Option to spew subprocess streams during tests

This supersedes PR  #4268.

Run the tests with '--subproc-streams show' to show output from
subprocesses (kernels, IPython.parallel components) in the terminal, or
with '--subproc-streams discard' to send it to /dev/null. By default (or
with '--subproc-streams capture') the output is piped, captured and
displayed only when tests fail. But in some situations, a test fails
because of an error which actually occurred earlier, so you have to see
all the output.

IPython/kernel/tests/utils.py

@@ -43,11 +43,10 b' KC = None'
 def start_new_kernel(argv=None):
     """start a new kernel, and return its Manager and Client"""
     km = KernelManager()
-    kwargs = dict(stdout=nose.ipy_stream_capturer.writefd, stderr=STDOUT)
+    kwargs = dict(stdout=nose.iptest_stdstreams_fileno(), stderr=STDOUT)
     if argv:
         kwargs['extra_arguments'] = argv
     km.start_kernel(**kwargs)
-    nose.ipy_stream_capturer.ensure_started()
     kc = km.client()
     kc.start_channels()
     

IPython/parallel/tests/__init__.py

@@ -37,16 +37,15 b' class TestProcessLauncher(LocalProcessLauncher):'
     """subclass LocalProcessLauncher, to prevent extra sockets and threads being created on Windows"""
     def start(self):
         if self.state == 'before':
+            # Store stdout & stderr to show with failing tests.
+            # This is defined in IPython.testing.iptest
             self.process = Popen(self.args,
-                stdout=nose.ipy_stream_capturer.writefd, stderr=STDOUT,
+                stdout=nose.iptest_stdstreams_fileno(), stderr=STDOUT,
                 env=os.environ,
                 cwd=self.work_dir
             )
             self.notify_start(self.process.pid)
             self.poll = self.process.poll
-            # Store stdout & stderr to show with failing tests.
-            # This is defined in IPython.testing.iptest
-            nose.ipy_stream_capturer.ensure_started()
         else:
             s = 'The process was already started and has state: %r' % self.state
             raise ProcessStateError(s)

IPython/testing/iptest.py

@@ -407,14 +407,25 b' class SubprocessStreamCapturePlugin(Plugin):'
     def __init__(self):
         Plugin.__init__(self)
         self.stream_capturer = StreamCapturer()
+        self.destination = os.environ.get('IPTEST_SUBPROC_STREAMS', 'capture')
         # This is ugly, but distant parts of the test machinery need to be able
         # to redirect streams, so we make the object globally accessible.
-        nose.ipy_stream_capturer = self.stream_capturer
+        nose.iptest_stdstreams_fileno = self.get_write_fileno
+
+    def get_write_fileno(self):
+        if self.destination == 'capture':
+            self.stream_capturer.ensure_started()
+            return self.stream_capturer.writefd
+        elif self.destination == 'discard':
+            return os.open(os.devnull, os.O_WRONLY)
+        else:
+            return sys.__stdout__.fileno()
     
     def configure(self, options, config):
         Plugin.configure(self, options, config)
         # Override nose trying to disable plugin.
-        self.enabled = True
+        if self.destination == 'capture':
+            self.enabled = True
     
     def startTest(self, test):
         # Reset log capture

IPython/testing/iptestcontroller.py

@@ -237,20 +237,23 b' def prepare_controllers(options):'
     c_py = [PyTestController(name) for name in py_testgroups]
 
     configure_py_controllers(c_py, xunit=options.xunit,
-            coverage=options.coverage, extra_args=options.extra_args)
+            coverage=options.coverage, subproc_streams=options.subproc_streams,
+            extra_args=options.extra_args)
 
     controllers = c_py + c_js
     to_run = [c for c in controllers if c.will_run]
     not_run = [c for c in controllers if not c.will_run]
     return to_run, not_run
 
-def configure_py_controllers(controllers, xunit=False, coverage=False, extra_args=()):
+def configure_py_controllers(controllers, xunit=False, coverage=False,
+                             subproc_streams='capture', extra_args=()):
     """Apply options for a collection of TestController objects."""
     for controller in controllers:
         if xunit:
             controller.add_xunit()
         if coverage:
             controller.add_coverage()
+        controller.env['IPTEST_SUBPROC_STREAMS'] = subproc_streams
         controller.cmd.extend(extra_args)
 
 def do_run(controller):
@@ -469,6 +472,9 b" argparser.add_argument('--xunit', action='store_true',"
 argparser.add_argument('--coverage', nargs='?', const=True, default=False,
                     help="Measure test coverage. Specify 'html' or "
                     "'xml' to get reports.")
+argparser.add_argument('--subproc-streams', default='capture',
+                    help="What to do with stdout/stderr from subprocesses. "
+                    "'capture' (default), 'show' and 'discard' are the options.")
 
 def default_options():
     """Get an argparse Namespace object with the default arguments, to pass to