Merge pull request #4099 from hvitved/java/dataflow/unique-encl-callable

Java: Use `unique` aggregate in `Node::getEnclosingCallable()`

Author: aschackmull

docs/ql-libraries/dataflow/dataflow.md

@@ -98,6 +98,10 @@ Recommendations:
   also work, but the upside of `use-use` steps is that sources defined in terms
   of variable reads just work out of the box. It also makes certain
   barrier-implementations simpler.
+* A predicate `DataFlowCallable Node::getEnclosingCallable()` is required, and in
+  order to ensure appropriate join-orders, it is important that the QL compiler knows
+  that this predicate is functional. It can therefore be necessary to enclose the body
+  of this predicate in a `unique` aggregate.
 
 The shared library does not use `localFlowStep` nor `localFlow` but users of
 `DataFlow.qll` may expect the existence of `DataFlow::localFlowStep` and

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

@@ -78,15 +78,19 @@ class Node extends TNode {
     result = this.(ImplicitPostUpdateNode).getPreUpdateNode().getType()
   }
 
-  /** Gets the callable in which this node occurs. */
-  Callable getEnclosingCallable() {
+  private Callable getEnclosingCallableImpl() {
     result = this.asExpr().getEnclosingCallable() or
     result = this.asParameter().getCallable() or
     result = this.(ImplicitVarargsArray).getCall().getEnclosingCallable() or
     result = this.(InstanceParameterNode).getCallable() or
     result = this.(ImplicitInstanceAccess).getInstanceAccess().getEnclosingCallable() or
     result = this.(MallocNode).getClassInstanceExpr().getEnclosingCallable() or
-    result = this.(ImplicitPostUpdateNode).getPreUpdateNode().getEnclosingCallable()
+    result = this.(ImplicitPostUpdateNode).getPreUpdateNode().getEnclosingCallableImpl()
+  }
+
+  /** Gets the callable in which this node occurs. */
+  Callable getEnclosingCallable() {
+    result = unique(DataFlowCallable c | c = this.getEnclosingCallableImpl() | c)
   }
 
   private Type getImprovedTypeBound() {