unified: Add consistency exclusion, but record the miss in a test case

Author: asgerf

unified/ql/consistency-queries/DataFlowConsistency.ql

@@ -6,6 +6,12 @@ module ConsistencyInput implements InputSig<Location, DataFlowInput> {
   predicate argHasPostUpdateExclude(DataFlowInput::ArgumentNode n) {
     not exists(n.getBasicBlock()) // ignore unreachable data flow nodes
   }
+
+  predicate reverseReadExclude(DataFlow::Node n) {
+    // When read steps are contributed by a language plugin we currently don't expect them to
+    // have post-update nodes for reverse-reads.
+    any(DataFlowPlugin p).step(n, any(Step s | s.read(_)), _)
+  }
 }
 
 module ConsistencyOutput =

unified/ql/test/library-tests/controlflow/CONSISTENCY/DataFlowConsistency.expected

@@ -1,3 +0,0 @@
-reverseRead
-| cfg.swift:130:13:130:15 | opt | Origin of readStep is missing a PostUpdateNode. |
-| cfg.swift:131:13:131:15 | opt | Origin of readStep is missing a PostUpdateNode. |

unified/ql/test/library-tests/dataflow/test.swift

@@ -221,3 +221,10 @@ func t25() {
     }
 
 }
+
+func t26() {
+    // TODO: This flow requires reverse-read through the "!" operator
+    var opt = Optional.some(("x", "y"))
+    opt!.0 = source("t26.1")
+    sink(opt!.0)  // $ MISSING: hasValueFlow=t26.1
+}