Java: Implement clearsContent()

Author: hvitved

java/ql/src/semmle/code/java/dataflow/internal/DataFlowPrivate.qll

@@ -195,6 +195,15 @@ predicate readStep(Node node1, Content f, Node node2) {
   )
 }
 
+/**
+ * Holds if values stored inside content `c` are cleared at node `n`. For example,
+ * any value stored inside `f` is cleared at the pre-update node associated with `x`
+ * in `x.f = newValue`.
+ */
+predicate clearsContent(Node n, Content c) {
+  n = any(PostUpdateNode pun | storeStep(_, c, pun)).getPreUpdateNode()
+}
+
 /**
  * Gets a representative (boxed) type for `t` for the purpose of pruning
  * possible flow. A single type is used for all numeric types to account for

java/ql/test/library-tests/dataflow/fields/F.java

@@ -13,12 +13,12 @@ private void m() {
     f.Field2 = o;
     f.Field2 = null;
     sink(f.Field1); // flow
-    sink(f.Field2); // no flow [FALSE POSITIVE]
+    sink(f.Field2); // no flow
 
     f = new F();
     f.Field2 = null;
     sink(f.Field1); // flow
-    sink(f.Field2); // no flow [FALSE POSITIVE]
+    sink(f.Field2); // no flow
 
     f = new F();
     o = new Object();
@@ -31,7 +31,7 @@ private void m2(F f)
   {
     f.Field2 = null;
     sink(f.Field1); // flow
-    sink(f.Field2); // no flow [FALSE POSITIVE]
+    sink(f.Field2); // no flow
   }
 
   public static void sink(Object o) { }

java/ql/test/library-tests/dataflow/fields/flow.expected

@@ -26,13 +26,6 @@
 | E.java:2:32:2:43 | new Object(...) | E.java:21:10:21:24 | bh2.buf.content |
 | E.java:2:32:2:43 | new Object(...) | E.java:24:10:24:28 | p2.data.buf.content |
 | E.java:2:32:2:43 | new Object(...) | E.java:30:10:30:27 | p.data.buf.content |
-| F.java:5:14:5:25 | new Object(...) | F.java:15:10:15:17 | f.Field1 |
 | F.java:5:14:5:25 | new Object(...) | F.java:20:10:20:17 | f.Field1 |
-| F.java:5:14:5:25 | new Object(...) | F.java:33:10:33:17 | f.Field1 |
-| F.java:6:14:6:25 | new Object(...) | F.java:16:10:16:17 | f.Field2 |
-| F.java:6:14:6:25 | new Object(...) | F.java:21:10:21:17 | f.Field2 |
-| F.java:6:14:6:25 | new Object(...) | F.java:34:10:34:17 | f.Field2 |
 | F.java:10:16:10:27 | new Object(...) | F.java:15:10:15:17 | f.Field1 |
-| F.java:10:16:10:27 | new Object(...) | F.java:16:10:16:17 | f.Field2 |
 | F.java:24:9:24:20 | new Object(...) | F.java:33:10:33:17 | f.Field1 |
-| F.java:24:9:24:20 | new Object(...) | F.java:34:10:34:17 | f.Field2 |