# -*- coding: utf-8 -*-
"""Job control for the xonsh shell."""
import os
import sys
import time
import ctypes
import signal
import builtins
import subprocess
import collections

from xonsh.lazyasd import LazyObject
from xonsh.platform import FD_STDERR, ON_DARWIN, ON_WINDOWS, ON_CYGWIN, ON_MSYS, LIBC
from xonsh.tools import unthreadable


tasks = LazyObject(collections.deque, globals(), "tasks")
# Track time stamp of last exit command, so that two consecutive attempts to
# exit can kill all jobs and exit.
_last_exit_time = None


if ON_DARWIN:

    def _send_signal(job, signal):
        # On OS X, os.killpg() may cause PermissionError when there are
        # any zombie processes in the process group.
        # See github issue #1012 for details
        for pid in job["pids"]:
            if pid is None:  # the pid of an aliased proc is None
                continue
            try:
                os.kill(pid, signal)
            except ProcessLookupError:
                pass


elif ON_WINDOWS:
    pass
elif ON_CYGWIN or ON_MSYS:
    # Similar to what happened on OSX, more issues on Cygwin
    # (see Github issue #514).
    def _send_signal(job, signal):
        try:
            os.killpg(job["pgrp"], signal)
        except Exception:
            for pid in job["pids"]:
                try:
                    os.kill(pid, signal)
                except Exception:
                    pass


else:

    def _send_signal(job, signal):
        pgrp = job["pgrp"]
        if pgrp is None:
            for pid in job["pids"]:
                try:
                    os.kill(pid, signal)
                except Exception:
                    pass
        else:
            os.killpg(job["pgrp"], signal)


if ON_WINDOWS:

    def _continue(job):
        job["status"] = "running"

    def _kill(job):
        subprocess.check_output(["taskkill", "/F", "/T", "/PID", str(job["obj"].pid)])

    def ignore_sigtstp():
        pass

    def give_terminal_to(pgid):
        pass

    def wait_for_active_job(last_task=None, backgrounded=False):
        """
        Wait for the active job to finish, to be killed by SIGINT, or to be
        suspended by ctrl-z.
        """
        _clear_dead_jobs()
        active_task = get_next_task()
        # Return when there are no foreground active task
        if active_task is None:
            return last_task
        obj = active_task["obj"]
        _continue(active_task)
        while obj.returncode is None:
            try:
                obj.wait(0.01)
            except subprocess.TimeoutExpired:
                pass
            except KeyboardInterrupt:
                _kill(active_task)
        return wait_for_active_job(last_task=active_task)


else:

    def _continue(job):
        _send_signal(job, signal.SIGCONT)

    def _kill(job):
        _send_signal(job, signal.SIGKILL)

    def ignore_sigtstp():
        signal.signal(signal.SIGTSTP, signal.SIG_IGN)

    _shell_pgrp = os.getpgrp()

    _block_when_giving = LazyObject(
        lambda: (signal.SIGTTOU, signal.SIGTTIN, signal.SIGTSTP, signal.SIGCHLD),
        globals(),
        "_block_when_giving",
    )

    if ON_CYGWIN or ON_MSYS:
        # on cygwin, signal.pthread_sigmask does not exist in Python, even
        # though pthread_sigmask is defined in the kernel.  thus, we use
        # ctypes to mimic the calls in the "normal" version below.
        LIBC.pthread_sigmask.restype = ctypes.c_int
        LIBC.pthread_sigmask.argtypes = [
            ctypes.c_int,
            ctypes.POINTER(ctypes.c_ulong),
            ctypes.POINTER(ctypes.c_ulong),
        ]

        def _pthread_sigmask(how, signals):
            mask = 0
            for sig in signals:
                mask |= 1 << sig
            oldmask = ctypes.c_ulong()
            mask = ctypes.c_ulong(mask)
            result = LIBC.pthread_sigmask(
                how, ctypes.byref(mask), ctypes.byref(oldmask)
            )
            if result:
                raise OSError(result, "Sigmask error.")

