Forbid stream self-dependency.

HISTORY.rst

@@ -6,6 +6,8 @@ Changelog
 
 **API Changes**
 
+- Throw ``PriorityLoop`` when inserting or reprioritising a stream that
+  depends on itself.
 - Throw ``BadWeightError`` when creating or reprioritising a stream with a
   weight that is not an integer between 1 and 256, inclusive.
 - Throw ``PseudoStreamError`` when trying to reprioritise, remove, block or

src/priority/priority.py

@@ -348,6 +348,10 @@ def insert_stream(self,
 
         if not depends_on:
             depends_on = 0
+        elif depends_on == stream_id:
+            raise PriorityLoop(
+                "Stream %d must not depend on itself." % stream_id
+            )
 
         if exclusive:
             parent_stream = self._get_or_insert_parent(depends_on)
@@ -389,6 +393,11 @@ def reprioritize(self,
         # own dependents. Then, we remove this stream from its current parent
         # and move it to its new parent, taking its children with it.
         if depends_on:
+            if depends_on == stream_id:
+                raise PriorityLoop(
+                    "Stream %d must not depend on itself" % stream_id
+                )
+
             new_parent = self._get_or_insert_parent(depends_on)
             cycle = _stream_cycle(new_parent, current_stream)
         else:

test/test_priority.py

@@ -455,6 +455,31 @@ def test_stream_with_out_of_bounds_weight_is_error(self, weight):
             err.value.args[0] ==
             'Stream weight must be between 1 and 256 (inclusive)')
 
+    @pytest.mark.parametrize('exclusive', (True, False))
+    @pytest.mark.parametrize('stream_id', (1, 5, 20, 32, 256))
+    def test_stream_depending_on_self_is_error(self, stream_id, exclusive):
+        """
+        Inserting a stream that is dependent on itself is rejected.
+        """
+        p = priority.PriorityTree()
+        with pytest.raises(priority.PriorityLoop):
+            p.insert_stream(
+                stream_id=stream_id, depends_on=stream_id, exclusive=exclusive
+            )
+
+    @pytest.mark.parametrize('exclusive', (True, False))
+    @pytest.mark.parametrize('stream_id', (1, 5, 20, 32, 256))
+    def test_reprioritize_depend_on_self_is_error(self, stream_id, exclusive):
+        """
+        Reprioritizing a stream to make it dependent on itself is an error.
+        """
+        p = priority.PriorityTree()
+        p.insert_stream(stream_id=stream_id)
+        with pytest.raises(priority.PriorityLoop):
+            p.reprioritize(
+                stream_id=stream_id, depends_on=stream_id, exclusive=exclusive
+            )
+
 
 class TestPriorityTreeOutput(object):
     """