Merge pull request #143 from valschneider/invoke_bg

Add option to run target.invoke() in the background
diff --git a/devlib/target.py b/devlib/target.py
index f3aaea0..65d5600 100644
--- a/devlib/target.py
+++ b/devlib/target.py
@@ -351,6 +351,38 @@
             command = 'cd {} && {}'.format(in_directory, command)
         return self.execute(command, as_root=as_root, timeout=timeout)
 
+    def background_invoke(self, binary, args=None, in_directory=None,
+                          on_cpus=None, as_root=False):
+        """
+        Executes the specified binary as a background task under the
+        specified conditions.
+
+        :binary: binary to execute. Must be present and executable on the device.
+        :args: arguments to be passed to the binary. The can be either a list or
+               a string.
+        :in_directory:  execute the binary in the  specified directory. This must
+                        be an absolute path.
+        :on_cpus:  taskset the binary to these CPUs. This may be a single ``int`` (in which
+                   case, it will be interpreted as the mask), a list of ``ints``, in which
+                   case this will be interpreted as the list of cpus, or string, which
+                   will be interpreted as a comma-separated list of cpu ranges, e.g.
+                   ``"0,4-7"``.
+        :as_root: Specify whether the command should be run as root
+
+        :returns: the subprocess instance handling that command
+        """
+        command = binary
+        if args:
+            if isiterable(args):
+                args = ' '.join(args)
+            command = '{} {}'.format(command, args)
+        if on_cpus:
+            on_cpus = bitmask(on_cpus)
+            command = '{} taskset 0x{:x} {}'.format(self.busybox, on_cpus, command)
+        if in_directory:
+            command = 'cd {} && {}'.format(in_directory, command)
+        return self.background(command, as_root=as_root)
+
     def kick_off(self, command, as_root=False):
         raise NotImplementedError()
 
diff --git a/doc/target.rst b/doc/target.rst
index d14942e..5339a72 100644
--- a/doc/target.rst
+++ b/doc/target.rst
@@ -265,6 +265,24 @@
    :param timeout: If this is specified and invocation does not terminate within this number 
            of seconds, an exception will be raised.
 
+.. method:: Target.background_invoke(binary [, args [, in_directory [, on_cpus [, as_root ]]]])
+
+   Execute the specified binary on target (must already be installed) as a background
+   task, under the specified conditions and return the :class:`subprocess.Popen`
+   instance for the command.
+
+   :param binary: binary to execute. Must be present and executable on the device.
+   :param args: arguments to be passed to the binary. The can be either a list or
+          a string.
+   :param in_directory:  execute the binary in the  specified directory. This must
+                   be an absolute path.
+   :param on_cpus:  taskset the binary to these CPUs. This may be a single ``int`` (in which
+          case, it will be interpreted as the mask), a list of ``ints``, in which
+          case this will be interpreted as the list of cpus, or string, which
+          will be interpreted as a comma-separated list of cpu ranges, e.g.
+          ``"0,4-7"``.
+   :param as_root: Specify whether the command should be run as root
+
 .. method:: Target.kick_off(command [, as_root])
 
    Kick off the specified command on the target and return immediately. Unlike