Avoid runtime attach user conflicts (#1704)

* allow multiple users with same agent version

Author: SylvainJuge

CHANGELOG.asciidoc

@@ -52,6 +52,7 @@ any further configuration. Supports log4j1, log4j2 and Logback. {pull}1261[#1261
 * fix sample rate rounded to zero when lower than precision - {pull}1655[#1655]
 * fixed a couple of bugs with the external plugin mechanism (not documented until now) - {pull}1660[#1660]
 * Update to async-profiler 1.8.4 to prevent crashes on Java 7 - {pull}1678[#1678]
+* Fix runtime attach conflict with multiple users - {pull}1704[#1704]
 
 [[release-notes-1.x]]
 === Java Agent version 1.x

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

@@ -202,7 +202,12 @@ private static File getAgentJarFile() {
                     throw new IllegalStateException("Agent jar not found");
                 }
                 String hash = md5Hash(ElasticApmAttacher.class.getResourceAsStream("/elastic-apm-agent.jar"));
-                File tempAgentJar = new File(System.getProperty("java.io.tmpdir"), "elastic-apm-agent-" + hash + ".jar");
+
+                // we have to include current user name as multiple copies of the same agent could be attached
+                // to multiple JVMs, each running under a different user. Also, we have to make it path-friendly.
+                String user = md5Hash(System.getProperty("user.name"));
+
+                File tempAgentJar = new File(System.getProperty("java.io.tmpdir"), String.format("elastic-apm-agent-%s-%s.jar", user, hash));
                 if (!tempAgentJar.exists()) {
                     try (FileOutputStream out = new FileOutputStream(tempAgentJar)) {
                         FileChannel channel = out.getChannel();
@@ -238,4 +243,10 @@ static String md5Hash(InputStream resourceAsStream) throws IOException, NoSuchAl
             return String.format("%032x", new BigInteger(1, md.digest()));
         }
     }
+
+    static String md5Hash(String s) throws NoSuchAlgorithmException {
+        MessageDigest md = MessageDigest.getInstance("MD5");
+        md.update(s.getBytes());
+        return String.format("%032x", new BigInteger(1, md.digest()));
+    }
 }