Make Apache v4 interception reliable enough to intercept Gradle

Not really because Gradle is especially interesting, more that it's sufficiently
common that it's likely to be a test case for many people, and it would be good
if that worked! Previously it did basically work, AFAICT, but with loud errors
during setup when HttpRoute couldn't be resolved. We now ensure that that's not
required, so there's no errors: just perfectly intercepted HTTP(S).

Author: pimterry

src/main/java/tech/httptoolkit/javaagent/advice/apacheclient/ApacheV4ReturnProxyRouteAdvice.java

@@ -1,23 +1,30 @@
 package tech.httptoolkit.javaagent.advice.apacheclient;
 
 import net.bytebuddy.asm.Advice;
+import net.bytebuddy.implementation.bytecode.assign.Assigner;
 import org.apache.http.HttpHost;
 import org.apache.http.conn.routing.HttpRoute;
-import tech.httptoolkit.javaagent.HttpProxyAgent;
+
+import java.net.*;
 
 public class ApacheV4ReturnProxyRouteAdvice {
     @Advice.OnMethodExit
     public static void determineRoute(
-            @Advice.Return(readOnly = false) HttpRoute returnValue
+        // We type this dynamically, because in some cases (notably Gradle) we seemingly can't reach the
+        // HttpRoute type from ByteBuddy, only at runtime.
+        @Advice.Return(readOnly = false, typing = Assigner.Typing.DYNAMIC) Object returnValue
     ) {
+        HttpRoute existingValue = (HttpRoute) returnValue;
+        // We guarantee that the default proxy selector is always our own. This ensures that we can
+        // always grab the proxy URL without needing to access our injected classes.
+        Proxy proxy = ProxySelector.getDefault().select(URI.create("https://example.com")).get(0);
+        InetSocketAddress proxyAddress = (InetSocketAddress) proxy.address();
+
         returnValue = new HttpRoute(
-            returnValue.getTargetHost(),
-            returnValue.getLocalAddress(),
-            new HttpHost(
-                HttpProxyAgent.getAgentProxyHost(),
-                HttpProxyAgent.getAgentProxyPort()
-            ),
-            returnValue.isSecure()
+            existingValue.getTargetHost(),
+            existingValue.getLocalAddress(),
+            new HttpHost(proxyAddress.getHostString(), proxyAddress.getPort()),
+            existingValue.isSecure()
         );
     }
 }