Allow to pass None as a timeout value to disable timeout logic

In some cases there is needed to disable timeout logic but keep it in
convenient code structure without ugly if-branching.

aiohttp/helpers.py

@@ -460,7 +460,7 @@ class Timeout:
     ...         await r.text()
 
 
-    :param timeout: timeout value in seconds
+    :param timeout: timeout value in seconds or None to disable timeout logic
     :param loop: asyncio compatible event loop
     """
     def __init__(self, timeout, *, loop=None):
@@ -477,17 +477,19 @@ def __enter__(self):
         if self._task is None:
             raise RuntimeError('Timeout context manager should be used '
                                'inside a task')
-        self._cancel_handler = self._loop.call_later(
-            self._timeout, self._cancel_task)
+        if self._timeout:
+            self._cancel_handler = self._loop.call_later(
+                self._timeout, self._cancel_task)
         return self
 
     def __exit__(self, exc_type, exc_val, exc_tb):
         if exc_type is asyncio.CancelledError and self._cancelled:
             self._cancel_handler = None
             self._task = None
             raise asyncio.TimeoutError
-        self._cancel_handler.cancel()
-        self._cancel_handler = None
+        if self._timeout:
+            self._cancel_handler.cancel()
+            self._cancel_handler = None
         self._task = None
 
     def _cancel_task(self):

tests/test_timeout.py

@@ -50,6 +50,19 @@ def run():
     loop.run_until_complete(run())
 
 
+@pytest.mark.parametrize("delay", [None, 0])
+@pytest.mark.run_loop
+def test_timeout_disable(loop, delay):
+    @asyncio.coroutine
+    def long_running_task():
+        yield from asyncio.sleep(0.1, loop=loop)
+        return 'done'
+
+    with Timeout(delay, loop=loop):
+        resp = yield from long_running_task()
+    assert resp == 'done'
+
+
 @pytest.mark.run_loop
 def test_timeout_not_relevant_exception(loop):
     yield from asyncio.sleep(0, loop=loop)