lush/lcmd.c

/*
** $Id: lcmd.c $
** Command execution for backtick syntax
** See Copyright Notice in lua.h
*/

#define lcmd_c
#define LUA_LIB

#include "lprefix.h"

#include <errno.h>
#include <signal.h>
#include <stdlib.h>
#include <string.h>
#include <sys/select.h>
#include <sys/wait.h>
#include <unistd.h>

#include "lua.h"
#include "lauxlib.h"
#include "lcmd.h"


/* ===== argv parser ===== */

typedef struct {
  char **argv;
  char *buf;
  int argc;
} ParsedArgs;


static void free_argv (ParsedArgs *pa) {
  free(pa->buf);
  free(pa->argv);
}


/*
** Tokenize command string into argv array.
** Handles: whitespace splitting, single quotes (literal),
** double quotes (with \", \\, \n, \t, \$ escapes), backslash escaping.
** Returns 0 on success, -1 on error (unterminated quote).
*/
static int parse_argv (const char *cmd, ParsedArgs *result) {
  size_t len = strlen(cmd);
  char *buf = (char *)malloc(len + 1);
  int capacity = 8;
  char **argv = (char **)malloc((size_t)capacity * sizeof(char *));
  int argc = 0;
  size_t bp = 0;  /* position in buf */
  const char *p = cmd;

  if (buf == NULL || argv == NULL) {
    free(buf);
    free(argv);
    return -1;
  }

  while (*p != '\0') {
    /* skip whitespace */
    while (*p == ' ' || *p == '\t' || *p == '\n' || *p == '\r')
      p++;
    if (*p == '\0')
      break;

    /* start of a token */
    if (argc >= capacity - 1) {
      capacity *= 2;
      argv = (char **)realloc(argv, (size_t)capacity * sizeof(char *));
      if (argv == NULL) { free(buf); return -1; }
    }
    argv[argc++] = buf + bp;

    while (*p != '\0') {
      if (*p == ' ' || *p == '\t' || *p == '\n' || *p == '\r')
        break;

      if (*p == '\'') {
        /* single quote: literal until closing quote */
        p++;
        while (*p != '\0' && *p != '\'')
          buf[bp++] = *p++;
        if (*p == '\'') p++;
        else { free(buf); free(argv); return -1; }
      }
      else if (*p == '"') {
        /* double quote: with escape sequences */
        p++;
        while (*p != '\0' && *p != '"') {
          if (*p == '\\' && p[1] != '\0') {
            p++;
            switch (*p) {
              case '"':  buf[bp++] = '"';  break;
              case '\\': buf[bp++] = '\\'; break;
              case 'n':  buf[bp++] = '\n'; break;
              case 't':  buf[bp++] = '\t'; break;
              case '$':  buf[bp++] = '$';  break;
              default:   buf[bp++] = '\\'; buf[bp++] = *p; break;
            }
            p++;
          }
          else {
            buf[bp++] = *p++;
          }
        }
        if (*p == '"') p++;
        else { free(buf); free(argv); return -1; }
      }
      else if (*p == '\\' && p[1] != '\0') {
        /* backslash escape outside quotes */
        p++;
        buf[bp++] = *p++;
      }
      else {
        buf[bp++] = *p++;
      }
    }

    buf[bp++] = '\0';
  }

  argv[argc] = NULL;
  result->argv = argv;
  result->buf = buf;
  result->argc = argc;
  return 0;
}


/* ===== pipeline splitter ===== */

#define MAX_PIPELINE_STAGES 64

/*
** Split a command string on unquoted '|' into pipeline stages.
** Reuses the same quote-tracking logic as parse_argv().
** Returns 0 on success, -1 on error (empty stage, leading/trailing pipe).
** Writes stage pointers into stages[] and count into *nstages.
** Caller must free stages[0] (the backing buffer).
*/
static int split_pipeline (const char *cmd, char **stages, int *nstages) {
  size_t len = strlen(cmd);
  char *buf = (char *)malloc(len + 1);
  int count = 0;
  const char *p = cmd;
  size_t bp = 0;

  if (buf == NULL) return -1;

  stages[count++] = buf;

