Ctrl-C no longer exits OSH, at least under some circumstances.

We were using Python's default SIGINT handler, which sets a flag and
then throws KeyboardInterrupt at the current execution point.

Now we have our own signal handler.  It seems to work, although it's
hard to test automatically.

OSH does behave a little differently than other shells: it doesn't
reprint the prompt after Ctrl-C.  I'm not sure other shells do that.  I
looked at dash and it does have special handling for SIGINT.

Addresses issue #36.

bin/oil.py

@@ -103,13 +103,9 @@ def InteractiveLoop(opts, ex, c_parser, arena):

  status = 0
  while True:
    # Why is this the way to handle Control-C?
    # Also, we shouldn't exit the shell!
    try:
      w = c_parser.Peek()
    except KeyboardInterrupt:
      print('Ctrl-C')
      continue
    # NOTE: We no longer need to catch KeyboardInterrupt here, because we
    # handle SIGINT ourselves.
    w = c_parser.Peek()

    c_id = word.CommandId(w)
    if c_id == Id.Op_Newline:
@@ -195,6 +191,8 @@ def OshMain(argv0, argv, login_shell):
    _ShowVersion()
    return 0

  builtin.RegisterSigIntHandler()

  trace_state = util.TraceState()
  if 'cmd-parse' == opts.trace:
    util.WrapMethods(cmd_parse.CommandParser, trace_state)
@@ -208,11 +206,6 @@ def OshMain(argv0, argv, login_shell):
  else:
    dollar0 = argv[opt_index]  # the script name, or the arg after -c

  # TODO: Create a --parse action or 'osh parse' or 'oil osh-parse'
  # osh-fix
  # It uses a different memory-management model.  It's a batch program and not
  # an interactive program.

  pool = alloc.Pool()
  arena = pool.NewArena()

core/builtin.py

@@ -1033,6 +1033,26 @@ def UnAlias(argv, aliases):
import signal


def _SigIntHandler(unused, unused_frame):
  """
  Either this handler is installed, or the user's handler is installed.
  Python's default handler of raising KeyboardInterrupt should never be
  installed.
  """
  # TODO: It might be nice to write diagnostic messages when invokved with
  # 'osh --debug-pipe=/path'.
  #
  # NOTE: I think dash and POSIX somehow set the exit code to 128 + exit code?

  #print('Ctrl-C')
  pass


def RegisterSigIntHandler():
  #log('Registering')
  signal.signal(signal.SIGINT, _SigIntHandler)


class _TrapHandler(object):
  """A function that is called by Python's signal module.

@@ -1143,7 +1163,10 @@ def Trap(argv, traps, nodes_to_run, ex):
        pass

      # Restore default
      signal.signal(sig_val, signal.SIG_DFL)
      if sig_val == signal.SIGINT:
        RegisterSigIntHandler()
      else:
        signal.signal(sig_val, signal.SIG_DFL)
      return 0

    util.error("Can't remove invalid trap %r" % sig_spec)

core/process.py

@@ -454,6 +454,20 @@ def ClosePipe(self):

  def Start(self):
    """Start this process with fork(), haandling redirects."""
    # TODO: If OSH were a job control shell, we might need to call some of
    # these here.  They control the distribution of signals, some of which
    # originate from a terminal.  All the processes in a pipeline should be in
    # a single process group.
    #
    # - os.setpgid()
    # - os.setpgrp() 
    # - os.tcsetpgrp()
    #
    # NOTE: os.setsid() isn't called by the shell; it's should be called by the
    # login program that starts the shell.
    #
    # The whole job control mechanism is complicated and hacky.

    pid = os.fork()
    if pid < 0:
      # When does this happen?
@@ -666,15 +680,25 @@ def Register(self, pid, callback):

  def Wait(self):
    # This is a list of async jobs
    try:
      pid, status = os.wait()
    except OSError as e:
      if e.errno == errno.ECHILD:
        #log('WAIT ECHILD')
        return False  # nothing to wait for caller should stop
    while True:
      try:
        pid, status = os.wait()
      except OSError as e:
        #log('wait() error: %s', e)
        if e.errno == errno.ECHILD:
          return False  # nothing to wait for caller should stop
        elif e.errno == errno.EINTR:
          # This happens when we register a handler for SIGINT, and thus never
          # get the KeyboardInterrupt exception?  Not sure why.
          # Try
          # $ cat   # Now hit Ctrl-C
          #log('Continuing')
          continue  # try again
        else:
          # An error we don't know about.
          raise
      else:
        # What else can go wrong?
        raise
        break  # no exception thrown, so no need to retry

    #log('WAIT got %s %s', pid, status)