runtime attach: prevent NPE with null value (#3712)

* runtime attach: prevent NPE with null value

Author: SylvainJuge

CHANGELOG.asciidoc

@@ -35,6 +35,7 @@ Use subheadings with the "=====" level for adding notes for unreleased changes:
 ===== Bug fixes
 * Restore compatibility with Java 7 - {pull}3657[#3657]
 * Avoid `ClassCastException` and issue warning when trying to use otel span links - {pull}3672[#3672]
+* Avoid `NullPointerException` with runtime attach API and invalid map entries - {pull}3712[#3712]
 
 [float]
 ===== Features

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

@@ -150,7 +150,14 @@ public static void attach(String pid, Map<String, String> configuration) {
      */
     public static void attach(String pid, Map<String, String> configuration, File agentJarFile) {
         // making a copy of provided configuration as user might have used an immutable map impl.
-        Map<String, String> config = new HashMap<>(configuration);
+        // and guard against user-provided null keys and values
+        Map<String, String> config = new HashMap<>();
+        for (Map.Entry<String, String> entry : configuration.entrySet()) {
+            if (entry.getKey() != null && entry.getValue() != null) {
+                config.put(entry.getKey(), entry.getValue());
+            }
+        }
+
         if (!config.containsKey("activation_method")) {
             config.put("activation_method", "PROGRAMMATIC_SELF_ATTACH");
         }

integration-tests/runtime-attach/runtime-attach-app/src/main/java/co/elastic/apm/testapp/AppMain.java

@@ -79,6 +79,10 @@ public static void main(String[] args) {
                 Map<String, String> config = new HashMap<>();
                 config.put("service_name", "self-attach-external-config");
                 config.put("config_file", agentConfig.getAbsolutePath());
+
+                // user-provided map might have null values and be un-modifiable
+                // thus we must test it to ensure it's working as expected
+                config.put("null_config_entry", null);
                 ElasticApmAttacher.attach(Collections.unmodifiableMap(config));
             }
         }