Support for XDG variables with multiple paths

[robinkrahl]

Issue for app_dirs

The XDG Base Directory Specification allows multiple values for the XDG_DATA_DIRS and XDG_CONFIG_DIRS variables that store the location of the shared data. Implementers of the specification should look for configuration files in all of the specified directories.

app_dirs2 currently always uses the first entry of the variables and ignores the rest. To make app_dirs2 compliant with the specification, you would have to make two changes:

1. app_dirs2 should at least check whether an application directory already exists in one of the specified directories. This would only require changes to the internal lookup implementations and would not break the public API.
2. app_dirs2 should make it possible to query all available data directories.
   1. This could be done by changing the return type of the existing functions. But this would break the public API and would make it harder to use the crate with single directories, i. e. on all non-XDG platforms and for all data types but SharedConfig and SharedData.
   2. Alternatively, you could add new functions to query multiple directories, e. g. get_data_roots, get_app_roots and get_app_dirs.

What do you think? Would you accept a PR that implements 1 + 2.ii.?

[kornelski]

Yes, 1 + 2.ii. sounds great.

[robinkrahl]

Okay, I’m working on a PR.

The implementation functions and the new multi-path getter functions will return a Vec<PathBuf> with at least one element. Should I use vec1 to express this invariant using the type system or is it enough to add it to the documentation?

[kornelski]

vec1 is an overkill. Regular vec should be enough.