JS: Add helper library for call graph exploration

Author: asgerf

javascript/ql/src/semmle/javascript/explore/CallGraph.qll

@@ -0,0 +1,85 @@
+/**
+ * Provides predicates for visualizing the call paths leading to or from a specific function.
+ *
+ * It defines three predicates: `callEdge`, `isStartOfCallPath` and `isEndOfCallPath`,
+ * as well as the `nodes` and `edges` predicates needed for a path problem query.
+ *
+ * To use this library, make sure the query has `@kind path-problem`
+ * and selects columns appropriate for a path problem query.
+ * For example:
+ * ```
+ * import javascript
+ * import semmle.javascript.explore.CallGraph
+ * import DataFlow
+ *
+ * from InvokeNode invoke, FunctionNode function
+ * where callEdge*(invoke, function)
+ *   and isStartOfCallPath(invoke)
+ *   and function.getName() = "targetFunction"
+ * select invoke, invoke, function, "Call path to 'targetFunction'"
+ * ```
+ */
+import javascript
+private import DataFlow
+
+/**
+ * Holds if `pred -> succ` is an edge in the call graph.
+ *
+ * There are edges from calls to their callees,
+ * and from functions to their contained calls and in some cases
+ * their inner functions.
+ */
+predicate callEdge(Node pred, Node succ) {
+    exists(InvokeNode invoke, Function f |
+      invoke.getACallee() = f and
+      pred = invoke and
+      succ = f.flow()
+      or
+      invoke.getContainer() = f and
+      pred = f.flow() and
+      succ = invoke
+    )
+    or
+    exists(Function inner, Function outer |
+      inner.getEnclosingContainer() = outer and
+      not inner = outer.getAReturnedExpr() and
+      pred = outer.flow() and
+      succ = inner.flow()
+    )
+}
+
+/** Holds if `pred -> succ` is an edge in the call graph. */
+query predicate edges = callEdge/2;
+
+/** Holds if `node` is part of the call graph. */
+query predicate nodes(Node node) {
+    node instanceof InvokeNode or
+    node instanceof FunctionNode
+}
+
+/** Gets a call in a function that has no known call sites. */
+private InvokeNode rootCall() {
+  not any(InvokeNode i).getACallee() = result.getContainer()
+}
+
+/**
+ * Holds if `invoke` should be used as the starting point of a call path.
+ */
+predicate isStartOfCallPath(InvokeNode invoke) {
+  // `invoke` should either be a root call or be part of a cycle with no root.
+  // An equivalent requirement is that `invoke` is not reachable from a root.
+  not callEdge+(rootCall(), invoke)
+}
+
+/** Gets a function contains no calls to other functions. */
+private FunctionNode leafFunction() {
+  not callEdge(result, _)
+}
+
+/**
+ * Holds if `invoke` should be used as the starting point of a call path.
+ */
+predicate isEndOfCallPath(FunctionNode fun) {
+  // `fun` should either be a leaf function or part of a cycle with no leaves.
+  not callEdge+(fun, leafFunction())
+}