  while (*p != '\0') {
    if (*p == '\'' ) {
      /* single-quoted string: copy through including quotes */
      buf[bp++] = *p++;
      while (*p != '\0' && *p != '\'')
        buf[bp++] = *p++;
      if (*p == '\'') buf[bp++] = *p++;
    }
    else if (*p == '"') {
      /* double-quoted string: copy through, respecting backslash */
      buf[bp++] = *p++;
      while (*p != '\0' && *p != '"') {
        if (*p == '\\' && p[1] != '\0') {
          buf[bp++] = *p++;
          buf[bp++] = *p++;
        } else {
          buf[bp++] = *p++;
        }
      }
      if (*p == '"') buf[bp++] = *p++;
    }
    else if (*p == '\\' && p[1] != '\0') {
      /* backslash escape: copy both chars */
      buf[bp++] = *p++;
      buf[bp++] = *p++;
    }
    else if (*p == '|') {
      /* unquoted pipe — split here */
      buf[bp++] = '\0';
      if (count >= MAX_PIPELINE_STAGES) { free(buf); return -1; }
      stages[count++] = buf + bp;
      p++;
    }
    else {
      buf[bp++] = *p++;
    }
  }
  buf[bp] = '\0';

  /* validate: no empty stages (only for multi-stage pipelines) */
  if (count > 1) {
    for (int i = 0; i < count; i++) {
      const char *s = stages[i];
      /* skip whitespace */
      while (*s == ' ' || *s == '\t' || *s == '\n' || *s == '\r') s++;
      if (*s == '\0') { free(buf); return -1; }
    }
  }

  *nstages = count;
  return 0;
}


/* ===== pipe reader ===== */

#define READ_CHUNK 4096

typedef struct {
  char *data;
  size_t len;
  size_t cap;
} DynBuf;


static void dynbuf_init (DynBuf *b) {
  b->data = NULL;
  b->len = 0;
  b->cap = 0;
}


static void dynbuf_free (DynBuf *b) {
  free(b->data);
  b->data = NULL;
  b->len = b->cap = 0;
}


static int dynbuf_grow (DynBuf *b, size_t need) {
  if (b->len + need > b->cap) {
    size_t newcap = (b->cap == 0) ? READ_CHUNK : b->cap;
    char *p;
    while (newcap < b->len + need)
      newcap *= 2;
    p = (char *)realloc(b->data, newcap);
    if (p == NULL) return -1;
    b->data = p;
    b->cap = newcap;
  }
  return 0;
}


/*
** Read from two fds simultaneously into DynBufs using select().
** Avoids deadlock when child fills one pipe buffer.
*/
static void read_pipes (int fd_out, int fd_err,
                        DynBuf *buf_out, DynBuf *buf_err) {
  int nfds = (fd_out > fd_err ? fd_out : fd_err) + 1;
  int open_out = 1, open_err = 1;

  while (open_out || open_err) {
    fd_set rfds;
    int ret;

    FD_ZERO(&rfds);
    if (open_out) FD_SET(fd_out, &rfds);
    if (open_err) FD_SET(fd_err, &rfds);

    ret = select(nfds, &rfds, NULL, NULL, NULL);
    if (ret < 0) {
      if (errno == EINTR) continue;
      break;  /* unexpected error */
    }

    if (open_out && FD_ISSET(fd_out, &rfds)) {
      ssize_t n;
      if (dynbuf_grow(buf_out, READ_CHUNK) != 0) break;
      n = read(fd_out, buf_out->data + buf_out->len, READ_CHUNK);
      if (n > 0)
        buf_out->len += (size_t)n;
      else {
        close(fd_out);
        open_out = 0;
      }
    }

    if (open_err && FD_ISSET(fd_err, &rfds)) {
      ssize_t n;
      if (dynbuf_grow(buf_err, READ_CHUNK) != 0) break;
      n = read(fd_err, buf_err->data + buf_err->len, READ_CHUNK);
      if (n > 0)
        buf_err->len += (size_t)n;
      else {
        close(fd_err);
        open_err = 0;
      }
    }
  }
}


/* ===== pipeline execution ===== */

/*
** Build result table {code=N, stdout=S, stderr=S} on the Lua stack.
*/
static void push_result_table (lua_State *L, int code,
                               DynBuf *buf_out, DynBuf *buf_err) {
  lua_createtable(L, 0, 3);
  lua_pushinteger(L, code);
  lua_setfield(L, -2, "code");
  lua_pushlstring(L, buf_out->data ? buf_out->data : "", buf_out->len);
  lua_setfield(L, -2, "stdout");
  lua_pushlstring(L, buf_err->data ? buf_err->data : "", buf_err->len);
  lua_setfield(L, -2, "stderr");
}


