Add a config agent.enable(default: true) to support disabling agent

CHANGES.md

@@ -24,6 +24,8 @@ Release Notes.
 * Support to customize the collect period of JVM relative metrics.
 * Upgrade netty-codec-http2 to 4.1.86.Final.
 * Put `Agent-Version` property reading in the premain stage to avoid deadlock when using `jarsigner`.
+* Add a config `agent.enable`(default: true) to support disabling the agent through system property `-Dskywalking.agent.disable=false` 
+  or system environment variable setting `SW_AGENT_ENABLE=false`. 
 
 #### Documentation
 

apm-sniffer/apm-agent-core/src/main/java/org/apache/skywalking/apm/agent/core/conf/Config.java

@@ -183,6 +183,11 @@ public static class Agent {
          * Agent version. This is set by the agent kernel through reading MANIFEST.MF file in the skywalking-agent.jar.
          */
         public static String VERSION = "UNKNOWN";
+
+        /**
+         * Enable the agent kernel services and instrumentation.
+         */
+        public static boolean ENABLE = true;
     }
 
     public static class OsInfo {

apm-sniffer/apm-agent/src/main/java/org/apache/skywalking/apm/agent/SkyWalkingAgent.java

@@ -23,7 +23,6 @@
 import java.util.Collections;
 import java.util.List;
 import java.util.Map;
-
 import net.bytebuddy.ByteBuddy;
 import net.bytebuddy.agent.builder.AgentBuilder;
 import net.bytebuddy.description.NamedElement;
@@ -70,13 +69,18 @@ public static void premain(String agentArgs, Instrumentation instrumentation) th
         } catch (Exception e) {
             // try to resolve a new logger, and use the new logger to write the error log here
             LogManager.getLogger(SkyWalkingAgent.class)
-                    .error(e, "SkyWalking agent initialized failure. Shutting down.");
+                      .error(e, "SkyWalking agent initialized failure. Shutting down.");
             return;
         } finally {
             // refresh logger again after initialization finishes
             LOGGER = LogManager.getLogger(SkyWalkingAgent.class);
         }
 
+        if (!Config.Agent.ENABLE) {
+            LOGGER.warn("SkyWalking agent is disabled.");
+            return;
+        }
+
         try {
             pluginFinder = new PluginFinder(new PluginBootstrap().loadPlugins());
         } catch (AgentPackageNotFoundException ape) {
@@ -90,15 +94,15 @@ public static void premain(String agentArgs, Instrumentation instrumentation) th
         final ByteBuddy byteBuddy = new ByteBuddy().with(TypeValidation.of(Config.Agent.IS_OPEN_DEBUGGING_CLASS));
 
         AgentBuilder agentBuilder = new AgentBuilder.Default(byteBuddy).ignore(
-                nameStartsWith("net.bytebuddy.")
-                        .or(nameStartsWith("org.slf4j."))
-                        .or(nameStartsWith("org.groovy."))
-                        .or(nameContains("javassist"))
-                        .or(nameContains(".asm."))
-                        .or(nameContains(".reflectasm."))
-                        .or(nameStartsWith("sun.reflect"))
-                        .or(allSkyWalkingAgentExcludeToolkit())
-                        .or(ElementMatchers.isSynthetic()));
+            nameStartsWith("net.bytebuddy.")
+                .or(nameStartsWith("org.slf4j."))
+                .or(nameStartsWith("org.groovy."))
+                .or(nameContains("javassist"))
+                .or(nameContains(".asm."))
+                .or(nameContains(".reflectasm."))
+                .or(nameStartsWith("sun.reflect"))
+                .or(allSkyWalkingAgentExcludeToolkit())
+                .or(ElementMatchers.isSynthetic()));
 
         JDK9ModuleExporter.EdgeClasses edgeClasses = new JDK9ModuleExporter.EdgeClasses();
         try {
@@ -140,7 +144,7 @@ public static void premain(String agentArgs, Instrumentation instrumentation) th
         }
 
         Runtime.getRuntime()
-                .addShutdownHook(new Thread(ServiceManager.INSTANCE::shutdown, "skywalking service shutdown thread"));
+               .addShutdownHook(new Thread(ServiceManager.INSTANCE::shutdown, "skywalking service shutdown thread"));
     }
 
     private static class Transformer implements AgentBuilder.Transformer {
@@ -163,7 +167,7 @@ public DynamicType.Builder<?> transform(final DynamicType.Builder<?> builder,
                 EnhanceContext context = new EnhanceContext();
                 for (AbstractClassEnhancePluginDefine define : pluginDefines) {
                     DynamicType.Builder<?> possibleNewBuilder = define.define(
-                            typeDescription, newBuilder, classLoader, context);
+                        typeDescription, newBuilder, classLoader, context);
                     if (possibleNewBuilder != null) {
                         newBuilder = possibleNewBuilder;
                     }
@@ -233,7 +237,10 @@ public void onBatch(int index, List<Class<?>> batch, List<Class<?>> types) {
         }
 
         @Override
-        public Iterable<? extends List<Class<?>>> onError(int index, List<Class<?>> batch, Throwable throwable, List<Class<?>> types) {
+        public Iterable<? extends List<Class<?>>> onError(int index,
+                                                          List<Class<?>> batch,
+                                                          Throwable throwable,
+                                                          List<Class<?>> types) {
             LOGGER.error(throwable, "index={}, batch={}, types={}", index, batch, types);
             return Collections.emptyList();
         }

apm-sniffer/config/agent.config

@@ -87,6 +87,9 @@ agent.ssl_key_path=${SW_AGENT_SSL_KEY_PATH:}
 
 agent.ssl_cert_chain_path=${SW_AGENT_SSL_CERT_CHAIN_PATH:}
 
+# Enable the agent kernel services and instrumentation.
+agent.enable=${SW_AGENT_ENABLE:true}
+
 # Limit the length of the ipv4 list size.
 osinfo.ipv4_list_size=${SW_AGENT_OSINFO_IPV4_LIST_SIZE:10}