Added 'setdefault' subcommand to 'spack lmod'

Added a subcommand that sets the default module to be loaded, if
multiple installations of the same package are present.

lib/spack/spack/cmd/common/modules.py

@@ -35,7 +35,8 @@
 import spack.cmd
 import spack.modules
 import spack.repo
-from spack.cmd.common import arguments
+
+import spack.cmd.common.arguments as arguments
 
 description = "manipulate module files"
 section = "environment"
@@ -51,15 +52,6 @@
 callbacks = {}
 
 
-def subcommand(subparser_name):
-    """Registers a function in the callbacks dictionary"""
-    def decorator(callback):
-        callbacks[subparser_name] = callback
-        return callback
-    return decorator
-from . import arguments
-
-
 def setup_parser(subparser):
     sp = subparser.add_subparsers(metavar='SUBCOMMAND', dest='subparser_name')
 
@@ -105,6 +97,7 @@ def setup_parser(subparser):
     arguments.add_common_arguments(
         loads_parser, ['constraint', 'recurse_dependencies']
     )
+    return sp
 
 
 class MultipleSpecsMatch(Exception):

lib/spack/spack/cmd/lmod.py

@@ -22,6 +22,10 @@
 # License along with this program; if not, write to the Free Software
 # Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
 ##############################################################################
+import os
+
+import llnl.util.filesystem
+import spack.cmd.common.arguments
 import spack.cmd.common.modules
 
 description = "manipulate hierarchical module files"
@@ -33,10 +37,39 @@
 
 
 def setup_parser(subparser):
-    spack.cmd.common.modules.setup_parser(subparser)
+    sp = spack.cmd.common.modules.setup_parser(subparser)
+
+    # Set default module file for a package
+    setdefault = sp.add_parser(
+        'setdefault', help='set the default module file for a package'
+    )
+    spack.cmd.common.arguments.add_common_arguments(
+        setdefault, ['constraint']
+    )
+
+
+def setdefault(module_type, specs, args):
+    """Set the default module file, when multiple are present"""
+    # For details on the underlying mechanism see:
+    #
+    # https://lmod.readthedocs.io/en/latest/060_locating.html#marking-a-version-as-default
+    #
+    spack.cmd.common.modules.one_spec_or_raise(specs)
+    writer = spack.modules.module_types[_module_type](specs[0])
+
+    module_folder = os.path.dirname(writer.layout.filename)
+    module_basename = os.path.basename(writer.layout.filename)
+    with llnl.util.filesystem.working_dir(module_folder):
+        if os.path.exists('default') and os.path.islink('default'):
+            os.remove('default')
+        os.symlink(module_basename, 'default')
+
+
+callbacks = dict(spack.cmd.common.modules.callbacks.items())
+callbacks['setdefault'] = setdefault
 
 
 def lmod(parser, args):
     spack.cmd.common.modules.modules_cmd(
-        parser, args, module_type=_module_type
+        parser, args, module_type=_module_type, callbacks=callbacks
     )