/*
** Execute a multi-stage pipeline.
** stages[0..nstages-1] are command strings. Each is parsed with parse_argv().
** Inter-stage pipes connect stdout→stdin. Only the last stage's stdout/stderr
** are captured. Middle stages' stderr goes to the parent's stderr (inherited).
** Returns 1 (one result table on the Lua stack).
*/
static int exec_pipeline (lua_State *L, char **stages, int nstages,
                          int interactive) {
  ParsedArgs *pa = NULL;
  int (*inter_pipes)[2] = NULL;  /* inter_pipes[i] connects stage i → i+1 */
  int out_pipe[2] = {-1, -1}, err_pipe[2] = {-1, -1};
  pid_t *pids = NULL;
  int i, last_code = -1;
  DynBuf buf_out, buf_err;
  struct sigaction sa_old_pipe, sa_old_int, sa_old_quit, sa_new;

  /* parse all stages */
  pa = (ParsedArgs *)calloc((size_t)nstages, sizeof(ParsedArgs));
  if (pa == NULL) return luaL_error(L, "out of memory");

  for (i = 0; i < nstages; i++) {
    if (parse_argv(stages[i], &pa[i]) != 0) {
      for (int j = 0; j < i; j++) free_argv(&pa[j]);
      free(pa);
      return luaL_error(L, "unterminated quote in pipeline stage %d", i + 1);
    }
    if (pa[i].argc == 0) {
      for (int j = 0; j <= i; j++) free_argv(&pa[j]);
      free(pa);
      return luaL_error(L, "empty command in pipeline stage %d", i + 1);
    }
  }

  /* allocate inter-stage pipes (nstages-1 of them) */
  if (nstages > 1) {
    inter_pipes = (int (*)[2])calloc((size_t)(nstages - 1), sizeof(int [2]));
    if (inter_pipes == NULL) {
      for (i = 0; i < nstages; i++) free_argv(&pa[i]);
      free(pa);
      return luaL_error(L, "out of memory");
    }
    for (i = 0; i < nstages - 1; i++) {
      if (pipe(inter_pipes[i]) != 0) {
        for (int j = 0; j < i; j++) { close(inter_pipes[j][0]); close(inter_pipes[j][1]); }
        free(inter_pipes);
        for (int j = 0; j < nstages; j++) free_argv(&pa[j]);
        free(pa);
        return luaL_error(L, "pipe() failed: %s", strerror(errno));
      }
    }
  }

  /* capture pipes for last stage (only in non-interactive mode) */
  if (!interactive) {
    if (pipe(out_pipe) != 0) {
      if (inter_pipes) {
        for (i = 0; i < nstages - 1; i++) { close(inter_pipes[i][0]); close(inter_pipes[i][1]); }
        free(inter_pipes);
      }
      for (i = 0; i < nstages; i++) free_argv(&pa[i]);
      free(pa);
      return luaL_error(L, "pipe() failed: %s", strerror(errno));
    }
    if (pipe(err_pipe) != 0) {
      close(out_pipe[0]); close(out_pipe[1]);
      if (inter_pipes) {
        for (i = 0; i < nstages - 1; i++) { close(inter_pipes[i][0]); close(inter_pipes[i][1]); }
        free(inter_pipes);
      }
      for (i = 0; i < nstages; i++) free_argv(&pa[i]);
      free(pa);
      return luaL_error(L, "pipe() failed: %s", strerror(errno));
    }
  }

  /* ignore signals in parent */
  memset(&sa_new, 0, sizeof(sa_new));
  sa_new.sa_handler = SIG_IGN;
  sigemptyset(&sa_new.sa_mask);
  sigaction(SIGPIPE, &sa_new, &sa_old_pipe);
  if (interactive) {
    sigaction(SIGINT, &sa_new, &sa_old_int);
    sigaction(SIGQUIT, &sa_new, &sa_old_quit);
  }

  /* allocate pid array */
  pids = (pid_t *)calloc((size_t)nstages, sizeof(pid_t));
  if (pids == NULL) {
    if (!interactive) {
      close(out_pipe[0]); close(out_pipe[1]);
      close(err_pipe[0]); close(err_pipe[1]);
    }
    if (inter_pipes) {
      for (i = 0; i < nstages - 1; i++) { close(inter_pipes[i][0]); close(inter_pipes[i][1]); }
      free(inter_pipes);
    }
    for (i = 0; i < nstages; i++) free_argv(&pa[i]);
    free(pa);
    sigaction(SIGPIPE, &sa_old_pipe, NULL);
    if (interactive) {
      sigaction(SIGINT, &sa_old_int, NULL);
      sigaction(SIGQUIT, &sa_old_quit, NULL);
    }
    return luaL_error(L, "out of memory");
  }

