Merge branch 'develop' into features/shared

Done to resolve merge conflicts that had arisen since work on this
feature completed.

etc/spack/defaults/config.yaml

@@ -37,18 +37,33 @@ config:
 
   # Temporary locations Spack can try to use for builds.
   #
-  # Spack will use the first one it finds that exists and is writable.
-  # You can use $tempdir to refer to the system default temp directory
-  # (as returned by tempfile.gettempdir()).
+  # Recommended options are given below.
   #
-  # A value of $spack/var/spack/stage indicates that Spack should run
-  # builds directly inside its install directory without staging them in
-  # temporary space.
+  # Builds can be faster in temporary directories on some (e.g., HPC) systems.
+  # Specifying `$tempdir` will ensure use of the default temporary directory
+  # (i.e., ``$TMP` or ``$TMPDIR``).
   #
-  # The build stage can be purged with `spack clean --stage`.
+  # Another option that prevents conflicts and potential permission issues is
+  # to specify `~/.spack/stage`, which ensures each user builds in their home
+  # directory.
+  #
+  # A more traditional path uses the value of `$spack/var/spack/stage`, which
+  # builds directly inside Spack's instance without staging them in a
+  # temporary space.  Problems with specifying a path inside a Spack instance
+  # are that it precludes its use as a system package and its ability to be
+  # pip installable.
+  #
+  # In any case, if the username is not already in the path, Spack will append
+  # the value of `$user` in an attempt to avoid potential conflicts between
+  # users in shared temporary spaces.
+  #
+  # The build stage can be purged with `spack clean --stage` and
+  # `spack clean -a`, so it is important that the specified directory uniquely
+  # identifies Spack staging to avoid accidentally wiping out non-Spack work.
   build_stage:
-    - $tempdir
-    - ~/.spack/var/spack/stage
+    - $tempdir/$user/spack-stage
+    - ~/.spack/stage
+  # - $spack/var/spack/stage
 
 
   # Cache directory for already downloaded source tarballs and archived

etc/spack/defaults/release.yaml

@@ -1,16 +0,0 @@
-# -------------------------------------------------------------------------
-# This is the default spack release spec set.
-# -------------------------------------------------------------------------
-spec-set:
-    include: []
-    exclude: []
-    matrix:
-        - packages:
-            xsdk:
-                versions: [0.4.0]
-        - compilers:
-            gcc:
-                versions: [5.5.0]
-            clang:
-                versions: [6.0.0, '6.0.0-1ubuntu2']
-    cdash: ["https://spack.io/cdash/submit.php?project=spack"]