            return {
                sig
                for sig in getattr(signal, "Signals", range(0, 65))
                if (oldmask.value >> sig) & 1
            }

    else:
        _pthread_sigmask = signal.pthread_sigmask

    # give_terminal_to is a simplified version of:
    #    give_terminal_to from bash 4.3 source, jobs.c, line 4030
    # this will give the terminal to the process group pgid
    def give_terminal_to(pgid):
        if pgid is None:
            return False
        oldmask = _pthread_sigmask(signal.SIG_BLOCK, _block_when_giving)
        try:
            os.tcsetpgrp(FD_STDERR, pgid)
            return True
        except ProcessLookupError:
            # when the process finished before giving terminal to it,
            # see issue #2288
            return False
        except OSError as e:
            if e.errno == 22:  # [Errno 22] Invalid argument
                # there are cases that all the processes of pgid have
                # finished, then we don't need to do anything here, see
                # issue #2220
                return False
            elif e.errno == 25:  # [Errno 25] Inappropriate ioctl for device
                # There are also cases where we are not connected to a
                # real TTY, even though we may be run in interactive
                # mode. See issue #2267 for an example with emacs
                return False
            else:
                raise
        finally:
            _pthread_sigmask(signal.SIG_SETMASK, oldmask)

    def wait_for_active_job(last_task=None, backgrounded=False):
        """
        Wait for the active job to finish, to be killed by SIGINT, or to be
        suspended by ctrl-z.
        """
        _clear_dead_jobs()
        active_task = get_next_task()
        # Return when there are no foreground active task
        if active_task is None:
            return last_task
        obj = active_task["obj"]
        backgrounded = False
        try:
            _, wcode = os.waitpid(obj.pid, os.WUNTRACED)
        except ChildProcessError:  # No child processes
            return wait_for_active_job(last_task=active_task, backgrounded=backgrounded)
        if os.WIFSTOPPED(wcode):
            print("^Z")
            active_task["status"] = "stopped"
            backgrounded = True
        elif os.WIFSIGNALED(wcode):
            print()  # get a newline because ^C will have been printed
            obj.signal = (os.WTERMSIG(wcode), os.WCOREDUMP(wcode))
            obj.returncode = None
        else:
            obj.returncode = os.WEXITSTATUS(wcode)
            obj.signal = None
        return wait_for_active_job(last_task=active_task, backgrounded=backgrounded)


def get_next_task():
    """ Get the next active task and put it on top of the queue"""
    selected_task = None
    for tid in tasks:
        task = get_task(tid)
        if not task["bg"] and task["status"] == "running":
            selected_task = tid
            break
    if selected_task is None:
        return
    tasks.remove(selected_task)
    tasks.appendleft(selected_task)
    return get_task(selected_task)


def get_task(tid):
    return builtins.__xonsh__.all_jobs[tid]


def _clear_dead_jobs():
    to_remove = set()
    for tid in tasks:
        obj = get_task(tid)["obj"]
        if obj is None or obj.poll() is not None:
            to_remove.add(tid)
    for job in to_remove:
        tasks.remove(job)
        del builtins.__xonsh__.all_jobs[job]


def print_one_job(num, outfile=sys.stdout):
    """Print a line describing job number ``num``."""
    try:
        job = builtins.__xonsh__.all_jobs[num]
    except KeyError:
        return
    pos = "+" if tasks[0] == num else "-" if tasks[1] == num else " "
    status = job["status"]
    cmd = [" ".join(i) if isinstance(i, list) else i for i in job["cmds"]]
    cmd = " ".join(cmd)
    pid = job["pids"][-1]
    bg = " &" if job["bg"] else ""
    print("[{}]{} {}: {}{} ({})".format(num, pos, status, cmd, bg, pid), file=outfile)


