BUG: Ensure that kernel can be spawned from a backgrounded process.

IPython/zmq/entry_point.py

@@ -1,158 +1,167
 """ Defines helper functions for creating kernel entry points and process
 launchers.
 """
 
 # Standard library imports.
 import atexit
 import os
 import socket
 from subprocess import Popen, PIPE
 import sys
 
 # Local imports.
 from parentpoller import ParentPollerWindows
 
 
 def base_launch_kernel(code, shell_port=0, iopub_port=0, stdin_port=0, hb_port=0,
                         ip=None, stdin=None, stdout=None, stderr=None,
                         executable=None, independent=False, extra_arguments=[]):
     """ Launches a localhost kernel, binding to the specified ports.
 
     Parameters
     ----------
     code : str,
         A string of Python code that imports and executes a kernel entry point.
 
     shell_port : int, optional
         The port to use for XREP channel.
 
     iopub_port : int, optional
         The port to use for the SUB channel.
 
     stdin_port : int, optional
         The port to use for the REQ (raw input) channel.
 
     hb_port : int, optional
         The port to use for the hearbeat REP channel.
 
     ip  : str, optional
         The ip address the kernel will bind to.
 
     stdin, stdout, stderr : optional (default None)
         Standards streams, as defined in subprocess.Popen.
 
     executable : str, optional (default sys.executable)
         The Python executable to use for the kernel process.
 
     independent : bool, optional (default False) 
         If set, the kernel process is guaranteed to survive if this process
         dies. If not set, an effort is made to ensure that the kernel is killed
         when this process dies. Note that in this case it is still good practice
         to kill kernels manually before exiting.
 
     extra_arguments = list, optional
         A list of extra arguments to pass when executing the launch code.
 
     Returns
     -------
     A tuple of form:
         (kernel_process, shell_port, iopub_port, stdin_port, hb_port)
     where kernel_process is a Popen object and the ports are integers.
     """
     # Find open ports as necessary.
     ports = []
     ports_needed = int(shell_port <= 0) + int(iopub_port <= 0) + \
                    int(stdin_port <= 0) + int(hb_port <= 0)
     for i in xrange(ports_needed):
         sock = socket.socket()
         sock.bind(('', 0))
         ports.append(sock)
     for i, sock in enumerate(ports):
         port = sock.getsockname()[1]
         sock.close()
         ports[i] = port
     if shell_port <= 0:
         shell_port = ports.pop(0)
     if iopub_port <= 0:
         iopub_port = ports.pop(0)
     if stdin_port <= 0:
         stdin_port = ports.pop(0)
     if hb_port <= 0:
         hb_port = ports.pop(0)
 
     # Build the kernel launch command.
     if executable is None:
         executable = sys.executable
     arguments = [ executable, '-c', code, '--shell=%i'%shell_port,
                   '--iopub=%i'%iopub_port, '--stdin=%i'%stdin_port,
-                  '--hb=%i'%hb_port
-    ]
+                  '--hb=%i'%hb_port ]
     if ip is not None:
         arguments.append('--ip=%s'%ip)
     arguments.extend(extra_arguments)
 
+    # Popen will fail (sometimes with a deadlock) if stdin, stdout, and stderr
+    # are invalid. Unfortunately, there is in general no way to detect whether
+    # they are valid.  The following two blocks redirect them to (temporary)
+    # pipes in certain important cases.
+
+    # If this process has been backgrounded, our stdin is invalid. Since there
+    # is no compelling reason for the kernel to inherit our stdin anyway, we'll
+    # place this one safe and always redirect.
+    redirect_in = True
+    _stdin = PIPE if stdin is None else stdin
+
+    # If this process in running on pythonw, we know that stdin, stdout, and
+    # stderr are all invalid.
+    redirect_out = sys.executable.endswith('pythonw.exe')
+    if redirect_out:        
+        _stdout = PIPE if stdout is None else stdout
+        _stderr = PIPE if stderr is None else stderr
+    else:
+        _stdout, _stderr = stdout, stderr 
+
     # Spawn a kernel.
     if sys.platform == 'win32':
         # Create a Win32 event for interrupting the kernel.
         interrupt_event = ParentPollerWindows.create_interrupt_event()
         arguments += [ '--interrupt=%i'%interrupt_event ]
 
-        # If this process in running on pythonw, stdin, stdout, and stderr are
-        # invalid. Popen will fail unless they are suitably redirected. We don't
-        # read from the pipes, but they must exist.
-        if sys.executable.endswith('pythonw.exe'):
-            redirect = True
-            _stdin = PIPE if stdin is None else stdin
-            _stdout = PIPE if stdout is None else stdout
-            _stderr = PIPE if stderr is None else stderr
-        else:
-            redirect = False
-            _stdin, _stdout, _stderr = stdin, stdout, stderr
-
         # If the kernel is running on pythonw and stdout/stderr are not been
         # re-directed, it will crash when more than 4KB of data is written to
         # stdout or stderr. This is a bug that has been with Python for a very
         # long time; see http://bugs.python.org/issue706263.
         # A cleaner solution to this problem would be to pass os.devnull to
         # Popen directly. Unfortunately, that does not work.
         if executable.endswith('pythonw.exe'):
             if stdout is None:
                 arguments.append('--no-stdout')
             if stderr is None:
                 arguments.append('--no-stderr')
 
         # Launch the kernel process.
         if independent:
             proc = Popen(arguments, 
                          creationflags=512, # CREATE_NEW_PROCESS_GROUP
                          stdin=_stdin, stdout=_stdout, stderr=_stderr)
         else:
             from _subprocess import DuplicateHandle, GetCurrentProcess, \
                 DUPLICATE_SAME_ACCESS
             pid = GetCurrentProcess()
             handle = DuplicateHandle(pid, pid, pid, 0, 
                                      True, # Inheritable by new processes.
                                      DUPLICATE_SAME_ACCESS)
             proc = Popen(arguments + ['--parent=%i'%int(handle)],
                          stdin=_stdin, stdout=_stdout, stderr=_stderr)
 
         # Attach the interrupt event to the Popen objet so it can be used later.
         proc.win32_interrupt_event = interrupt_event
 
-        # Clean up pipes created to work around Popen bug.
-        if redirect:
-            if stdin is None:
-                proc.stdin.close()
-            if stdout is None:
-                proc.stdout.close()
-            if stderr is None:
-                proc.stderr.close()
-
     else:
         if independent:
             proc = Popen(arguments, preexec_fn=lambda: os.setsid(),
-                         stdin=stdin, stdout=stdout, stderr=stderr)
+                         stdin=_stdin, stdout=_stdout, stderr=_stderr)
         else:
             proc = Popen(arguments + ['--parent=1'],
-                         stdin=stdin, stdout=stdout, stderr=stderr)
+                         stdin=_stdin, stdout=_stdout, stderr=_stderr)
+
+    # Clean up pipes created to work around Popen bug.
+    if redirect_in:
+        if stdin is None:
+            proc.stdin.close()
+    if redirect_out:
+        if stdout is None:
+            proc.stdout.close()
+        if stderr is None:
+            proc.stderr.close()
+            
     return proc, shell_port, iopub_port, stdin_port, hb_port