bugfix: make _source_single_file work in venvs (#14569)

Using `sys.executable` to run Python in a sub-shell doesn't always work in a virtual environment as the `sys.executable` Python is not necessarily compatible with any loaded spack/other virtual environment.

- revert use of sys.executable to print out subshell environment (#14496)
- try instead to use an available python, then if there *is not* one, use `sys.executable`
- this addresses RHEL8 (where there is no `python` and `PYTHONHOME` issue in a simpler way

lib/spack/spack/util/environment.py

@@ -17,7 +17,6 @@
 
 import llnl.util.tty as tty
 import spack.util.executable as executable
-from spack.util.module_cmd import py_cmd
 
 from llnl.util.lang import dedupe
 
@@ -919,8 +918,14 @@ def _source_single_file(file_and_args, environment):
         source_file.extend(x for x in file_and_args)
         source_file = ' '.join(source_file)
 
-        dump_environment = 'PYTHONHOME="{0}" "{1}" -c "{2}"'.format(
-            sys.prefix, sys.executable, py_cmd)
+        # If the environment contains 'python' use it, if not
+        # go with sys.executable. Below we just need a working
+        # Python interpreter, not necessarily sys.executable.
+        python_cmd = executable.which('python3', 'python', 'python2')
+        python_cmd = python_cmd.name if python_cmd else sys.executable
+
+        dump_cmd = 'import os, json; print(json.dumps(dict(os.environ)))'
+        dump_environment = python_cmd + ' -c "{0}"'.format(dump_cmd)
 
         # Try to source the file
         source_file_arguments = ' '.join([

lib/spack/spack/util/module_cmd.py

@@ -18,7 +18,7 @@
 # This list is not exhaustive. Currently we only use load and unload
 # If we need another option that changes the environment, add it here.
 module_change_commands = ['load', 'swap', 'unload', 'purge', 'use', 'unuse']
-py_cmd = 'import os; import json; print(json.dumps(dict(os.environ)))'
+py_cmd = "'import os;import json;print(json.dumps(dict(os.environ)))'"
 
 # This is just to enable testing. I hate it but we can't find a better way
 _test_mode = False
@@ -32,8 +32,7 @@ def module(*args):
     if args[0] in module_change_commands:
         # Do the module manipulation, then output the environment in JSON
         # and read the JSON back in the parent process to update os.environ
-        module_cmd += ' > /dev/null; PYTHONHOME="{0}" "{1}" -c "{2}"'.format(
-            sys.prefix, sys.executable, py_cmd)
+        module_cmd += ' >/dev/null;' + sys.executable + ' -c %s' % py_cmd
         module_p  = subprocess.Popen(module_cmd,
                                      stdout=subprocess.PIPE,
                                      stderr=subprocess.STDOUT,