ANDROID: Additional error checking on JNI #386

1) Android 9/10 restricts access to JNI and Jave reflection API's. This adds additional checks. Commented out an possible optimization, which could be assumed to works, but does not for me so far.  Please leave this comment in, so we recall before we wonder why and try over and over again.  Once in a while we should ponder if it still does not work or why.
2) Clear Java exceptions. This enables to continue to run when an exception occured during a JNI call.  (Future versions shall forward this as an exception in Gambit.)

Author: 0-8-15

loaders/android/bootstrap.c.in

@@ -90,19 +90,37 @@ jint JNI_OnLoad(JavaVM* vm, void* reserved){
 JNIEnv* GetJNIEnv(){
   int error=0;
   JNIEnv* env = NULL;
-  if (s_vm) error=(*s_vm)->AttachCurrentThread(s_vm, &env, NULL);
-  if (!error&&(*env)->ExceptionCheck(env)) return NULL;
+  /* static `env` does NOT work!  Once in a while we should ponder if
+     it still does not work or why.
+
+  if(env) {
+    if((*env)->ExceptionCheck(env)) (*env)->ExceptionClear(env);
+    return env;
+  }
+  */
+  if(s_vm) error=(*s_vm)->AttachCurrentThread(s_vm, &env, NULL);
+  if(!error) error = JNI_forward_exception_to_gambit(env);
   return (error?NULL:env);
 }
 
+int JNI_forward_exception_to_gambit(JNIEnv*env) {
+  // TBD: actually forward, not only clear!
+  if((*env)->ExceptionCheck(env)) {
+    (*env)->ExceptionClear(env);
+    return 1;
+  }
+  return 0;
+}
+
 // url launcher ffi
 void android_launch_url(char* urlstring){
   JNIEnv *env = GetJNIEnv();
-  jstring jurlstring = (*env)->NewStringUTF(env,urlstring);
   if (env&&globalObj) {
+    jstring jurlstring = (*env)->NewStringUTF(env,urlstring);
     jclass cls = (*env)->FindClass(env, "@SYS_PACKAGE_SLASH@/@SYS_APPNAME@");
-    jmethodID method = (*env)->GetMethodID(env, cls, "openURL", "(Ljava/lang/String;)V");
-    (*env)->CallVoidMethod(env, globalObj, method, jurlstring);
+    jmethodID method = cls ? (*env)->GetMethodID(env, cls, "openURL", "(Ljava/lang/String;)V") : NULL;
+    if (method) (*env)->CallVoidMethod(env, globalObj, method, jurlstring);
+    JNI_forward_exception_to_gambit(env);
   }
 }