Discussion about multi-interceptors supporting in SkyWalking

[daimingzhi]

• Discussion : multi-interceptors supporting in SkyWalking

As I have mentioned in https://github.com/apache/skywalking/issues/6932#issuecomment-895825310。

For extensibility of SkyWaling, we should support multi-interceptors。

I have two ways of doing this：

the first one

We can construct multiple ClassFileTransformer instances. In each ClassFileTransformer, there will only be one pluginDefine for each class.To do this, we should group the plugindefine,and we will construct a ClassFileTransformer instance for each group.
The code might look something like this

List<PluginFinder> pluginFinders = groupByPluginDefine();
for (PluginFinder finder : pluginFinders) {
    agentBuilder.type(finder.buildMatch())
        .transform(new Transformer(finder))
        .with(AgentBuilder.RedefinitionStrategy.RETRANSFORMATION)
        .with(new Listener())
        .installOn(instrumentation);
}

What the method named groupByPluginDefine method does is group the pluginDefine, and promise only one pluginDefine for each class per group.

the second one

I have debugged code, and found the reason is every time a class is enhanced, it will delegate the original method to a new InstMethodsInter instance. So, the previous is overridden.

To solve it, maybe we can cache the interceptor and merge them.

Finally, I also looked for new apis in ByteBuddy as an alternative to MethodDelegation. but,I am newer for it.This will be a long time.
This is my whole idea.I hope some seniors can give me some advice. Thanks.

[wu-sheng]

The first solution may not work. Because the delegate mechanism only supports to do once.

Finally, I also looked for new apis in ByteBuddy as an alternative to MethodDelegation. but,I am newer for it.This will be a long time.

This should not work, I checked with the author before.

I have debugged code, and found the reason is every time a class is enhanced, it will delegate the original method to a new InstMethodsInter instance. So, the previous is overridden.

Yes, this is the reason.

You missed one important port, EnhancedInstance#dynamicField, which has only one value. But different interceptors could access(set/get) with or without other plugins. In this case, you will face ·race condition, visibility of memory operations across threads, even type cast exception` in the runtime.

[daimingzhi]

Because the delegate mechanism only supports to do once

I didn't fully grasp the meaning of this。but,I tested the byteBuddy API like this

• code of agent

public class ByteBuddyAgent {

    public static void premain(String agentArgs, Instrumentation inst) {
        // install first time
        new AgentBuilder.Default()
            .type(named("com.easy4coding.bytebuddy.Bar"))
            .transform(new AgentBuilder.Transformer() {
                @Override
                public DynamicType.Builder transform(DynamicType.Builder builder,
                                                     TypeDescription typeDescription,
                                                     ClassLoader classloader,
                                                     JavaModule javaModule) {
                    return builder.method(named("m"))
                        .intercept(MethodDelegation.to(new BarInterceptorFirst()));
                }
            }).installOn(inst);
		
        // install second time
        new AgentBuilder.Default()
            .type(named("com.easy4coding.bytebuddy.Bar"))
            .transform(new AgentBuilder.Transformer() {
                @Override
                public DynamicType.Builder transform(DynamicType.Builder builder,
                                                     TypeDescription typeDescription,
                                                     ClassLoader classloader,
                                                     JavaModule javaModule) {
                    return builder.method(named("m"))
                        .intercept(MethodDelegation.to(new BarInterceptorSecond()));
                }
            }).installOn(inst);
    }
}

public class BarInterceptorFirst {

	@RuntimeType
	public Object intercept(@This Object obj, @AllArguments Object[] allArguments, @SuperCall Callable<?> zuper,
	                        @Origin Method method) {

		System.out.println("before method first");
		Object ret = null;
		try {
			ret = zuper.call();
		} catch (Exception e) {
			e.printStackTrace();
		}
		System.out.println("after method first");

		return ret;
	}

}

public class BarInterceptorSecond {

	@RuntimeType
	public Object intercept(@This Object obj, @AllArguments Object[] allArguments, @SuperCall Callable<?> zuper,
	                        @Origin Method method) {

		System.out.println("before method Second");
		Object ret = null;
		try {
			ret = zuper.call();
		} catch (Exception e) {
			e.printStackTrace();
		}
		System.out.println("after method Second");

		return ret;
	}

}

• testDemo

public class DuplicateInstallDemo {
	public static void main(String[] args) {
		new Bar().m();
	}
}

The program output is as follows：

before method 2
before method
method named m invoked
after method
after method 2

What puzzles me most about the first one is, how do I group pluginDefine?

What I did in my company was adjust the directory structure of the project and extend the AgentClassLoader。

like this:

But, this way is not suitable for skywaling obviously。

You missed one important port, EnhancedInstance#dynamicField, which has only one value.

I'm not really familiar with this piece of logic now.I will spend more time learning about it,Also, I might have to learn more about ByteBuddy

[wu-sheng]

This is not about bytebuddy. SkyWalking agent is not bytebuddy agent. I think you mixed these.
I never said Bytebuddy can't do this, it definitely can. I am from SkyWalking perspective.

[wu-sheng]

I think you skipped the details, and assume SkyWalking agent is as simple as Bytebuddy. If so, you wouldn't see why there are so many codes for SkyWalking agent core. We are building on the top of Bytebuddy.