def get_next_job_number():
    """Get the lowest available unique job number (for the next job created).
    """
    _clear_dead_jobs()
    i = 1
    while i in builtins.__xonsh__.all_jobs:
        i += 1
    return i


def add_job(info):
    """Add a new job to the jobs dictionary."""
    num = get_next_job_number()
    info["started"] = time.time()
    info["status"] = "running"
    tasks.appendleft(num)
    builtins.__xonsh__.all_jobs[num] = info
    if info["bg"] and builtins.__xonsh__.env.get("XONSH_INTERACTIVE"):
        print_one_job(num)


def clean_jobs():
    """Clean up jobs for exiting shell

    In non-interactive mode, kill all jobs.

    In interactive mode, check for suspended or background jobs, print a
    warning if any exist, and return False. Otherwise, return True.
    """
    jobs_clean = True
    if builtins.__xonsh__.env["XONSH_INTERACTIVE"]:
        _clear_dead_jobs()

        if builtins.__xonsh__.all_jobs:
            global _last_exit_time
            hist = builtins.__xonsh__.history
            if hist is not None and len(hist.tss) > 0:
                last_cmd_start = hist.tss[-1][0]
            else:
                last_cmd_start = None

            if _last_exit_time and last_cmd_start and _last_exit_time > last_cmd_start:
                # Exit occurred after last command started, so it was called as
                # part of the last command and is now being called again
                # immediately. Kill jobs and exit without reminder about
                # unfinished jobs in this case.
                kill_all_jobs()
            else:
                if len(builtins.__xonsh__.all_jobs) > 1:
                    msg = "there are unfinished jobs"
                else:
                    msg = "there is an unfinished job"

                if builtins.__xonsh__.env["SHELL_TYPE"] != "prompt_toolkit":
                    # The Ctrl+D binding for prompt_toolkit already inserts a
                    # newline
                    print()
                print("xonsh: {}".format(msg), file=sys.stderr)
                print("-" * 5, file=sys.stderr)
                jobs([], stdout=sys.stderr)
                print("-" * 5, file=sys.stderr)
                print(
                    'Type "exit" or press "ctrl-d" again to force quit.',
                    file=sys.stderr,
                )
                jobs_clean = False
                _last_exit_time = time.time()
    else:
        kill_all_jobs()

    return jobs_clean


def kill_all_jobs():
    """
    Send SIGKILL to all child processes (called when exiting xonsh).
    """
    _clear_dead_jobs()
    for job in builtins.__xonsh__.all_jobs.values():
        _kill(job)


def jobs(args, stdin=None, stdout=sys.stdout, stderr=None):
    """
    xonsh command: jobs

    Display a list of all current jobs.
    """
    _clear_dead_jobs()
    for j in tasks:
        print_one_job(j, outfile=stdout)
    return None, None


def resume_job(args, wording):
    """
    used by fg and bg to resume a job either in the foreground or in the background.
    """
    _clear_dead_jobs()
    if len(tasks) == 0:
        return "", "There are currently no suspended jobs"

    if len(args) == 0:
        tid = tasks[0]  # take the last manipulated task by default
    elif len(args) == 1:
        try:
            if args[0] == "+":  # take the last manipulated task
                tid = tasks[0]
            elif args[0] == "-":  # take the second to last manipulated task
                tid = tasks[1]
            else:
                tid = int(args[0])
        except (ValueError, IndexError):
            return "", "Invalid job: {}\n".format(args[0])

        if tid not in builtins.__xonsh__.all_jobs:
            return "", "Invalid job: {}\n".format(args[0])
    else:
        return "", "{} expects 0 or 1 arguments, not {}\n".format(wording, len(args))

    # Put this one on top of the queue
    tasks.remove(tid)
    tasks.appendleft(tid)

    job = get_task(tid)
    job["bg"] = False
    job["status"] = "running"
    if builtins.__xonsh__.env.get("XONSH_INTERACTIVE"):
        print_one_job(tid)
    pipeline = job["pipeline"]
    pipeline.resume(job)


