CtMethod.insertBefore messes up stack map when inserting unconditional "return null;"

[kriegaex]

I think this issue is similar to fixed issue #328. I originally found the problem description on StackOverflow, please also read the related discussion.

Here is an MCVE reproducing the problem with both 3.27.0-GA and latest master 1c4e31b:
https://github.com/kriegaex/SO_Javassist_VerifyErrorReplacingMethodReturnValue_64561436

The problem is inserting return null; into a method by insertBefore(). This fails in some cases, causing a VerifyError. A workaround is to insert if (true) return null; instead, then the error goes away. See here:

  private void replaceReturnValue(CtClass targetClass) throws CannotCompileException {
    // Fails in Javassist without ASM stack map repair
    String injectedCode = "return null;";
    // Works in Javassist without ASM stack map repair
    // String injectedCode = "if (true) return null;";

    for (CtMethod ctMethod : targetClass.getDeclaredMethods()) {
      if (LOG_RETURN_VALUE_REPLACER) {
        log("Replacing return value for method " + ctMethod.getLongName());
        log("Injected code: " + injectedCode);
      }
      ctMethod.insertBefore(injectedCode);
    }
  }

In some method bodies it works, e.g.

  public String greet_JavassistWorks1(String recipient) {
    return "Hello " + recipient + "!";
  }

  public String greet_JavassistWorks2(String recipient) {
    String prefix = "Hello ";
    String postfix = "!";
    return prefix + recipient + postfix;
  }

  public String greet_JavassistWorks3(String recipient) {
    if (new Random().nextBoolean()) {
      String prefix = "Hello ";
      String postfix = "!";
      return prefix + recipient + postfix;
    }
    else {
      return "Lazy to greet today...";
    }
  }

This one fails, though:

  public String greet_JavassistFails(String recipient) {
    if (new Random().nextBoolean()) {
      String prefix = "Hello ";
      String postfix = "!";
      return prefix + recipient + postfix;
    }
    else {
      String greeting = "Lazy to greet today...";
      return greeting;
    }
  }

See class TargetClass.

If I repair the broken class Javassist produces via ASM, setting REPAIR_STACK_MAP_USING_ASM = true; in class JavassistMethodReturnValueReplacer, it works.

If you also set DUMP_CLASS_FILES = true, the sample program will output both the Javassist-generated, unrepaired class file and the ASM-repaired version. You can compare them using a byte code viewer and will notice differences in stack map frame generation, similar to #328.