  /* fork all stages */
  for (i = 0; i < nstages; i++) {
    pids[i] = fork();
    if (pids[i] < 0) {
      /* fork failed — kill already-forked children */
      for (int j = 0; j < i; j++) {
        kill(pids[j], SIGTERM);
        waitpid(pids[j], NULL, 0);
      }
      if (!interactive) {
        close(out_pipe[0]); close(out_pipe[1]);
        close(err_pipe[0]); close(err_pipe[1]);
      }
      if (inter_pipes) {
        for (int j = 0; j < nstages - 1; j++) { close(inter_pipes[j][0]); close(inter_pipes[j][1]); }
        free(inter_pipes);
      }
      for (int j = 0; j < nstages; j++) free_argv(&pa[j]);
      free(pa); free(pids);
      sigaction(SIGPIPE, &sa_old_pipe, NULL);
      if (interactive) {
        sigaction(SIGINT, &sa_old_int, NULL);
        sigaction(SIGQUIT, &sa_old_quit, NULL);
      }
      return luaL_error(L, "fork() failed: %s", strerror(errno));
    }

    if (pids[i] == 0) {
      /* === child process for stage i === */
      int j;

      /* restore default signals */
      sa_new.sa_handler = SIG_DFL;
      sigaction(SIGPIPE, &sa_new, NULL);
      if (interactive) {
        sigaction(SIGINT, &sa_new, NULL);
        sigaction(SIGQUIT, &sa_new, NULL);
      }

      /* set up stdin */
      if (i > 0) {
        dup2(inter_pipes[i - 1][0], STDIN_FILENO);
      }

      /* set up stdout */
      if (i < nstages - 1) {
        dup2(inter_pipes[i][1], STDOUT_FILENO);
      } else if (!interactive) {
        /* last stage: capture stdout and stderr */
        dup2(out_pipe[1], STDOUT_FILENO);
        dup2(err_pipe[1], STDERR_FILENO);
      }
      /* else: interactive last stage inherits terminal */

      /* close all pipe fds in child */
      if (inter_pipes) {
        for (j = 0; j < nstages - 1; j++) {
          close(inter_pipes[j][0]);
          close(inter_pipes[j][1]);
        }
      }
      if (!interactive) {
        close(out_pipe[0]); close(out_pipe[1]);
        close(err_pipe[0]); close(err_pipe[1]);
      }

      execvp(pa[i].argv[0], pa[i].argv);

      /* exec failed */
      {
        const char *err = strerror(errno);
        size_t namelen = strlen(pa[i].argv[0]);
        size_t errlen = strlen(err);
        (void)write(STDERR_FILENO, pa[i].argv[0], namelen);
        (void)write(STDERR_FILENO, ": ", 2);
        (void)write(STDERR_FILENO, err, errlen);
        (void)write(STDERR_FILENO, "\n", 1);
      }
      _exit(127);
    }
  }

  /* === parent: close all write ends === */
  if (inter_pipes) {
    for (i = 0; i < nstages - 1; i++) {
      close(inter_pipes[i][0]);
      close(inter_pipes[i][1]);
    }
    free(inter_pipes);
  }

  /* free parsed args (children have already exec'd) */
  for (i = 0; i < nstages; i++) free_argv(&pa[i]);
  free(pa);

  dynbuf_init(&buf_out);
  dynbuf_init(&buf_err);

  if (!interactive) {
    close(out_pipe[1]);
    close(err_pipe[1]);
    /* read captured output from last stage */
    read_pipes(out_pipe[0], err_pipe[0], &buf_out, &buf_err);
  }

  /* wait for all children, keep last stage's exit code */
  for (i = 0; i < nstages; i++) {
    int status;
    waitpid(pids[i], &status, 0);
    if (i == nstages - 1) {
      if (WIFEXITED(status))
        last_code = WEXITSTATUS(status);
      else if (WIFSIGNALED(status))
        last_code = 128 + WTERMSIG(status);
      else
        last_code = -1;
    }
  }
  free(pids);

