Do not allow to execute empty lists. Fix serious bug leading to an infinite loop when iterating through a stack trace.

Author: objecthub

Sources/LispKit/Compiler/Compiler.swift

@@ -683,6 +683,8 @@ public final class Compiler {
                                          inTailPos tail: Bool) throws -> Bool {
     let cp = self.checkpointer.checkpoint()
     switch expr {
+      case .null:
+        throw RuntimeError.eval(.executeEmptyList)
       case .symbol(let sym):
         try self.pushValueOf(sym, in: env)
       case .pair(.symbol(let sym), let cdr):

Sources/LispKit/Compiler/EvalError.swift

@@ -54,6 +54,7 @@ public enum EvalError: Int, Hashable {
   case notBoundInEnvironment
   case bindingImmutable
   case nonApplicativeValue
+  case executeEmptyList
   case illegalRadix
   case argumentError
   case noMatchingCase
@@ -221,6 +222,8 @@ public enum EvalError: Int, Hashable {
         return "unable to assign value to immutable binding for symbol $0"
       case .nonApplicativeValue:
         return "cannot apply arguments to $0"
+      case .executeEmptyList:
+        return "cannot execute empty application"
       case .illegalRadix:
         return "illegal radix: $0"
       case .argumentError:

Sources/LispKit/Runtime/VirtualMachine.swift

@@ -559,7 +559,7 @@ public final class VirtualMachine: ManagedObject {
       }
       stackTrace.append(proc)
       if fp > 2 {
-        guard case .fixnum(let newfp) = self.stack[fp &- 3] else {
+        guard case .fixnum(let newfp) = self.stack[fp &- 3], Int(newfp) < fp else {
           // This may happen if an error is thrown
           return stackTrace
         }