nixos: tarsnap - separate archive cachedirs

Tarsnap locks the cachedir during backup, meaning if you specify
multiple backups with a shared cache that might overlap (for example,
one backup may take an hour), secondary backups will fail. This isn't
very nice behavior for the obvious reasons.

This splits the cache dirs for each archive appropriately. Note that
this will require a rebuild of your archive caches (although if you were
only using one archive for your whole system, you can just move the
directory).

Signed-off-by: Austin Seipp <[e-mail]>

nixos/modules/services/backup/tarsnap.nix

···
 let
   cfg = config.services.tarsnap;
 
-  configFile = cfg: ''
-    cachedir ${config.services.tarsnap.cachedir}
+  configFile = name: cfg: ''
+    cachedir ${config.services.tarsnap.cachedir}/${name}
     keyfile  ${config.services.tarsnap.keyfile}
     ${optionalString cfg.nodump "nodump"}
     ${optionalString cfg.printStats "print-stats"}
···
           Should the cache become desynchronized or corrupted, tarsnap
           will refuse to run until you manually rebuild the cache with
           <command>tarsnap --fsck</command>.
+
+          Note that each individual archive (specified below) has its own cache
+          directory specified under <literal>cachedir</literal>; this is because
+          tarsnap locks the cache during backups, meaning multiple services
+          archives cannot be backed up concurrently or overlap with a shared
+          cache.
 
           Set to <literal>null</literal> to disable caching.
         '';
···
         mkdir -p -m 0700 ${cfg.cachedir}
         chown root:root ${cfg.cachedir}
         chmod 0700 ${cfg.cachedir}
+        mkdir -p -m 0700 ${cfg.cachedir}/$1
         DIRS=`cat /etc/tarsnap/$1.dirs`
         exec tarsnap --configfile /etc/tarsnap/$1.conf -c -f $1-$(date +"%Y%m%d%H%M%S") $DIRS
       '';
···
 
     environment.etc =
       (mapAttrs' (name: cfg: nameValuePair "tarsnap/${name}.conf"
-        { text = configFile cfg;
+        { text = configFile name cfg;
         }) cfg.archives) //
       (mapAttrs' (name: cfg: nameValuePair "tarsnap/${name}.dirs"
         { text = concatStringsSep " " cfg.directories;