Add option to try other caches when NEAREST_CACHE fails

[matyasselmeci]

Now that #118 added trying multiple caches, it would be useful to have an environment variable that tells stashcp to try NEAREST_CACHE first, but still fall back to other caches. The use case for this is to set NEAREST_CACHE to a private cache, but fall back to the public caches if it's down or overloaded.

We could have it off by default and name it something like NEAREST_CACHE_FALLBACK=1, or on by default and name it something like NEAREST_CACHE_ONLY=1.

[djw8605]

Just to be clear on this. There are 2 options you are proposing:

1. If NEAREST_CACHE environment variable is set, then use that as the first cache to try. But continue to fall back to the others. So, just add it to the front of the list of nearest caches (with duplicate removal if it's already in the list)
2. If NEAREST_CACHE is set, check if NEAREST_CACHE_ONLY is set and if so, only attempt the nearest cache. This would have the same effect as specifying --cache on the command line.

[matyasselmeci]

Yep. Either one of those would work for me.

[djw8605]

I'm confused by your "either one", do you not want both?

[matyasselmeci]

(2) means do (1) and also add NEAREST_CACHE_ONLY, right? For me (2) is optional, it just gives people a way to get the current behavior back once we do (1).

[djw8605]

Ok, makes sense. I forgot we already have NEAREST_CACHE functionality.