Unable to use existing tag because system thinks I'm trying to create it?

Question

A user on our site is trying to use the r2-d2 and is getting an error message telling him he can't create the tag because already exists:

There is currently a tag synonym from r2d2 to r2-d2, but trying to use r2d2 doesn't automatically redirect to the master tag, it just gives the same error message. Note that r2-d2 does not show up when you browse the tag list, only the synonym does. I suspect it was the target of a tag cleanup effort to get rid of it.

It was also the subject of some moderator-ification in the past (to get the synonym in the first place):

Is there a way to add a hyphen on a already existing tag? (R2D2 -> R2-D2)

Is there something broken with this tag that we can get fixed? Is there a reason that it can't be used, but is also not getting deleted by the tag cleanup script?

Answer 1

The problem is that the r2-d2 tag itself does not exist. There is only a synonym pointing towards it, and trying to rewrite a synonym to a non-existent master tag simply doesn't work. At some point after that synonym was created, all the questions tagged with it got retagged to something else and the master tag was automatically deleted.

This particular case, though, causes something interesting to happen. Technically synonyms shouldn't exist pointing to tags that don't exist themselves. So rather than the system picking up on that, it's assuming you're trying to create a version tag because of the "2" on the end of it. All it's seeing is the "2" and the fact that it's being rejected for some reason it's not programmed to recognize and it's just going with that reason because the "conditions" match. (Sidenote: if it hadn't ended in a number, I believe the system would have created the tag again, which causes another whole problem in that the synonym system lets users without the create new tags privilege bypass it and create tags which are masters of a synonym.)

I've gone ahead and just deleted the synonym, which should cause everything to return to normal regarding that tag. If anything, we should probably just automatically cleanup tag synonyms which involve tags that don't exist anymore. This is already covered when a moderator merges a tag, which will automatically transfer all synonyms over to the new master tag during the merge process.