Add a coroutine_wrapper decorator

This decorator checks for the coroutine=True keyword arg and wraps the
result of the function call in a coroutine if it isn’t.

This allows to have constructs like:

@coroutine_wrapper
def toto(xmpp, *, coroutine=False):
    if xmpp.cached:
        return xmpp.cached
    else:
        return xmpp.make_iq_get().send(coroutine=coroutine)

@asyncio.coroutine
def main(xmpp):
    result = yield from toto(xmpp, coroutine=True)
    xmpp.cached = result
    result2 = yield from toto(xmpp, coroutine=True)

If the wrapper wasn’t there, the second fetch would fail. This decorator
does not do anything if the coroutine argument is False.

slixmpp/xmlstream/asyncio.py

@@ -8,6 +8,7 @@ call_soon() ones. These callback are called only once each.
 
 import asyncio
 from asyncio import events
+from functools import wraps
 
 import collections
 
@@ -32,3 +33,23 @@ cls.idle_call = idle_call
 real_run_once = cls._run_once
 cls._run_once = my_run_once
 
+
+def coroutine_wrapper(func):
+    """
+    Make sure the result of a function call is a coroutine
+    if the ``coroutine`` keyword argument is true.
+    """
+    def wrap_coro(result):
+        if asyncio.iscoroutinefunction(result):
+            return result
+        else:
+            return asyncio.coroutine(lambda: result)()
+
+    @wraps(func)
+    def wrapper(*args, **kwargs):
+        if kwargs.get('coroutine', False):
+            return wrap_coro(func(*args, **kwargs))
+        else:
+            return func(*args, **kwargs)
+
+    return wrapper