Added downgrade support for the core and plugins.

--------
Merged revisions 32975,33588,33697,34342 via svnmerge from 
https://www.dev.java.net/svn/hudson/branches/downgrade

........
  r32975 | kohsuke | 2010-07-16 21:27:59 +0200 (Fri, 16 Jul 2010) | 1 line
  
  committed my partially modified version
........
  r33588 | dienomight | 2010-08-05 09:42:36 +0200 (Thu, 05 Aug 2010) | 13 lines
  
  Added getBackupVersion (UpdateCenter)
  downgrade button now shows version of backup war file
  
  Added HudsonDowngradeJob (UpdateCenter)
  downgrading process is works now as HudsonUpgradeJob
  
  Changed rewriteHudsonWar (Lifecycle) 
  backup file is deleted when downgrading
  
  hudson/model/Hudson/downgrade_success.jelly is no longer necessary
  
  Changed rewriteHudsonWar(WindowsServiceLifecycle)
  hudson run as windows service creates backup during upgrading
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  r33697 | dienomight | 2010-08-11 17:09:48 +0200 (Wed, 11 Aug 2010) | 8 lines
  
  committed changes:
  "plugins downgrade functionality"
  added: 
  * PluginDowngradeJob in UpdateCenter
  * getCanDowngrade & getBackupPath & getBackupVersion in PluginWrapper
  * additional "previous version" column in installed.jelly (PluginManager) with downgrade button (when it's possible to downgrade)
  * doDowngrade in PluginManager
  * deployBackup in UpdateSite
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  r34342 | dienomight | 2010-09-03 09:52:13 +0200 (Fri, 03 Sep 2010) | 12 lines
  
  Code improvements: 
  jelly files:
  *I put text inside ${%} so it would be available for translation.
  UpdateCenter: 
  *changed getCanDowngrade to isDowngradable
  *getBackupVersion use JarFile class to get version of .war backup
  PluginWrapper:
  *changed getBackupPath to getBackupFile
  *getBackupVersion use JarFile class to get version of plugin backup
  PluginManager:
  *changed condition in doDowngrade method
........


git-svn-id: https://hudson.dev.java.net/svn/hudson/trunk/hudson/main@34543 71c3de6d-444a-0410-be80-ed276b4c234a

core/src/main/java/hudson/PluginWrapper.java

@@ -37,7 +37,6 @@ import java.io.OutputStream;
 import java.io.Closeable;
 import java.net.URL;
 import java.util.ArrayList;
-import java.util.Enumeration;
 import java.util.List;
 import java.util.jar.Manifest;
 import java.util.logging.Logger;
@@ -47,6 +46,9 @@ import org.apache.commons.logging.LogFactory;
 import org.kohsuke.stapler.HttpResponse;
 import org.kohsuke.stapler.HttpResponses;
 
+import java.util.Enumeration;
+import java.util.jar.JarFile;
+
 /**
  * Represents a Hudson plug-in and associated control information
  * for Hudson to control {@link Plugin}.
@@ -464,6 +466,37 @@ public class PluginWrapper implements Comparable<PluginWrapper> {
         return shortName.compareToIgnoreCase(pw.shortName);
     }
 
+    /**
+     * returns true if backup of previous version of plugin exists
+     */
+    public boolean isDowngradable() {
+        return getBackupFile().exists();
+    }
+
+    /**
+     * Where is the backup file?
+     */
+    public File getBackupFile() {
+        return new File(Hudson.getInstance().getRootDir(),"plugins/"+getShortName() + ".bak");
+    }
+
+    /**
+     * returns the version of the backed up plugin,
+     * or null if there's no back up.
+     */
+    public String getBackupVersion() {
+        if (getBackupFile().exists()) {
+            try {
+                JarFile backupPlugin = new JarFile(getBackupFile());
+                return backupPlugin.getManifest().getMainAttributes().getValue("Plugin-Version");
+            } catch (IOException e) {
+                LOGGER.log(WARNING, "Failed to get backup version ", e);
+                return null;
+            }
+        } else {
+            return null;
+        }
+    }
 //
 //
 // Action methods