Python: Add attributeClearStep

Author: RasmusWL

python/ql/lib/semmle/python/dataflow/new/internal/DataFlowPrivate.qll

@@ -1973,6 +1973,18 @@ predicate clearsContent(Node n, Content c) {
   kwOverflowClearStep(n, c)
   or
   matchClearStep(n, c)
+  or
+  attributeClearStep(n, c)
+}
+
+/**
+ * Holds if values stored inside attribute `c` are cleared at node `n`.
+ *
+ * In `obj.foo = x` the any old value stored in `foo` is cleared at the pre-update node
+ * associated with `obj`
+ */
+predicate attributeClearStep(Node n, AttributeContent c) {
+  exists(PostUpdateNode post | post.getPreUpdateNode() = n | attributeStoreStep(_, c, post))
 }
 
 //--------

python/ql/test/experimental/dataflow/fieldflow/test.py

@@ -67,15 +67,15 @@ def test_direct_assign_overwrite():
     myobj = MyObj(NONSOURCE)
     myobj.foo = SOURCE
     myobj.foo = NONSOURCE
-    SINK_F(myobj.foo) # $ SPURIOUS: flow="SOURCE, l:-2 -> myobj.foo"
+    SINK_F(myobj.foo)
 
 
 def test_direct_if_assign(cond = False):
     myobj = MyObj(NONSOURCE)
     myobj.foo = SOURCE
     if cond:
         myobj.foo = NONSOURCE
-        SINK_F(myobj.foo) # $ SPURIOUS: flow="SOURCE, l:-3 -> myobj.foo"
+        SINK_F(myobj.foo)
     SINK(myobj.foo) # $ flow="SOURCE, l:-4 -> myobj.foo"
 
 
@@ -85,11 +85,11 @@ def test_direct_if_always_assign(cond = True):
     myobj.foo = SOURCE
     if cond:
         myobj.foo = NONSOURCE
-        SINK_F(myobj.foo) # $ SPURIOUS: flow="SOURCE, l:-3 -> myobj.foo"
+        SINK_F(myobj.foo)
     else:
         myobj.foo = NONSOURCE
-        SINK_F(myobj.foo) # $ SPURIOUS: flow="SOURCE, l:-6 -> myobj.foo"
-    SINK_F(myobj.foo) # $ SPURIOUS: flow="SOURCE, l:-7 -> myobj.foo"
+        SINK_F(myobj.foo)
+    SINK_F(myobj.foo)
 
 
 def test_getattr():