// Copyright (C) 2015 The Android Open Source Project
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

// makeparallel communicates with the GNU make jobserver
// (http://make.mad-scientist.net/papers/jobserver-implementation/)
// in order claim all available jobs, and then passes the number of jobs
// claimed to a subprocess with -j<jobs>.

#include <errno.h>
#include <fcntl.h>
#include <poll.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/time.h>
#include <sys/types.h>
#include <sys/wait.h>

#include <string>
#include <vector>

#ifdef __linux__
#include <error.h>
#endif

#ifdef __APPLE__
#include <err.h>
#define error(code, eval, fmt, ...) errc(eval, code, fmt, ##__VA_ARGS__)
// Darwin does not interrupt syscalls by default.
#define TEMP_FAILURE_RETRY(exp) (exp)
#endif

// Throw an error if fd is not valid.
static void CheckFd(int fd) {
  int ret = fcntl(fd, F_GETFD);
  if (ret < 0) {
    if (errno == EBADF) {
      error(errno, 0, "no jobserver pipe, prefix recipe command with '+'");
    } else {
      error(errno, errno, "fnctl failed");
    }
  }
}

// Extract --jobserver-fds= argument from MAKEFLAGS environment variable.
static int GetJobserver(int* in_fd, int* out_fd) {
  const char* makeflags_env = getenv("MAKEFLAGS");
  if (makeflags_env == nullptr) {
    return false;
  }

  std::string makeflags = makeflags_env;

  const std::string jobserver_fds_arg = "--jobserver-fds=";
  size_t start = makeflags.find(jobserver_fds_arg);

  if (start == std::string::npos) {
    return false;
  }

  start += jobserver_fds_arg.size();

  std::string::size_type end = makeflags.find(' ', start);

  std::string::size_type len;
  if (end == std::string::npos) {
    len = std::string::npos;
  } else {
    len = end - start;
  }

  std::string jobserver_fds = makeflags.substr(start, len);

  if (sscanf(jobserver_fds.c_str(), "%d,%d", in_fd, out_fd) != 2) {
    return false;
  }

  CheckFd(*in_fd);
  CheckFd(*out_fd);

  return true;
}

// Read a single byte from fd, with timeout in milliseconds.  Returns true if
// a byte was read, false on timeout.  Throws away the read value.
// Non-reentrant, uses timer and signal handler global state, plus static
// variable to communicate with signal handler.
//
// Uses a SIGALRM timer to fire a signal after timeout_ms that will interrupt
// the read syscall if it hasn't yet completed.  If the timer fires before the
// read the read could block forever, so read from a dup'd fd and close it from
// the signal handler, which will cause the read to return EBADF if it occurs
// after the signal.
// The dup/read/close combo is very similar to the system described to avoid
// a deadlock between SIGCHLD and read at
// http://make.mad-scientist.net/papers/jobserver-implementation/
static bool ReadByteTimeout(int fd, int timeout_ms) {
  // global variable to communicate with the signal handler
  static int dup_fd = -1;

  // dup the fd so the signal handler can close it without losing the real one
  dup_fd = dup(fd);
  if (dup_fd < 0) {
    error(errno, errno, "dup failed");
  }

  // set up a signal handler that closes dup_fd on SIGALRM
  struct sigaction action = {};
  action.sa_flags = SA_SIGINFO,
  action.sa_sigaction = [](int, siginfo_t*, void*) {
    close(dup_fd);
  };
  struct sigaction oldaction = {};
  int ret = sigaction(SIGALRM, &action, &oldaction);
  if (ret < 0) {
    error(errno, errno, "sigaction failed");
  }

  // queue a SIGALRM after timeout_ms
  const struct itimerval timeout = {{}, {0, timeout_ms * 1000}};
  ret = setitimer(ITIMER_REAL, &timeout, NULL);
  if (ret < 0) {
    error(errno, errno, "setitimer failed");
  }

  // start the blocking read
  char buf;
  int read_ret = read(dup_fd, &buf, 1);
  int read_errno = errno;

