Update center will create backup files to make it easier for manual roll back of botched upgrades.

git-svn-id: https://hudson.dev.java.net/svn/hudson/trunk/hudson/main@21980 71c3de6d-444a-0410-be80-ed276b4c234a

core/src/main/java/hudson/Util.java

@@ -1044,6 +1044,19 @@ public class Util {
         }
     }
 
+    /**
+     * Returns a file name by changing its extension.
+     *
+     * @param ext
+     *      For example, ".zip"
+     */
+    public static File changeExtension(File dst, String ext) {
+        String p = dst.getPath();
+        int pos = p.lastIndexOf('.');
+        if (pos<0)  return new File(p+ext);
+        else        return new File(p.substring(0,pos)+ext);
+    }
+
     public static final FastDateFormat XS_DATETIME_FORMATTER = FastDateFormat.getInstance("yyyy-MM-dd'T'HH:mm:ss'Z'",new SimpleTimeZone(0,"GMT"));
 
     // Note: RFC822 dates must not be localized!

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

@@ -835,7 +835,10 @@ public class UpdateCenter extends AbstractModelObject {
          * the old file with the new file.
          */
         protected void replace(File dst, File src) throws IOException {
-            dst.delete();
+            File bak = Util.changeExtension(dst,".bak");
+            bak.delete();
+            dst.renameTo(bak);
+            dst.delete(); // any failure up to here is no big deal
             if(!src.renameTo(dst)) {
                 throw new IOException("Failed to rename "+src+" to "+dst);
             }