ClearAllSignalHandlers();

  ASSERT(signal_mutex != nullptr);
  delete signal_mutex;
  signal_mutex = nullptr;

  ASSERT(Process::global_exit_code_mutex_ != nullptr);
  delete Process::global_exit_code_mutex_;
  Process::global_exit_code_mutex_ = nullptr;

  ProcessInfoList::Cleanup();
  ExitCodeHandler::Cleanup();
}

}  // namespace bin
}  // namespace dart

#endif  // defined(DART_HOST_OS_LINUX) || defined(DART_HOST_OS_ANDROID)