Merge pull request #4474 from tausbn/python-fix-tostring-divergence

Python: Fix divergence in tuple/subscripted type `toString`

python/ql/src/semmle/python/objects/Classes.qll

@@ -311,7 +311,8 @@ class SubscriptedTypeInternal extends ObjectInternal, TSubscriptedType {
   override string getName() { result = this.getGeneric().getName() }
 
   override string toString() {
-    result = this.getGeneric().toString() + "[" + this.getSpecializer().toString() + "]"
+    result =
+      bounded_toString(this.getGeneric()) + "[" + bounded_toString(this.getSpecializer()) + "]"
   }
 
   override predicate introducedAt(ControlFlowNode node, PointsToContext context) {

python/ql/src/semmle/python/objects/Instances.qll

@@ -379,7 +379,8 @@ private predicate cls_descriptor(ClassObjectInternal cls, string name, ObjectInt
 /** A class representing an instance of the `super` class */
 class SuperInstance extends TSuperInstance, ObjectInternal {
   override string toString() {
-    result = "super(" + this.getStartClass().toString() + ", " + this.getSelf().toString() + ")"
+    result =
+      "super(" + this.getStartClass().toString() + ", " + bounded_toString(this.getSelf()) + ")"
   }
 
   override boolean booleanValue() { result = true }

python/ql/src/semmle/python/objects/ObjectInternal.qll

@@ -515,7 +515,7 @@ class DecoratedFunction extends ObjectInternal, TDecoratedFunction {
   override string getName() { result = this.decoratedObject().getName() }
 
   override string toString() {
-    result = "Decorated " + this.decoratedObject().toString()
+    result = "Decorated " + bounded_toString(this.decoratedObject())
     or
     not exists(this.decoratedObject()) and result = "Decorated function"
   }
@@ -592,3 +592,18 @@ pragma[nomagic]
 predicate receiver_type(AttrNode attr, string name, ObjectInternal value, ClassObjectInternal cls) {
   PointsToInternal::pointsTo(attr.getObject(name), _, value, _) and value.getClass() = cls
 }
+
+/**
+ * Returns a string representation of `obj`. Because some classes have (mutually) recursive
+ * `toString` implementations, this predicate acts as a stop for these classes, preventing an
+ * unbounded `toString` from being materialized.
+ */
+string bounded_toString(ObjectInternal obj) {
+  if
+    obj instanceof DecoratedFunction or
+    obj instanceof TupleObjectInternal or
+    obj instanceof SubscriptedTypeInternal or
+    obj instanceof SuperInstance
+  then result = "(...)"
+  else result = obj.toString()
+}

python/ql/src/semmle/python/objects/Sequences.qll

@@ -54,10 +54,7 @@ abstract class TupleObjectInternal extends SequenceObjectInternal {
   }
 
   private string item(int n) {
-    exists(ObjectInternal item | item = this.getItem(n) |
-      // To avoid infinite recursion, nested tuples are replaced with the string "...".
-      if item instanceof TupleObjectInternal then result = "(...)" else result = item.toString()
-    )
+    result = bounded_toString(this.getItem(n))
     or
     n in [0 .. this.length() - 1] and
     not exists(this.getItem(n)) and