[FIXED JENKINS-4220] fixed a dead lock.

While the original report is in https://cloudbees.zendesk.com/tickets/427, this fix also takes care of JENKINS-4220.

changelog.html

@@ -61,6 +61,9 @@ Upcoming changes</a>
   <li class=bug>
     Environment variable not available for Maven build/POM parsing.
     (<a href="http://issues.jenkins-ci.org/browse/JENKINS-8865">issue 8865</a>)
+  <li class=bug>
+    Fixed a dead lock in concurrent builds of the same Maven projects.
+      (<a href="http://issues.jenkins-ci.org/browse/JENKINS-4220">issue 4220</a>)
   <li class=rfe>
     thread dump now reports all the threads from all the slaves, not just the master.
   <li class=bug>

maven-plugin/src/main/java/hudson/maven/MavenModuleSetBuild.java

@@ -126,6 +126,8 @@ public class MavenModuleSetBuild extends AbstractMavenBuild<MavenModuleSet,Maven
 
     private String mavenVersionUsed;
 
+    private transient Object notifyModuleBuildLock = new Object();
+
     public MavenModuleSetBuild(MavenModuleSet job) throws IOException {
         super(job);
     }
@@ -134,6 +136,12 @@ public class MavenModuleSetBuild extends AbstractMavenBuild<MavenModuleSet,Maven
         super(project, buildDir);
     }
 
+    @Override
+    protected void onLoad() {
+        super.onLoad();
+        notifyModuleBuildLock = new Object();
+    }
+
     /**
      * Exposes {@code MAVEN_OPTS} to forked processes.
      *
@@ -457,7 +465,9 @@ public class MavenModuleSetBuild extends AbstractMavenBuild<MavenModuleSet,Maven
 
             // actions need to be replaced atomically especially
             // given that two builds might complete simultaneously.
-            synchronized(this) {
+            // use a separate lock object since this synchronized block calls into plugins,
+            // which in turn can access other MavenModuleSetBuild instances, which will result in a dead lock.
+            synchronized(notifyModuleBuildLock) {
                 boolean modified = false;
 
                 List<Action> actions = getActions();
@@ -1318,7 +1328,7 @@ public class MavenModuleSetBuild extends AbstractMavenBuild<MavenModuleSet,Maven
      * Extra verbose debug switch.
      */
     public static boolean debug = Boolean.getBoolean( "hudson.maven.debug" );
-    
+
     @Override
     public MavenModuleSet getParent() {// don't know why, but javac wants this
         return super.getParent();