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Wow, bad week for ColdFusion/S3 bugs. I reported a few days ago about a ColdFusion/S3 issue with metadata. Today I verified a bug reported by Stefan Richter on cf-talk concerning folders. I'll keep it short and sweet. If you use ColdFusion's cfdirectory tag to list the contents of a bucket that has a subfolder with no lastdate modified value, you will get an error in ColdFusion. How do you end up with a folder with no lastdate modified value? It's simple enough in the S3 Web tool. Just make a new folder. Obviously this folder was never modified so it makes sense.
Unfortunately this is a pretty critical issue. If you use ColdFusion to work with buckets you have no control over, then you will need to use another solution (like perhaps this open source S3 wrapper) to safely navigate your data. If your ColdFusion application is the only "client" for a bucket, then you probably don't need to worry.
I've filed a bug report here for the issue.
I was about to hit submit on this when I decided to quickly test something. If you use directoryList() instead of cfdirectory, and use listInfo=path, you do not get an error. Unfortunately you don't get information about the results in terms of them being a file or a directory, but if you want to assume anything without an extension is a folder, you are probably safe.