Add PrintDfg.qll

Adding this directly in the DataFlow module would expose it publicly via the `DataFlow::` prefix which does not seem desirable. We just want to be able to access it ourselves, so I've put it in its own file.

Author: asgerf

shared/dataflow/codeql/dataflow/DataFlow.qll

@@ -361,6 +361,12 @@ signature module InputSig<LocationSig Location> {
    * visible.
    */
   default predicate isEvaluatingInOverlay() { none() }
+
+  /**
+   * Gets a string to distinguish nodes that have the same location and toString value,
+   * for use when generating graphs with `PrintDfg.qll`.
+   */
+  default string nodeGetOrderDisambiguation(Node node) { result = "" }
 }
 
 module Configs<LocationSig Location, InputSig<Location> Lang> {

shared/dataflow/codeql/dataflow/PrintDfg.qll

@@ -0,0 +1,45 @@
+/**
+ * Provides a module for implementing the `View DFG` query based on inputs to the data flow library.
+ */
+
+private import codeql.util.Location
+private import codeql.dataflow.DataFlow as DF
+private import codeql.dataflow.TaintTracking as TT
+
+module MakePrintDfg<
+  LocationSig Location, DF::InputSig<Location> DataFlowLang,
+  TT::InputSig<Location, DataFlowLang> TaintTrackingLang>
+{
+  private import DataFlowLang
+  private import codeql.util.PrintGraph as Pp
+
+  final private class FinalNode = Node;
+
+  private module PrintGraphInput implements Pp::InputSig<Location> {
+    class Callable = DataFlowLang::DataFlowCallable;
+
+    class Node extends FinalNode {
+      string getOrderDisambiguation() { result = DataFlowLang::nodeGetOrderDisambiguation(this) }
+
+      Callable getEnclosingCallable() { result = DataFlowLang::nodeGetEnclosingCallable(this) }
+    }
+
+    predicate edge(Node node1, string label, Node node2) {
+      simpleLocalFlowStep(node1, node2, _) and label = "value"
+      or
+      jumpStep(node1, node2) and label = "jump"
+      or
+      TaintTrackingLang::defaultAdditionalTaintStep(node1, node2, _) and label = "taint"
+      or
+      exists(ContentSet c |
+        readStep(node1, c, node2) and label = "read[" + c.toString() + "]"
+        or
+        storeStep(node1, c, node2) and label = "store[" + c.toString() + "]"
+      )
+      or
+      node1 = node2.(PostUpdateNode).getPreUpdateNode() and label = "post-update"
+    }
+  }
+
+  import Pp::PrintGraph<Location, PrintGraphInput>
+}