@unthreadable
def fg(args, stdin=None):
    """
    xonsh command: fg

    Bring the currently active job to the foreground, or, if a single number is
    given as an argument, bring that job to the foreground. Additionally,
    specify "+" for the most recent job and "-" for the second most recent job.
    """
    return resume_job(args, wording="fg")


def bg(args, stdin=None):
    """xonsh command: bg

    Resume execution of the currently active job in the background, or, if a
    single number is given as an argument, resume that job in the background.
    """
    res = resume_job(args, wording="bg")
    if res is None:
        curtask = get_task(tasks[0])
        curtask["bg"] = True
        _continue(curtask)
    else:
        return res

xonsh/jobs.py
# -*- coding: utf-8 -*-
"""Job control for the xonsh shell."""
import os
import sys
import time
import ctypes
import signal
import builtins
import subprocess
import collections

from xonsh.lazyasd import LazyObject
from xonsh.platform import FD_STDERR, ON_DARWIN, ON_WINDOWS, ON_CYGWIN, ON_MSYS, LIBC
from xonsh.tools import unthreadable


tasks = LazyObject(collections.deque, globals(), "tasks")
# Track time stamp of last exit command, so that two consecutive attempts to
# exit can kill all jobs and exit.
_last_exit_time = None


if ON_DARWIN:

    def _send_signal(job, signal):
        # On OS X, os.killpg() may cause PermissionError when there are
        # any zombie processes in the process group.
        # See github issue #1012 for details
        for pid in job["pids"]:
            if pid is None:  # the pid of an aliased proc is None
                continue
            try:
                os.kill(pid, signal)
            except ProcessLookupError:
                pass


elif ON_WINDOWS:
    pass
elif ON_CYGWIN or ON_MSYS:
    # Similar to what happened on OSX, more issues on Cygwin
    # (see Github issue #514).
    def _send_signal(job, signal):
        try:
            os.killpg(job["pgrp"], signal)
        except Exception:
            for pid in job["pids"]:
                try:
                    os.kill(pid, signal)
                except Exception:
                    pass


else:

    def _send_signal(job, signal):
        pgrp = job["pgrp"]
        if pgrp is None:
            for pid in job["pids"]:
                try:
                    os.kill(pid, signal)
                except Exception:
                    pass
        else:
            os.killpg(job["pgrp"], signal)


if ON_WINDOWS:

    def _continue(job):
        job["status"] = "running"

    def _kill(job):
        subprocess.check_output(["taskkill", "/F", "/T", "/PID", str(job["obj"].pid)])

    def ignore_sigtstp():
        pass

    def give_terminal_to(pgid):
        pass

    def wait_for_active_job(last_task=None, backgrounded=False):
        """
        Wait for the active job to finish, to be killed by SIGINT, or to be
        suspended by ctrl-z.
        """
        _clear_dead_jobs()
        active_task = get_next_task()
        # Return when there are no foreground active task
        if active_task is None:
            return last_task
        obj = active_task["obj"]
        _continue(active_task)
        while obj.returncode is None:
            try:
                obj.wait(0.01)
            except subprocess.TimeoutExpired:
                pass
            except KeyboardInterrupt:
                _kill(active_task)
        return wait_for_active_job(last_task=active_task)


else:

    def _continue(job):
        _send_signal(job, signal.SIGCONT)

    def _kill(job):
        _send_signal(job, signal.SIGKILL)

    def ignore_sigtstp():
        signal.signal(signal.SIGTSTP, signal.SIG_IGN)

    _shell_pgrp = os.getpgrp()

