[Condor-devel] Changing Condor externals caching default

[Jaime Frey <jfrey@xxxxxxxxxxx>]

Having the externals location default to /scratch/condor_externals for non-proper builds has several annoyances. First, it doesn't work for a development machine used by more than one developer. The second developer who tries to build runs into file permission errors. They have to disable caching in order to get their builds to work.
Second, most machines outside of the UW Comp Sci department don't have a /scratch directory. The default build configuration shouldn't require its existence. Creating it may not be an option in some locations (e.g. if the person building doesn't have root).
Third, there's no easy way to specify an alternate externals cache location. If you don't want to use /scratch/condor_externals, you have to disable caching.

I propose two changes that address these annoyances:
1) On windows, the environment variable CONDOR_BLD_EXTERNAL_STAGE specifies where to place the externals cache. It should be useable on all platforms. If it's set, externals caching is enabled and it gives the location.
2) If CONDOR_BLD_EXTERNAL_STAGE isn't set, externals caching is disabled by default (meaning externals are installed in the build tree). If externals caching is explicitly enabled (via the cmake option CACHED_EXTERNALS), then the cache is placed under /scratch/condor_externals, as before.

As a result, in the normal case, developers won't run into each other's externals and not special hard-coded directories need to exist. And it's easy for a developer to set up an externals cache where they want that's used by all of their build directories.

If there are no objections, I'll commit the change to V7_8-branch next week.

+--------------------------------+-----------------------------------+
|           Jaime Frey           | I used to be a heavy gambler.     |
|       jfrey@xxxxxxxxxxx        | But now I just make mental bets.  |
|                                | That's how I lost my mind.        |
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