Fix Netty interception when launched in Docker

Within static fields, it was not possible to access project dependencies
at runtime. Specifically, when launching the spring petclinic
microservices demo, the API gateway that uses HttpClient crashed because
reactor.netty.transport.ProxyProvider wasn't available when evaluating
the static field.

I'm fairly sure this is due to the app being launched with
org.springframework.boot.loader.JarLauncher, which reconfigures the
classpath during startup. To work around this, we defer loading these
inaccessible types until the intercepted HttpClient constructor. That
probably creates a very minor performance hit, but nothing significant.

If this does become significant in future, we could probably preserve
the fields, and make constructor initialization a one-off, but it's more
complicated and risky, so let's not bother for now. Only matters if
you're creating very very large numbers of HttpClients in a hot loop,
which seems unlikely.

Author: pimterry

src/main/java/tech/httptoolkit/javaagent/advice/reactornetty/ReactorNettyResetAllConfigAdvice.java

@@ -13,30 +13,36 @@
 
 public class ReactorNettyResetAllConfigAdvice {
 
-    public static final ProxyProvider agentProxyProvider = ProxyProvider.builder()
-        .type(ProxyProvider.Proxy.HTTP)
-        .address(new InetSocketAddress(
-            HttpProxyAgent.getAgentProxyHost(),
-            HttpProxyAgent.getAgentProxyPort()
-        ))
-        .build();
-
-    public static final SslProvider agentSslProvider;
-
-    public static final Field configSslField;
-    public static final Field proxyProviderField;
+    // Netty's HTTP Client works by creating a new client subclass, and passing the existing client's
+    // config. We hook that here: we rewrite the config whenever it's used, affecting all clients
+    // involved, and in practice anybody using config anywhere.
 
-    static {
-        try {
-            // Initialize our intercepted SSL provider:
-            agentSslProvider = SslProvider.builder()
-                .sslContext(
+    @Advice.OnMethodEnter
+    public static void beforeConstructor(
+        @Advice.Argument(value=0) HttpClientConfig baseHttpConfig
+    ) throws Exception {
+        // It would be nice to do this setup statically, but due to how some classloader configs work (e.g. Spring) it
+        // seems this can fail when run in a static block, so we just repeat the process for every interception:
+        final SslProvider agentSslProvider = SslProvider.builder()
+            .sslContext(
                     SslContextBuilder
-                    .forClient()
-                    .trustManager(HttpProxyAgent.getInterceptedTrustManagerFactory())
-                    .build()
-                ).build();
+                            .forClient()
+                            .trustManager(HttpProxyAgent.getInterceptedTrustManagerFactory())
+                            .build()
+            ).build();
+
+        final ProxyProvider agentProxyProvider = ProxyProvider.builder()
+            .type(ProxyProvider.Proxy.HTTP)
+            .address(new InetSocketAddress(
+                    HttpProxyAgent.getAgentProxyHost(),
+                    HttpProxyAgent.getAgentProxyPort()
+            ))
+            .build();
+
+        Field configSslField;
+        Field proxyProviderField;
 
+        try {
             // Rewrite the fields we want to mess with in the client config:
             configSslField = HttpClientConfig.class.getDeclaredField("sslProvider");
             configSslField.setAccessible(true);
@@ -46,16 +52,7 @@ public class ReactorNettyResetAllConfigAdvice {
         } catch (Exception e) {
             throw new RuntimeException(e);
         }
-    }
-
-    // Netty's HTTP Client works by creating a new client subclass, and passing the existing client's
-    // config. We hook that here: we rewrite the config whenever it's used, affecting all clients
-    // involved, and in practice anybody using config anywhere.
 
-    @Advice.OnMethodEnter
-    public static void beforeConstructor(
-        @Advice.Argument(value=0) HttpClientConfig baseHttpConfig
-    ) throws Exception {
         configSslField.set(baseHttpConfig, agentSslProvider);
         proxyProviderField.set(baseHttpConfig, agentProxyProvider);
     }