    _block_when_giving = LazyObject(
        lambda: (signal.SIGTTOU, signal.SIGTTIN, signal.SIGTSTP, signal.SIGCHLD),
        globals(),
        "_block_when_giving",
    )

    if ON_CYGWIN or ON_MSYS:
        # on cygwin, signal.pthread_sigmask does not exist in Python, even
        # though pthread_sigmask is defined in the kernel.  thus, we use
        # ctypes to mimic the calls in the "normal" version below.
        LIBC.pthread_sigmask.restype = ctypes.c_int
        LIBC.pthread_sigmask.argtypes = [
            ctypes.c_int,
            ctypes.POINTER(ctypes.c_ulong),
            ctypes.POINTER(ctypes.c_ulong),
        ]

        def _pthread_sigmask(how, signals):
            mask = 0
            for sig in signals:
                mask |= 1 << sig
            oldmask = ctypes.c_ulong()
            mask = ctypes.c_ulong(mask)
            result = LIBC.pthread_sigmask(
                how, ctypes.byref(mask), ctypes.byref(oldmask)
            )
            if result:
                raise OSError(result, "Sigmask error.")

            return {
                sig
                for sig in getattr(signal, "Signals", range(0, 65))
                if (oldmask.value >> sig) & 1
            }

    else:
        _pthread_sigmask = signal.pthread_sigmask

    # give_terminal_to is a simplified version of:
    #    give_terminal_to from bash 4.3 source, jobs.c, line 4030
    # this will give the terminal to the process group pgid
    def give_terminal_to(pgid):
        if pgid is None:
            return False
        oldmask = _pthread_sigmask(signal.SIG_BLOCK, _block_when_giving)
        try:
            os.tcsetpgrp(FD_STDERR, pgid)
            return True
        except ProcessLookupError:
            # when the process finished before giving terminal to it,
            # see issue #2288
            return False
        except OSError as e:
            if e.errno == 22:  # [Errno 22] Invalid argument
                # there are cases that all the processes of pgid have
                # finished, then we don't need to do anything here, see
                # issue #2220
                return False
            elif e.errno == 25:  # [Errno 25] Inappropriate ioctl for device
                # There are also cases where we are not connected to a
                # real TTY, even though we may be run in interactive
                # mode. See issue #2267 for an example with emacs
                return False
            else:
                raise
        finally:
            if oldmask:
                # only reset the mask if it is non-empty! See #2989
                _pthread_sigmask(signal.SIG_SETMASK, oldmask)

    def wait_for_active_job(last_task=None, backgrounded=False):
        """
        Wait for the active job to finish, to be killed by SIGINT, or to be
        suspended by ctrl-z.
        """
        _clear_dead_jobs()
        active_task = get_next_task()
        # Return when there are no foreground active task
        if active_task is None:
            return last_task
        obj = active_task["obj"]
        backgrounded = False
        try:
            _, wcode = os.waitpid(obj.pid, os.WUNTRACED)
        except ChildProcessError:  # No child processes
            return wait_for_active_job(last_task=active_task, backgrounded=backgrounded)
        if os.WIFSTOPPED(wcode):
            print("^Z")
            active_task["status"] = "stopped"
            backgrounded = True
        elif os.WIFSIGNALED(wcode):
            print()  # get a newline because ^C will have been printed
            obj.signal = (os.WTERMSIG(wcode), os.WCOREDUMP(wcode))
            obj.returncode = None
        else:
            obj.returncode = os.WEXITSTATUS(wcode)
            obj.signal = None
        return wait_for_active_job(last_task=active_task, backgrounded=backgrounded)


def get_next_task():
    """ Get the next active task and put it on top of the queue"""
    selected_task = None
    for tid in tasks:
        task = get_task(tid)
        if not task["bg"] and task["status"] == "running":
            selected_task = tid
            break
    if selected_task is None:
        return
    tasks.remove(selected_task)
    tasks.appendleft(selected_task)
    return get_task(selected_task)


def get_task(tid):
    return builtins.__xonsh__.all_jobs[tid]


def _clear_dead_jobs():
    to_remove = set()
    for tid in tasks:
        obj = get_task(tid)["obj"]
        if obj is None or obj.poll() is not None:
            to_remove.add(tid)
    for job in to_remove:
        tasks.remove(job)
        del builtins.__xonsh__.all_jobs[job]


