Summary:
This diff adds a model for NSFileManager.contentsOfDirectoryAtURL as returning a constant-length
collection.
The analyzer cannot know files in a directory. We have some options to handle such unknown data.
1. Use `Unknown` value, ie `top`
2. Use a symbolic value
3. Use a constant value
We had been used the first option. An upside of this is that the analyzer can remain as sound.
However, a downside of this is the top value can be propagated to other procedures, making their
costs top, thus we may miss some cost changes of them.
The second option is to introduce a symbolic value, ie. that for the number of files. A problem is
that the symbolic value will never be concretized. As a result, the symbol can be propagated to
other procedures, increasing the coefficient of the complexity or making top costs. Note that handling multiple
symbols is somewhat limited in Inferbo's interval domain.
The last option is to introduce a constant value. I think this is the best approach we can take among above.
Even though we may have FNs when there are a lot of files in a directory, we cannot reason or expect about
that at the analysis time anyway.
Reviewed By: ezgicicek
Differential Revision: D24418099
fbshipit-source-id: bf8cf3538
Sungkeun Cho