Use emulated attach as fallback (elastic#2385)

* use emulated attach as fallback on attach itself

* update changelog

Author: SylvainJuge

CHANGELOG.asciidoc

@@ -25,6 +25,7 @@ endif::[]
 
 [float]
 ===== Bug fixes
+* Fix runtime attach with some docker images - {pull}2385[#2385]
 
 [[release-notes-1.x]]
 === Java Agent version 1.x

apm-agent-attach/src/main/java/co/elastic/apm/attach/ElasticApmAttacher.java

@@ -151,14 +151,34 @@ public static void attach(String pid, Map<String, String> configuration, File ag
         File tempFile = createTempProperties(configuration, null);
         String agentArgs = tempFile == null ? null : TEMP_PROPERTIES_FILE_KEY + "=" + tempFile.getAbsolutePath();
 
-        ByteBuddyAgent.attach(agentJarFile, pid, agentArgs, ElasticAttachmentProvider.get());
+        attachWithFallback(agentJarFile, pid, agentArgs);
         if (tempFile != null) {
             if (!tempFile.delete()) {
                 tempFile.deleteOnExit();
             }
         }
     }
 
+    private static void attachWithFallback(File agentJarFile, String pid, String agentArgs) {
+        try {
+            // while the native providers may report to be supported and appear to work properly, in practice there are
+            // cases (Docker without '--init' option on some JDK images like 'openjdk:8-jdk-alpine') where the accessor
+            // returned by the provider will not work as expected at attachment time.
+            ByteBuddyAgent.attach(agentJarFile, pid, agentArgs, ElasticAttachmentProvider.get());
+        } catch (RuntimeException e1) {
+            try {
+                ByteBuddyAgent.attach(agentJarFile, pid, agentArgs, ElasticAttachmentProvider.getFallback());
+            } catch (RuntimeException e2) {
+                // output the two exceptions for debugging
+                System.err.println("Unable to attach with fallback provider:");
+                e2.printStackTrace();
+
+                System.err.println("Unable to attach with regular provider:");
+                e1.printStackTrace();
+            }
+        }
+    }
+
     /**
      * Attaches the agent to a remote JVM
      *
@@ -168,7 +188,7 @@ public static void attach(String pid, Map<String, String> configuration, File ag
      */
     @Deprecated
     public static void attach(String pid, String agentArgs) {
-        ByteBuddyAgent.attach(AgentJarFileHolder.INSTANCE.agentJarFile, pid, agentArgs, ElasticAttachmentProvider.get());
+        attachWithFallback(AgentJarFileHolder.INSTANCE.agentJarFile, pid, agentArgs);
     }
 
     public static File getBundledAgentJarFile() {

apm-agent-attach/src/main/java/co/elastic/apm/attach/ElasticAttachmentProvider.java

@@ -27,9 +27,10 @@ public class ElasticAttachmentProvider {
 
     private static ByteBuddyAgent.AttachmentProvider provider;
 
+    private static ByteBuddyAgent.AttachmentProvider fallback;
+
     /**
      * Initializes attachment provider, this method can only be called once as it loads native code.
-     *
      */
     private synchronized static void init() {
         if (provider != null) {
@@ -44,14 +45,21 @@ private synchronized static void init() {
             new CachedAttachmentProvider(ByteBuddyAgent.AttachmentProvider.ForStandardToolsJarVm.MACINTOSH),
             new CachedAttachmentProvider(ByteBuddyAgent.AttachmentProvider.ForUserDefinedToolsJar.INSTANCE),
             // only use emulated attach last, as native attachment providers should be preferred
-            ByteBuddyAgent.AttachmentProvider.ForEmulatedAttachment.INSTANCE);
+            getFallback());
 
 
         provider = new ByteBuddyAgent.AttachmentProvider.Compound(providers);
     }
 
+    private synchronized static void initFallback(){
+        if (fallback != null) {
+            throw new IllegalStateException("ElasticAttachmentProvider.initFallback() should only be called once");
+        }
+        fallback = ByteBuddyAgent.AttachmentProvider.ForEmulatedAttachment.INSTANCE;
+    }
+
     /**
-     * Get (and optionally initialize) attachment provider, will internally call {@link #init()} if not already called
+     * Get (and optionally initialize) attachment provider
      *
      * @return attachment provider
      */
@@ -62,4 +70,16 @@ public synchronized static ByteBuddyAgent.AttachmentProvider get() {
         return provider;
     }
 
+    /**
+     * Get (and optionally initialize) fallback (emulated) attachment provider
+     *
+     * @return fallback (emulated) attachment provider
+     */
+    public synchronized static ByteBuddyAgent.AttachmentProvider getFallback() {
+        if (fallback == null) {
+            initFallback();
+        }
+        return fallback;
+    }
+
 }