def print_one_job(num, outfile=sys.stdout):
    """Print a line describing job number ``num``."""
    try:
        job = builtins.__xonsh__.all_jobs[num]
    except KeyError:
        return
    pos = "+" if tasks[0] == num else "-" if tasks[1] == num else " "
    status = job["status"]
    cmd = [" ".join(i) if isinstance(i, list) else i for i in job["cmds"]]
    cmd = " ".join(cmd)
    pid = job["pids"][-1]
    bg = " &" if job["bg"] else ""
    print("[{}]{} {}: {}{} ({})".format(num, pos, status, cmd, bg, pid), file=outfile)


def get_next_job_number():
    """Get the lowest available unique job number (for the next job created).
    """
    _clear_dead_jobs()
    i = 1
    while i in builtins.__xonsh__.all_jobs:
        i += 1
    return i


def add_job(info):
    """Add a new job to the jobs dictionary."""
    num = get_next_job_number()
    info["started"] = time.time()
    info["status"] = "running"
    tasks.appendleft(num)
    builtins.__xonsh__.all_jobs[num] = info
    if info["bg"] and builtins.__xonsh__.env.get("XONSH_INTERACTIVE"):
        print_one_job(num)


def clean_jobs():
    """Clean up jobs for exiting shell

    In non-interactive mode, kill all jobs.

    In interactive mode, check for suspended or background jobs, print a
    warning if any exist, and return False. Otherwise, return True.
    """
    jobs_clean = True
    if builtins.__xonsh__.env["XONSH_INTERACTIVE"]:
        _clear_dead_jobs()

        if builtins.__xonsh__.all_jobs:
            global _last_exit_time
            hist = builtins.__xonsh__.history
            if hist is not None and len(hist.tss) > 0:
                last_cmd_start = hist.tss[-1][0]
            else:
                last_cmd_start = None

            if _last_exit_time and last_cmd_start and _last_exit_time > last_cmd_start:
                # Exit occurred after last command started, so it was called as
                # part of the last command and is now being called again
                # immediately. Kill jobs and exit without reminder about
                # unfinished jobs in this case.
                kill_all_jobs()
            else:
                if len(builtins.__xonsh__.all_jobs) > 1:
                    msg = "there are unfinished jobs"
                else:
                    msg = "there is an unfinished job"

                if builtins.__xonsh__.env["SHELL_TYPE"] != "prompt_toolkit":
                    # The Ctrl+D binding for prompt_toolkit already inserts a
                    # newline
                    print()
                print("xonsh: {}".format(msg), file=sys.stderr)
                print("-" * 5, file=sys.stderr)
                jobs([], stdout=sys.stderr)
                print("-" * 5, file=sys.stderr)
                print(
                    'Type "exit" or press "ctrl-d" again to force quit.',
                    file=sys.stderr,
                )
                jobs_clean = False
                _last_exit_time = time.time()
    else:
        kill_all_jobs()

    return jobs_clean


def kill_all_jobs():
    """
    Send SIGKILL to all child processes (called when exiting xonsh).
    """
    _clear_dead_jobs()
    for job in builtins.__xonsh__.all_jobs.values():
        _kill(job)


def jobs(args, stdin=None, stdout=sys.stdout, stderr=None):
    """
    xonsh command: jobs

    Display a list of all current jobs.
    """
    _clear_dead_jobs()
    for j in tasks:
        print_one_job(j, outfile=stdout)
    return None, None


def resume_job(args, wording):
    """
    used by fg and bg to resume a job either in the foreground or in the background.
    """
    _clear_dead_jobs()
    if len(tasks) == 0:
        return "", "There are currently no suspended jobs"

    if len(args) == 0:
        tid = tasks[0]  # take the last manipulated task by default
    elif len(args) == 1:
        try:
            if args[0] == "+":  # take the last manipulated task
                tid = tasks[0]
            elif args[0] == "-":  # take the second to last manipulated task
                tid = tasks[1]
            else:
                tid = int(args[0])
        except (ValueError, IndexError):
            return "", "Invalid job: {}\n".format(args[0])

        if tid not in builtins.__xonsh__.all_jobs:
            return "", "Invalid job: {}\n".format(args[0])
    else:
        return "", "{} expects 0 or 1 arguments, not {}\n".format(wording, len(args))

