Calling from C++ into Java with local_ref may crash

[shelefg]

Issue description

If a C++ call is made to a Java method whose reference is obtained by getMethod with local_ref arguments types, and that call throws an exception, it causes the process to crash. fbjni does not prevent specifying Java methods descriptors arguments in terms of local_ref.

This will only happen in debug builds because the crash is caused by a call to verifyReference in local_ref dtor, that is made inside an assert statement. verifyReference calls GetObjectRefType which is not allowed when there's a pending Java exception.

For release builds there is no issue because the actual JNI call to destroy a reference (DeleteLocalRef) is safe to call when there is a pending Java exception.

Code example

class JFoo : public facebook::jni::JavaClass<JFoo> {
  public:
  static constexpr auto kJavaDescriptor = "Lcom/example/Foo;";

  static void bar(facebook::jni::local_ref<JObject> jObj) {
    static auto method = javaClassStatic()->
                    getMethod<void(facebook::jni::local_ref<JObject>)>(
                    "bar");

    method(javaClassStatic(), jObj);
  }
};

package com.example
object Foo {
  @JvmStatic
  void bar(obj: Any) {
    throw Exception()
  }
}

Now a call such as this will crash:

JFoo::bar(facebook::jni::make_jstring(""));

[dreiss]

Thanks for the very clear bug report. If you change the getMethod call to getMethod<void(JObject)>, does that fix the issue?

[shelefg]

The following constructs seem to work:

getMethod<void(facebook::jni::alias_ref<JObject>)>

and

getMethod<void(JObject::javaobject)> 

The second requires calling get() on the jObj reference:

method(javaClassStatic(), jObj.get());

I think I saw both of these constructs used in React Native (which I used as an fbjni reference).