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

IPython/zmq/entry_point.py

@@ -85,30 +85,37 b' def base_launch_kernel(code, shell_port=0, iopub_port=0, stdin_port=0, hb_port=0'
         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
@@ -139,20 +146,22 b' def base_launch_kernel(code, shell_port=0, iopub_port=0, stdin_port=0, hb_port=0'
         # 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