bazil | file system that lets your data reside

The problem that I'm facing is that when an application wants to write to foo/bar inside my file system, it checks whether foo is a directory, then whether bar is a file. Unfortunately, I can't know upfront whether foo should be a file or a directory.

Given these constraints, solving your problem is impossible.

A file system node can be either a file or a directory; sometimes neither, but never both at the same time. Because your FUSE driver cannot know in advance whether an application performing the getattr request means to recurse inside the node until it actually tries to, you cannot know whether it should pretend to be a file or a directory.

Your best options seem to be:

• hardcode application-specific virtual directory structure into your FUSE driver
• implement application-specific heuristics in your FUSE driver
• make your FUSE driver memorise getattr requests and construct a virtual directory tree by trial and error (and keep re-running the application until it works)