Added pre-checkout method.

This method is called after assinging the workspace to the build, but before SCM checkout. Can be implemented by BuildWrappers.

core/src/main/java/hudson/model/AbstractBuild.java

@@ -412,6 +412,7 @@ public abstract class AbstractBuild<P extends AbstractProject<P,R>,R extends Abs
                 workspace = lease.path.getRemote();
                 node.getFileSystemProvisioner().prepareWorkspace(AbstractBuild.this,lease.path,listener);
 
+                preCheckout(launcher,listener);
                 checkout(listener);
 
                 if (!preBuild(listener,project.getProperties()))
@@ -489,6 +490,25 @@ public abstract class AbstractBuild<P extends AbstractProject<P,R>,R extends Abs
             return l;
         }
 
+        
+        /**
+         * Run preCheckout on {@link BuildWrapper}s
+         * 
+         * @param launcher
+         * 		The launcher, never null.
+         * @param listener
+         * 		Never null, connected to the main build output.
+         * @throws IOException
+         * @throws InterruptedException
+         */
+        private void preCheckout(Launcher launcher, BuildListener listener) throws IOException, InterruptedException{
+        	if (project instanceof BuildableItemWithBuildWrappers) {
+                BuildableItemWithBuildWrappers biwbw = (BuildableItemWithBuildWrappers) project;
+                for (BuildWrapper bw : biwbw.getBuildWrappersList())
+                    bw.preCheckout(AbstractBuild.this,launcher,listener);
+            }
+        }
+        
         private void checkout(BuildListener listener) throws Exception {
             try {
                 for (int retryCount=project.getScmCheckoutRetryCount(); ; retryCount--) {

core/src/main/java/hudson/tasks/BuildWrapper.java

@@ -201,6 +201,28 @@ public abstract class BuildWrapper extends AbstractDescribableImpl<BuildWrapper>
         return logger;
     }
 
+    /**
+     * Provides an opportunity for a {@link BuildWrapper} to perform some actions before SCM checkout.
+     *
+     * <p>
+     * This hook is called early on in the build (before {@link #setUp(AbstractBuild, Launcher, BuildListener)}, 
+     * but after {@link #decorateLauncher(AbstractBuild, Launcher, BuildListener)} is invoked.)
+     * The typical use is delete existing workspace before new build starts etc.
+     *
+     * <p>
+     * The default implementation is no-op.
+     * 
+     * @param build
+     *      The build in progress for which this {@link BuildWrapper} is called. Never null.
+     * @param launcher
+     *      The launcher. Never null. 
+     * @param listener
+     *      Connected to the build output. Never null. Can be used for error reporting.
+     *      
+     */
+    public void preCheckout(AbstractBuild build, Launcher launcher, BuildListener listener) throws IOException, InterruptedException{
+    }
+    
     /**
      * {@link Action} to be displayed in the job page.
      *