  /* restore signals */
  sigaction(SIGPIPE, &sa_old_pipe, NULL);
  if (interactive) {
    sigaction(SIGINT, &sa_old_int, NULL);
    sigaction(SIGQUIT, &sa_old_quit, NULL);
  }

  /* push result table */
  push_result_table(L, last_code, &buf_out, &buf_err);
  dynbuf_free(&buf_out);
  dynbuf_free(&buf_err);

  return 1;
}


/* ===== luaB_command ===== */

int luaB_command (lua_State *L) {
  const char *cmd = luaL_checkstring(L, 1);
  char *stages[MAX_PIPELINE_STAGES];
  int nstages;
  ParsedArgs pa;
  int out_pipe[2], err_pipe[2];
  pid_t pid;
  int status;
  DynBuf buf_out, buf_err;
  struct sigaction sa_old, sa_new;

  /* try to split into pipeline stages */
  if (split_pipeline(cmd, stages, &nstages) != 0)
    return luaL_error(L, "invalid pipeline syntax in command");

  /* multi-stage pipeline: delegate to exec_pipeline */
  if (nstages > 1) {
    int result = exec_pipeline(L, stages, nstages, 0);
    free(stages[0]);  /* free backing buffer */
    return result;
  }

  /* single stage: free pipeline buffer and use original path */
  free(stages[0]);

  /* parse command into argv */
  if (parse_argv(cmd, &pa) != 0)
    return luaL_error(L, "unterminated quote in command");

  /* empty command: return {code=0, stdout="", stderr=""} */
  if (pa.argc == 0) {
    free_argv(&pa);
    lua_createtable(L, 0, 3);
    lua_pushinteger(L, 0);
    lua_setfield(L, -2, "code");
    lua_pushliteral(L, "");
    lua_setfield(L, -2, "stdout");
    lua_pushliteral(L, "");
    lua_setfield(L, -2, "stderr");
    return 1;
  }

  /* create pipes */
  if (pipe(out_pipe) != 0) {
    free_argv(&pa);
    return luaL_error(L, "pipe() failed: %s", strerror(errno));
  }
  if (pipe(err_pipe) != 0) {
    close(out_pipe[0]); close(out_pipe[1]);
    free_argv(&pa);
    return luaL_error(L, "pipe() failed: %s", strerror(errno));
  }

  /* ignore SIGPIPE so parent doesn't crash if child exits early */
  memset(&sa_new, 0, sizeof(sa_new));
  sa_new.sa_handler = SIG_IGN;
  sigemptyset(&sa_new.sa_mask);
  sigaction(SIGPIPE, &sa_new, &sa_old);

  pid = fork();
  if (pid < 0) {
    close(out_pipe[0]); close(out_pipe[1]);
    close(err_pipe[0]); close(err_pipe[1]);
    free_argv(&pa);
    sigaction(SIGPIPE, &sa_old, NULL);
    return luaL_error(L, "fork() failed: %s", strerror(errno));
  }

  if (pid == 0) {
    /* child */
    close(out_pipe[0]);
    close(err_pipe[0]);
    dup2(out_pipe[1], STDOUT_FILENO);
    dup2(err_pipe[1], STDERR_FILENO);
    close(out_pipe[1]);
    close(err_pipe[1]);

    /* restore default SIGPIPE for child */
    sa_new.sa_handler = SIG_DFL;
    sigaction(SIGPIPE, &sa_new, NULL);

    execvp(pa.argv[0], pa.argv);

    /* exec failed — write error to stderr and exit */
    {
      const char *err = strerror(errno);
      size_t namelen = strlen(pa.argv[0]);
      size_t errlen = strlen(err);
      /* write "cmd: error\n" */
      (void)write(STDERR_FILENO, pa.argv[0], namelen);
      (void)write(STDERR_FILENO, ": ", 2);
      (void)write(STDERR_FILENO, err, errlen);
      (void)write(STDERR_FILENO, "\n", 1);
    }
    _exit(127);
  }

  /* parent */
  close(out_pipe[1]);
  close(err_pipe[1]);
  free_argv(&pa);

  dynbuf_init(&buf_out);
  dynbuf_init(&buf_err);
  read_pipes(out_pipe[0], err_pipe[0], &buf_out, &buf_err);

  waitpid(pid, &status, 0);

  /* restore old SIGPIPE handler */
  sigaction(SIGPIPE, &sa_old, NULL);

