JS: Add global post-update steps

javascript/ql/lib/semmle/javascript/dataflow/internal/DataFlowPrivate.qll

@@ -671,6 +671,8 @@ private predicate valuePreservingStep(Node node1, Node node2) {
   or
   FlowSteps::globalFlowStep(node1, node2)
   or
+  FlowSteps::globalPostUpdateStep(node1, node2)
+  or
   node2 = FlowSteps::getThrowTarget(node1)
   or
   FlowSummaryImpl::Private::Steps::summaryLocalStep(node1.(FlowSummaryNode).getSummaryNode(),

javascript/ql/lib/semmle/javascript/dataflow/internal/FlowSteps.qll

@@ -383,6 +383,14 @@ private module CachedSteps {
     result = DataFlow::valueNode(gv.getAnAccess())
   }
 
+  /**
+   * Gets a post-update of `gv` in `f`.
+   */
+  pragma[noinline]
+  private DataFlow::ExprPostUpdateNode getAPostUpdateIn(GlobalVariable gv, File f) {
+    result.getPreUpdateNode() = getAUseIn(gv, f)
+  }
+
   /**
    * Holds if there is a flow step from `pred` to `succ` through a global
    * variable. Both `pred` and `succ` must be in the same file.
@@ -395,6 +403,20 @@ private module CachedSteps {
     )
   }
 
+  /**
+   * Holds if `pred` is a post-update node for a use of a global variable, and `succ`
+   * is a use of the global variable in the same file.
+   */
+  cached
+  predicate globalPostUpdateStep(DataFlow::Node pred, DataFlow::Node succ) {
+    exists(GlobalVariable gv, File f |
+      pred = getAPostUpdateIn(gv, f) and
+      succ = getAUseIn(gv, f) and
+      // Remove some unnecessary steps
+      not succ = any(DataFlow::PropWrite write).getBase()
+    )
+  }
+
   /**
    * Holds if there is a write to property `prop` of global variable `gv`
    * in file `f`, where the right-hand side of the write is `rhs`.
@@ -438,6 +460,7 @@ private module CachedSteps {
   predicate basicStoreStep(DataFlow::Node pred, DataFlow::Node succ, string prop) {
     succ.(DataFlow::SourceNode).hasPropertyWrite(prop, pred)
     or
+    // Note that this case is handled by globalPostUpdateStep in dataflow2
     exists(GlobalVariable gv, File f |
       globalPropertyWrite(gv, f, prop, pred) and
       globalPropertyRead(gv, f, prop, succ)