  // cancel the alarm in case it hasn't fired yet
  const struct itimerval cancel = {};
  ret = setitimer(ITIMER_REAL, &cancel, NULL);
  if (ret < 0) {
    error(errno, errno, "reset setitimer failed");
  }

  // remove the signal handler
  ret = sigaction(SIGALRM, &oldaction, NULL);
  if (ret < 0) {
    error(errno, errno, "reset sigaction failed");
  }

  // clean up the dup'd fd in case the signal never fired
  close(dup_fd);
  dup_fd = -1;

  if (read_ret == 0) {
    error(EXIT_FAILURE, 0, "EOF on jobserver pipe");
  } else if (read_ret > 0) {
    return true;
  } else if (read_errno == EINTR || read_errno == EBADF) {
    return false;
  } else {
    error(read_errno, read_errno, "read failed");
  }
  abort();
}

// Measure the size of the jobserver pool by reading from in_fd until it blocks
static int GetJobserverTokens(int in_fd) {
  int tokens = 0;
  pollfd pollfds[] = {{in_fd, POLLIN, 0}};
  int ret;
  while ((ret = TEMP_FAILURE_RETRY(poll(pollfds, 1, 0))) != 0) {
    if (ret < 0) {
      error(errno, errno, "poll failed");
    } else if (pollfds[0].revents != POLLIN) {
      error(EXIT_FAILURE, 0, "unexpected event %d\n", pollfds[0].revents);
    }

    // There is probably a job token in the jobserver pipe.  There is a chance
    // another process reads it first, which would cause a blocking read to
    // block forever (or until another process put a token back in the pipe).
    // The file descriptor can't be set to O_NONBLOCK as that would affect
    // all users of the pipe, including the parent make process.
    // ReadByteTimeout emulates a non-blocking read on a !O_NONBLOCK socket
    // using a SIGALRM that fires after a short timeout.
    bool got_token = ReadByteTimeout(in_fd, 10);
    if (!got_token) {
      // No more tokens
      break;
    } else {
      tokens++;
    }
  }

  // This process implicitly gets a token, so pool size is measured size + 1
  return tokens;
}

// Return tokens to the jobserver pool.
static void PutJobserverTokens(int out_fd, int tokens) {
  // Return all the tokens to the pipe
  char buf = '+';
  for (int i = 0; i < tokens; i++) {
    int ret = TEMP_FAILURE_RETRY(write(out_fd, &buf, 1));
    if (ret < 0) {
      error(errno, errno, "write failed");
    } else if (ret == 0) {
      error(EXIT_FAILURE, 0, "EOF on jobserver pipe");
    }
  }
}

int main(int argc, char* argv[]) {
  int in_fd = -1;
  int out_fd = -1;
  int tokens = 0;

  const char* path = argv[1];
  std::vector<char*> args(&argv[1], &argv[argc]);

  if (GetJobserver(&in_fd, &out_fd)) {
    fcntl(in_fd, F_SETFD, FD_CLOEXEC);
    fcntl(out_fd, F_SETFD, FD_CLOEXEC);

    tokens = GetJobserverTokens(in_fd);
  }

  std::string jarg = "-j" + std::to_string(tokens + 1);
  args.push_back(strdup(jarg.c_str()));
  args.push_back(nullptr);

  pid_t pid = fork();
  if (pid < 0) {
    error(errno, errno, "fork failed");
  } else if (pid == 0) {
    // child
    int ret = execvp(path, args.data());
    if (ret < 0) {
      error(errno, errno, "exec failed");
    }
    abort();
  }

  // parent
  siginfo_t status = {};
  int exit_status = 0;
  int ret = waitid(P_PID, pid, &status, WEXITED);
  if (ret < 0) {
    error(errno, errno, "waitpid failed");
  } else if (status.si_code == CLD_EXITED) {
    exit_status = status.si_status;
  } else {
    exit_status = -(status.si_status);
  }

  if (tokens > 0) {
    PutJobserverTokens(out_fd, tokens);
  }
  exit(exit_status);
}