Improve exception handling and documentation of runtime attachment (#388)

Author: jeanbisutti

runtime-attach/README.md

@@ -4,12 +4,14 @@ If you can't update the JVM arguments to attach the [OpenTelemetry Java agent](h
 
 The `io.opentelemetry.contrib.attach.RuntimeAttach` class has an `attachJavaagentToCurrentJVM` method allowing to trigger the attachment of the OTel agent for Java.
 
-The attachment will not be initiated in the following cases:
+The attachment will _not_ be initiated in the following cases:
 * The `otel.javaagent.enabled` property is set to `false`
 * The `OTEL_JAVAAGENT_ENABLED` environment variable is set to `false`
 * The attachment is not requested from the _main_ thread
 * The attachment is not requested from the `public static void main(String[] args)` method
 * The agent is already attached
+* The application is running on a JRE (a JDK is necessary)
+* The application is running on a read-only file system
 
 _The attachment must be requested at the beginning of the `public static void main(String[] args)` method._ We give below an example for Spring Boot applications:
 

runtime-attach/runtime-attach-core/src/main/java/io/opentelemetry/contrib/attach/AgentFileProvider.java

@@ -40,7 +40,9 @@ private static Path createTempDir() {
     try {
       tempDir = Files.createTempDirectory("otel-agent");
     } catch (IOException e) {
-      throw new IllegalStateException("Runtime attachment can't create temp directory", e);
+      throw new RuntimeAttachException(
+          "Runtime attachment can't create a temp directory. Are you using a read-only file system?",
+          e);
     }
     return tempDir;
   }
@@ -50,12 +52,12 @@ private Path createTempAgentJarFileIn(Path tempDir) {
     try (InputStream jarAsInputStream =
         AgentFileProvider.class.getResourceAsStream(this.agentJarResourceName)) {
       if (jarAsInputStream == null) {
-        throw new IllegalStateException(this.agentJarResourceName + " resource can't be found");
+        throw new RuntimeAttachException(this.agentJarResourceName + " resource can't be found");
       }
       Files.copy(jarAsInputStream, agentJarPath);
     } catch (IOException e) {
-      throw new IllegalStateException(
-          "Runtime attachment can't create agent jar file in temp directory", e);
+      throw new RuntimeAttachException(
+          "Runtime attachment can't create an agent jar file in temp directory", e);
     }
     return agentJarPath;
   }

runtime-attach/runtime-attach-core/src/main/java/io/opentelemetry/contrib/attach/CoreRuntimeAttach.java

@@ -40,7 +40,12 @@ public void attachJavaagentToCurrentJVM() {
     AgentFileProvider agentFileProvider = new AgentFileProvider(agentJarResourceName);
 
     File javaagentFile = agentFileProvider.getAgentFile();
-    ByteBuddyAgent.attach(javaagentFile, getPid());
+
+    try {
+      ByteBuddyAgent.attach(javaagentFile, getPid());
+    } catch (RuntimeException e) {
+      handleByteBuddyException(e);
+    }
 
     if (!agentIsAttached()) {
       printError("Agent was not attached. An unexpected issue has happened.");
@@ -88,6 +93,24 @@ private static boolean agentIsDisabledWithEnvVar() {
     return "false".equals(agentEnabledEnvVarValue);
   }
 
+  private static void handleByteBuddyException(RuntimeException exception) {
+    handleNoAgentProvider(exception);
+    throw new RuntimeAttachException(
+        "A problem has occurred during the runtime attachment of the Java agent.", exception);
+  }
+
+  private static void handleNoAgentProvider(RuntimeException exception) {
+    if (exception instanceof IllegalStateException) {
+      String message = exception.getMessage();
+      if (message != null
+          && message.contains(
+              "No compatible attachment provider is available")) { // ByteBuddy message
+        throw new RuntimeAttachException(
+            "Runtime attachment has failed. Are you using a JRE (not a JDK)?", exception);
+      }
+    }
+  }
+
   private static boolean agentIsAttached() {
     try {
       Class.forName("io.opentelemetry.javaagent.OpenTelemetryAgent", false, null);

runtime-attach/runtime-attach-core/src/main/java/io/opentelemetry/contrib/attach/RuntimeAttachException.java

@@ -0,0 +1,24 @@
+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package io.opentelemetry.contrib.attach;
+
+/**
+ * Exception that gets thrown if a problem occurs during the attachment of the OpenTelemetry agent.
+ */
+public final class RuntimeAttachException extends RuntimeException {
+
+  private static final long serialVersionUID = 1982913847038355735L;
+
+  private RuntimeAttachException() {}
+
+  RuntimeAttachException(String message) {
+    super(message);
+  }
+
+  RuntimeAttachException(String message, Throwable cause) {
+    super(message, cause);
+  }
+}