Update log_output to handle cases where sys.stdout/stderr streams do not have

an associated file descriptor (e.g. holds for nose tests where sys.stdout is
assigned to a StringIO object)

lib/spack/llnl/util/tty/log.py

@@ -122,6 +122,10 @@ def __init__(self, stream, echo=False, force_color=False, debug=False):
         self.force_color = force_color
         self.debug = debug
 
+        # Default is to try file-descriptor reassignment unless the system 
+        # out/err streams do not have an associated file descriptor
+        self.directAssignment = False
+
     def trace(self, frame, event, arg):
         """Jumps to __exit__ on the child process."""
         raise _SkipWithBlock()
@@ -185,13 +189,21 @@ def __enter__(self):
             # Child: redirect output, execute the with block.
             os.close(read)
 
-            # Save old stdout and stderr
-            self._stdout = os.dup(sys.stdout.fileno())
-            self._stderr = os.dup(sys.stderr.fileno())
+            try:
+                # Save old stdout and stderr
+                self._stdout = os.dup(sys.stdout.fileno())
+                self._stderr = os.dup(sys.stderr.fileno())
 
-            # redirect to the pipe.
-            os.dup2(write, sys.stdout.fileno())
-            os.dup2(write, sys.stderr.fileno())
+                # redirect to the pipe.
+                os.dup2(write, sys.stdout.fileno())
+                os.dup2(write, sys.stderr.fileno())
+            except AttributeError:
+                self.directAssignment = True
+                self._stdout = sys.stdout
+                self._stderr = sys.stderr
+                output_redirect = os.fdopen(write, 'w')
+                sys.stdout = output_redirect
+                sys.stderr = output_redirect
 
             if self.force_color:
                 color._force_color = True
@@ -218,8 +230,12 @@ def __exit__(self, exc_type, exception, traceback):
                 #
                 # TODO: think about how this works outside install.
                 # TODO: ideally would propagate exception to parent...
-                os.dup2(self._stdout, sys.stdout.fileno())
-                os.dup2(self._stderr, sys.stderr.fileno())
+                if self.directAssignment:
+                    sys.stdout = self._stdout
+                    sys.stderr = self._stderr
+                else:
+                    os.dup2(self._stdout, sys.stdout.fileno())
+                    os.dup2(self._stderr, sys.stderr.fileno())                    
 
                 return False