src/jobclient.c: Cope if the jobserver pipe is set nonblocking.

[distorted-chroot] / src / jobclient.c

/* -*-c-*-
 *
 * Job client low-level magic
 *
 * (c) 2019 Mark Wooding
 */

/*----- Licensing notice --------------------------------------------------*
 *
 * This file is part of the distorted.org.uk chroot maintenance tools.
 *
 * distorted-chroot is free software: you can redistribute it and/or
 * modify it under the terms of the GNU General Public License as
 * published by the Free Software Foundation; either version 2 of the
 * License, or (at your option) any later version.
 *
 * distorted-chroot is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with distorted-chroot.  If not, write to the Free Software
 * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307,
 * USA.
 */

/*----- Header files ------------------------------------------------------*/

#include <assert.h>
#include <errno.h>
#include <signal.h>

#include <sys/types.h>
#include <sys/wait.h>
#include <sys/select.h>
#include <unistd.h>

#define PY_SSIZE_T_CLEAN
#include <Python.h>

/*----- Static variables --------------------------------------------------*/

static volatile int sigfd = -1;

/*----- Main code ---------------------------------------------------------*/

static void chld(int sig)
  { int fd = sigfd; if (fd != -1) { close(fd); sigfd = -1; } }

static PyObject *pymeth_jobclient(PyObject *me, PyObject *arg)
{
  PyObject *rc = 0;
  struct sigaction sa, oldsa;
  sigset_t mask, oldmask;
  fd_set infd;
  int rstrchld = 0;
  int fd, sfd;
  int kid, st;
  ssize_t n;
  char ch;

  /* Parse the arguments. */
  if (!PyArg_ParseTuple(arg, "i", &fd)) goto end;

  /* Prepare to be able to block `SIGCHLD'. */
  sigemptyset(&mask); sigaddset(&mask, SIGCHLD);
  if (sigprocmask(SIG_SETMASK, 0, &oldmask)) goto oserr;

  /* Establish the signal handler. */
  sa.sa_handler = chld;
  sigemptyset(&sa.sa_mask); sigaddset(&sa.sa_mask, SIGCHLD);
  sa.sa_flags = SA_NOCLDSTOP;
  if (sigaction(SIGCHLD, &sa, &oldsa)) goto oserr;
  rstrchld = 1;

  /* Establish a copy of the file descriptor.  If a signal occurs, we'll
   * clobber this descriptor: see below.  If we come back here, we'll
   * probably need to reinstate this, but check just in case: we don't want a
   * descriptor leak.
   */
again:
  if (sigprocmask(SIG_BLOCK, &mask, 0)) goto oserr;
  sfd = sigfd;
  if (sfd == -1) {
    sfd = dup(fd); if (sfd == -1) goto oserr;
    sigfd = sfd;
  }
  if (sigprocmask(SIG_UNBLOCK, &mask, 0)) goto oserr;

  /* Check for child processes.  If a child exit somehow failed to be
   * spotted earlier, this is our chance to notice and do something about it.
   */
  kid = waitpid(-1, &st, WNOHANG);
  if (kid == -1 && errno != ECHILD) goto oserr;
  else if (kid > 0)
    { rc = Py_BuildValue("(Oii)", Py_None, kid, st); goto end; }

  /* Read from the pipe.  If a child exits between the `waitpid' above and
   * the `read' here, the signal handler will pop and close the descriptor,
   * so `read' will report `EBADF'.  If a child exits while we're waiting for
   * a byte on the descriptor, then `read' will report `EINTR'.  If anything
   * like this happens, then we go back and try reaping children again.
   */
  Py_BEGIN_ALLOW_THREADS
  for (;;) {
    n = read(sigfd, &ch, 1); if (n >= 0 || errno != EAGAIN) break;
    FD_ZERO(&infd); FD_SET(sigfd, &infd);
    n = select(sigfd + 1, &infd, 0, 0, 0); if (n < 0) break;
  }
  Py_END_ALLOW_THREADS
  if (n == 1) rc = Py_BuildValue("(cOO)", ch, Py_None, Py_None);
  else if (!n) rc = Py_BuildValue("(sOO)", "", Py_None, Py_None);
  else if (n != -1) {
    PyErr_SetString(PyExc_SystemError, "impossible return from read");
    goto end;
  } else if (errno == EINTR || errno == EBADF) goto again;
  else goto oserr;

  /* We're done. */
  goto end;

oserr:
  /* Report a system error. */
  PyErr_SetFromErrnoWithFilename(PyExc_OSError, 0); goto end;

end:
  /* Clean up and return. */
  if (rstrchld) {
    sigprocmask(SIG_SETMASK, &oldmask, 0);
    sigaction(SIGCHLD, &oldsa, 0);
  }
  sfd = sigfd; if (sfd != -1) { close(sfd); sigfd = -1; }
  return (rc);
}

static PyMethodDef methods[] = {
  { "jobclient", pymeth_jobclient, METH_VARARGS,
    "jobclient(FD) -> (TOKEN, PID, STATUS)" },
  { 0 }
};

PyMODINIT_FUNC initjobclient(void)
  { Py_InitModule("jobclient", methods); }

/*----- That's all, folks -------------------------------------------------*/