Fixed all bugs related to cancelling, pausing or unpausing a (stopped…

…) Coroutine that never yielded any Deferreds

tests.py

@@ -203,6 +203,17 @@ def fn2():
     assert retval == 'someretval'
 
 
+def test_pause_unpause_cancel_on_coroutine_with_no_depends_on():
+    @coroutine
+    def fn():
+        yield
+
+    c = fn()
+    c.pause()
+    c.unpause()
+    c.cancel()
+
+
 @contextmanager
 def recursion_limit(n):
     old = sys.getrecursionlimit()

txcoroutine/__init__.py

@@ -154,11 +154,13 @@ class Coroutine(Deferred):
     depends_on = None
 
     def pause(self):
-        self.depends_on.pause()
+        if self.depends_on:
+            self.depends_on.pause()
         return Deferred.pause(self)
 
     def unpause(self):
-        self.depends_on.unpause()
+        if self.depends_on:
+            self.depends_on.unpause()
         return Deferred.unpause(self)
 
     def cancel(self):
@@ -172,8 +174,9 @@ def cancel(self):
         # will still get it.
         swallow_cancelled_error = lambda f: f.trap(CancelledError)
 
-        self.depends_on.addErrback(swallow_cancelled_error)
-        self.depends_on.cancel()
+        if self.depends_on:
+            self.depends_on.addErrback(swallow_cancelled_error)
+            self.depends_on.cancel()
 
         self.addErrback(swallow_cancelled_error)
         Deferred.cancel(self)