utils/misc: Add redirect_streams() helper

Update a command line to redirect standard streams as specified using the
parameters. This helper allows honoring streams specified in the same way as
subprocess.Popen, by doing it as much using shell redirections as possible.

devlib/utils/misc.py

@@ -46,6 +46,11 @@ try:
 except AttributeError:
     from contextlib2 import ExitStack
 
+try:
+    from shlex import quote
+except ImportError:
+    from pipes import quote
+
 from past.builtins import basestring
 
 # pylint: disable=redefined-builtin
@@ -232,6 +237,32 @@ def walk_modules(path):
             mods.append(submod)
     return mods
 
+def redirect_streams(stdout, stderr, command):
+    """
+    Update a command to redirect a given stream to /dev/null if it's
+    ``subprocess.DEVNULL``.
+
+    :return: A tuple (stdout, stderr, command) with stream set to ``subprocess.PIPE``
+        if the `stream` parameter was set to ``subprocess.DEVNULL``.
+    """
+    def redirect(stream, redirection):
+        if stream == subprocess.DEVNULL:
+            suffix = '{}/dev/null'.format(redirection)
+        elif stream == subprocess.STDOUT:
+            suffix = '{}&1'.format(redirection)
+            # Indicate that there is nothing to monitor for stderr anymore
+            # since it's merged into stdout
+            stream = subprocess.DEVNULL
+        else:
+            suffix = ''
+
+        return (stream, suffix)
+
+    stdout, suffix1 = redirect(stdout, '>')
+    stderr, suffix2 = redirect(stderr, '2>')
+
+    command = 'sh -c {} {} {}'.format(quote(command), suffix1, suffix2)
+    return (stdout, stderr, command)
 
 def ensure_directory_exists(dirpath):
     """A filter for directory paths to ensure they exist."""