Fixing slim attacher CLI with requested agent version for download (#3096)

* Fixing slim attacher CLI with requested agent version for download

---------

Co-authored-by: Sylvain Juge <[email protected]>

Author: eyalkoren

CHANGELOG.asciidoc

@@ -53,6 +53,7 @@ communication - {pull}2996[#2996]
 * Fix usage of `HttpUrlConnection.getResponseCode()` causing an error event due to exception capturing, even when it is internally handled - {pull}3024[#3024]
 * Fix source code jar to contain apm-agent sources - {pull}3063[#3063]
 * Fix security exception when security manager is used with `log_level=debug` - {pull}3077[#3077]
+* Fix slim attacher when downloading agent version - {pull}3096[#3096]
 
 [[release-notes-1.x]]
 === Java Agent version 1.x

apm-agent-attach-cli/src/main/java/co/elastic/apm/attach/AgentAttacher.java

@@ -380,7 +380,8 @@ private Arguments(DiscoveryRules rules, Set<String> includePids, Map<String, Str
                 if (!agentJar.canRead()) {
                     throw new IllegalArgumentException(String.format("Agent jar %s is not readable", agentJarString));
                 }
-            } else {
+            } else if (downloadAgentVersion == null) {
+                // this would fail if using the slim attacher CLI jar without providing either --agent-jar or --download-agent-version
                 this.agentJar = ElasticApmAttacher.getBundledAgentJarFile();
             }
             if (!config.isEmpty() && argsProvider != null) {

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

@@ -213,7 +213,7 @@ private static File getAgentJarFile() {
 
             // Running attacher without proper packaging is quite common when running it from the IDE without having
             // it packaged from CLI beforehand
-            throw new IllegalStateException("unable to get packaged agent within attacher jar, make sure to execute 'mvn clean package' first.");
+            throw new IllegalStateException("unable to get packaged agent within attacher jar.");
         }
     }