ClearSCM Inc.

What are Evil Twins?

Simply put, an evil twin is a condition that can be caused in Clearcase when a user attempts to add an element to source control that has a name that is the same as an element on another branch. If it is allowed to be created it will be difficult if not impossible to merge in the future. You don't want evil twins to be created to start with.

Let's look a little bit deeper into how this can happen. Let's assume that a user adds the element named "foo" to the directory bin. Sometime later that element name is removed with rmname. Finally assume that it is decided that foo is again needed. When it is recreated the second time you will be creating an evil twin because Clearcase will get confused between foo in say version 3 of the parent directory and this new foo destined for say version 7 of the parent directory.

Here's a few steps to create the evil twin scenario:

$ # First check out the current directory and create foo
$ ct co -nc .
$ echo "bar" > foo
$ ct mkelem -nc foo
$ # Now check it all in
$ ct ci -nc foo .
$ # Now check out the parent directory and rmname foo
$ ct co -nc .
$ ct rmname foo
$ ct ci -nc .
    

At this point we have a directory with foo in it and then the next version of the directory has foo rmnamed (i.e. uncataloged). Now let's attempt to create an evil twin:

$ # Let's create foo's evil twin
$ ct co -nc .
$ echo "Evil Twin" > foo
$ ct mkelem -nc foo
    

At this point we should see the following dialog box preventing the creation of the evil twin:

Insert dialog box here

This is telling us that we are about to create an evil twin. Note that it also tells us where it found the first twin in the view extended syntax (the part starting with @@). There is another dummy in the bin directory in "andys_branch" version 1.

Merging the Original Elements Back

The preferred way to resolve this problem is to merge the original elements back from the proper directory version of the parent directory into the current branch. To do this you must first locate the branch where the evil twin existed. From the above example that would be \main\Andrew_Integration\adefaria_Andrew\3. Locate that version of the parent directory (e.g. the adm\bin directory of andy vob in the above example) in the Clearcase Version Tree Browser. To be clear, locate the parent directory for the element dummy in the Clearcase Explorer, right click on it and select Version Tree then look in the version tree for the \main\Andrew_Integration\adefaria_Andrew\3 version.

Note: A good way to find this directory version in directory elements that have large or complicated verion trees is to use the Locate toolbar button (the button with the "flashlight" icon). You can search for versions by version name, branch, etc).

Next right click on that version (the version of the parent directory that has the original elements) and select Merge to. Your mouse cursor will change to a little "target" icon. Next select the version of the directory that your view selects (this can be found by locating the little "eye" icon).

Just before you select OK to start the merge make sure you toggle on the Merge the element graphically toggle. This will start cleardiffmrg and prompt you to select each merge.

This will bring up cleardiffmrg and allow you to confirm each merge of the diredtory. During the merge choose the entries from the parent directory for the elements that you wish to "recover" or "reinstate" .

Another way to resolve this condition is to hardlink to the previous version of this element but this is not always what you want to do. For one it can be confusing. In any event if you need help because you've hit and evil twin be sure to contact the Help Desk and we'll help you out.