  /* build result table */
  lua_createtable(L, 0, 3);

  if (WIFEXITED(status))
    lua_pushinteger(L, WEXITSTATUS(status));
  else if (WIFSIGNALED(status))
    lua_pushinteger(L, 128 + WTERMSIG(status));
  else
    lua_pushinteger(L, -1);
  lua_setfield(L, -2, "code");

  lua_pushlstring(L, buf_out.data ? buf_out.data : "", buf_out.len);
  lua_setfield(L, -2, "stdout");

  lua_pushlstring(L, buf_err.data ? buf_err.data : "", buf_err.len);
  lua_setfield(L, -2, "stderr");

  dynbuf_free(&buf_out);
  dynbuf_free(&buf_err);

  return 1;
}


/* ===== luaB_interactive ===== */

int luaB_interactive (lua_State *L) {
  const char *cmd = luaL_checkstring(L, 1);
  char *stages[MAX_PIPELINE_STAGES];
  int nstages;
  ParsedArgs pa;
  pid_t pid;
  int status, code;
  struct sigaction sa_old_pipe, sa_old_int, sa_old_quit, sa_new;
  DynBuf buf_out, buf_err;

  /* try to split into pipeline stages */
  if (split_pipeline(cmd, stages, &nstages) != 0)
    return luaL_error(L, "invalid pipeline syntax in command");

  /* multi-stage pipeline: delegate to exec_pipeline with interactive=1 */
  if (nstages > 1) {
    exec_pipeline(L, stages, nstages, 1);
    free(stages[0]);
    lua_setglobal(L, "_");
    return 0;
  }

  /* single stage */
  free(stages[0]);

  if (parse_argv(cmd, &pa) != 0)
    return luaL_error(L, "unterminated quote in command");

  if (pa.argc == 0) {
    free_argv(&pa);
    dynbuf_init(&buf_out);
    dynbuf_init(&buf_err);
    push_result_table(L, 0, &buf_out, &buf_err);
    lua_setglobal(L, "_");
    return 0;
  }

  /* ignore SIGINT, SIGQUIT, SIGPIPE in parent */
  memset(&sa_new, 0, sizeof(sa_new));
  sa_new.sa_handler = SIG_IGN;
  sigemptyset(&sa_new.sa_mask);
  sigaction(SIGPIPE, &sa_new, &sa_old_pipe);
  sigaction(SIGINT, &sa_new, &sa_old_int);
  sigaction(SIGQUIT, &sa_new, &sa_old_quit);

  pid = fork();
  if (pid < 0) {
    free_argv(&pa);
    sigaction(SIGPIPE, &sa_old_pipe, NULL);
    sigaction(SIGINT, &sa_old_int, NULL);
    sigaction(SIGQUIT, &sa_old_quit, NULL);
    return luaL_error(L, "fork() failed: %s", strerror(errno));
  }

  if (pid == 0) {
    /* child: restore signal defaults, inherit terminal */
    sa_new.sa_handler = SIG_DFL;
    sigaction(SIGPIPE, &sa_new, NULL);
    sigaction(SIGINT, &sa_new, NULL);
    sigaction(SIGQUIT, &sa_new, NULL);

    execvp(pa.argv[0], pa.argv);

    /* exec failed */
    {
      const char *err = strerror(errno);
      size_t namelen = strlen(pa.argv[0]);
      size_t errlen = strlen(err);
      (void)write(STDERR_FILENO, pa.argv[0], namelen);
      (void)write(STDERR_FILENO, ": ", 2);
      (void)write(STDERR_FILENO, err, errlen);
      (void)write(STDERR_FILENO, "\n", 1);
    }
    _exit(127);
  }

  /* parent */
  free_argv(&pa);
  waitpid(pid, &status, 0);

  /* restore signals */
  sigaction(SIGPIPE, &sa_old_pipe, NULL);
  sigaction(SIGINT, &sa_old_int, NULL);
  sigaction(SIGQUIT, &sa_old_quit, NULL);

  if (WIFEXITED(status))
    code = WEXITSTATUS(status);
  else if (WIFSIGNALED(status))
    code = 128 + WTERMSIG(status);
  else
    code = -1;

  /* build {code=N, stdout="", stderr=""} and set as global _ */
  dynbuf_init(&buf_out);
  dynbuf_init(&buf_err);
  push_result_table(L, code, &buf_out, &buf_err);
  lua_setglobal(L, "_");

  return 0;  /* void — no Lua return values */
}