    # Put this one on top of the queue
    tasks.remove(tid)
    tasks.appendleft(tid)

    job = get_task(tid)
    job["bg"] = False
    job["status"] = "running"
    if builtins.__xonsh__.env.get("XONSH_INTERACTIVE"):
        print_one_job(tid)
    pipeline = job["pipeline"]
    pipeline.resume(job)


@unthreadable
def fg(args, stdin=None):
    """
    xonsh command: fg

    Bring the currently active job to the foreground, or, if a single number is
    given as an argument, bring that job to the foreground. Additionally,
    specify "+" for the most recent job and "-" for the second most recent job.
    """
    return resume_job(args, wording="fg")


def bg(args, stdin=None):
    """xonsh command: bg

    Resume execution of the currently active job in the background, or, if a
    single number is given as an argument, resume that job in the background.
    """
    res = resume_job(args, wording="bg")
    if res is None:
        curtask = get_task(tasks[0])
        curtask["bg"] = True
        _continue(curtask)
    else:
        return res

xonsh/jobs.py
diff --git a/news/pv.rst b/news/pv.rst
new file mode 100644
index 0000000000..c1babae041
--- /dev/null
+++ b/news/pv.rst
@@ -0,0 +1,25 @@
+**Added:**
+
+* <news item>
+
+**Changed:**
+
+* <news item>
+
+**Deprecated:**
+
+* <news item>
+
+**Removed:**
+
+* <news item>
+
+**Fixed:**
+
+* Resolved issue where setting empty signal masks was causing the
+  terminal to close. This was problematic for certain command
+  pipelines. For example, ``pv /dev/urandom | head`` now works.
+
+**Security:**
+
+* <news item>
diff --git a/xonsh/jobs.py b/xonsh/jobs.py
index 6025072f9a..145c37a862 100644
--- a/xonsh/jobs.py
+++ b/xonsh/jobs.py
@@ -180,7 +180,9 @@ def give_terminal_to(pgid):
             else:
                 raise
         finally:
-            _pthread_sigmask(signal.SIG_SETMASK, oldmask)
+            if oldmask:
+                # only reset the mask if it is non-empty! See #2989
+                _pthread_sigmask(signal.SIG_SETMASK, oldmask)
 
     def wait_for_active_job(last_task=None, backgrounded=False):
         """

`pv /dev/urandom | head` kills terminal
## xonfig

```
+------------------+---------------------+
| xonsh            | 0.8.4               |
| Git SHA          | 8405b8dd            |
| Commit Date      | Dec 7 10:39:36 2018 |
| Python           | 3.6.7               |
| PLY              | 3.11                |
| have readline    | True                |
| prompt toolkit   | 2.0.7               |
| shell type       | prompt_toolkit2     |
| pygments         | 2.2.0               |
| on posix         | True                |
| on linux         | True                |
| distro           | unknown             |
| on darwin        | False               |
| on windows       | False               |
| on cygwin        | False               |
| on msys2         | False               |
| is superuser     | False               |
| default encoding | utf-8               |
| xonsh encoding   | utf-8               |
| encoding errors  | surrogateescape     |
+------------------+---------------------+
```

## Expected Behavior

`pv /dev/urandom | head` doesn't crash the terminal

## Current Behavior

`pv /dev/urandom | head` crashes the terminal, leading to the window getting closed

## Steps to Reproduce

```
sudo apt install pv -y
pv /dev/urandom | head
```
