Class `FieldBackedProvider` uses ByteBuddy to add a field and its respective getters and setters to store the context object.
Assuming that we have a class `A` that implements runnable and that we wrap with a `FieldBackedProvider`
`Runnable` interfaces, if later on we use a cglib's `Enancher` class on `A` then the our mechanism will kick in again and try
to add the methods again. This causes a `java.lang.ClassFormatError: Duplicate method name "get__datadogContext$java$lang$Runnable" with signature "()Ljava.lang.Object;"`
to be thrown because CGLIB already copied over those methods from the original class `A` to the newly created class.
With this commit we now check that method were not previously defined before adding them, and if they were then we avoid adding them
again.
The reason why it wasn't faiing before is that we only checked on context field existence, not methods and cglib do not copy over fields, it copies only methods. E.g.
```
public class Main {
public static class A {
private String name = "hey";
public String getName() {
return this.name;
}
}
public static void main(String[] args) {
System.out.println("----- 'A' declared fields -----");
A s = new A();
for (Field f : s.getClass().getDeclaredFields()) {
System.out.println("field: " + f.getName());
}
for (Method m : s.getClass().getDeclaredMethods()) {
System.out.println("method: " + m.getName());
}
System.out.println("----- Proxy declared fields -----");
Enhancer enhancer = new Enhancer();
enhancer.setSuperclass(A.class);
enhancer.setCallback((FixedValue) () -> null);
A proxy = (A) enhancer.create();
for (Field f : proxy.getClass().getDeclaredFields()) {
System.out.println("field: " + f.getName());
}
for (Method m : proxy.getClass().getDeclaredMethods()) {
System.out.println("method: " + m.getName());
}
}
}
```
Results in:
```
----- 'A' declared fields -----
field: name
method: getName
----- Proxy declared fields -----
field: CGLIB$BOUND
field: CGLIB$FACTORY_DATA
field: CGLIB$THREAD_CALLBACKS
field: CGLIB$STATIC_CALLBACKS
field: CGLIB$CALLBACK_0
field: CGLIB$CALLBACK_FILTER
method: equals
method: toString
method: hashCode
method: clone
method: getName
method: newInstance
method: newInstance
method: newInstance
method: setCallbacks
method: CGLIB$SET_STATIC_CALLBACKS
method: CGLIB$SET_THREAD_CALLBACKS
method: getCallback
method: getCallbacks
method: CGLIB$STATICHOOK1
method: CGLIB$BIND_CALLBACKS
method: setCallback
```
Tyler Benson
Luca Abbati
Nikolay Martynov