Add `spack env activate --temp` (#25388)

Creates an environment in a temporary directory and activates it, which
is useful for a quick ephemeral environment:

```
$ spack env activate -p --temp
[spack-1a203lyg] $ spack add zlib
==> Adding zlib to environment /tmp/spack-1a203lyg
==> Updating view at /tmp/spack-1a203lyg/.spack-env/view
```

2 files changed, 54 insertions, 16 deletions

diff --git a/lib/spack/spack/cmd/env.py b/lib/spack/spack/cmd/env.py
index ae45e105de..5b9caa0877 100644
--- a/lib/spack/spack/cmd/env.py
+++ b/lib/spack/spack/cmd/env.py
@@ -6,6 +6,7 @@
 import os
 import shutil
 import sys
+import tempfile
 
 import llnl.util.filesystem as fs
 import llnl.util.tty as tty
@@ -70,23 +71,37 @@ def env_activate_setup_parser(subparser):
         help="do not update PATH etc. with associated view")
 
     subparser.add_argument(
-        '-d', '--dir', action='store_true', default=False,
-        help="force spack to treat env as a directory, not a name")
-    subparser.add_argument(
         '-p', '--prompt', action='store_true', default=False,
         help="decorate the command line prompt when activating")
-    subparser.add_argument(
-        metavar='env', dest='activate_env',
+
+    env_options = subparser.add_mutually_exclusive_group()
+    env_options.add_argument(
+        '--temp', action='store_true', default=False,
+        help='create and activate an environment in a temporary directory')
+    env_options.add_argument(
+        '-d', '--dir', default=None,
+        help="activate the environment in this directory")
+    env_options.add_argument(
+        metavar='env', dest='activate_env', nargs='?', default=None,
         help='name of environment to activate')
 
 
+def create_temp_env_directory():
+    """
+    Returns the path of a temporary directory in which to
+    create an environment
+    """
+    return tempfile.mkdtemp(prefix="spack-")
+
+
 def env_activate(args):
-    env = args.activate_env
+    if not args.activate_env and not args.dir and not args.temp:
+        tty.die('spack env activate requires an environment name, directory, or --temp')
 
     if not args.shell:
         spack.cmd.common.shell_init_instructions(
             "spack env activate",
-            "    eval `spack env activate {sh_arg} %s`" % env,
+            "    eval `spack env activate {sh_arg} [...]`",
         )
         return 1
 
@@ -95,21 +110,32 @@ def env_activate(args):
         tty.die('Calling spack env activate with --env, --env-dir and --no-env '
                 'is ambiguous')
 
-    if ev.exists(env) and not args.dir:
-        spack_env = ev.root(env)
-        short_name = env
-        env_prompt = '[%s]' % env
+    env_name_or_dir = args.activate_env or args.dir
 
-    elif ev.is_env_dir(env):
+    # Temporary environment
+    if args.temp:
+        env = create_temp_env_directory()
         spack_env = os.path.abspath(env)
-        short_name = os.path.basename(os.path.abspath(env))
-        env_prompt = '[%s]' % short_name
+        short_name = os.path.basename(spack_env)
+        ev.Environment(env).write(regenerate=False)
+
+    # Named environment
+    elif ev.exists(env_name_or_dir) and not args.dir:
+        spack_env = ev.root(env_name_or_dir)
+        short_name = env_name_or_dir
+
+    # Environment directory
+    elif ev.is_env_dir(env_name_or_dir):
+        spack_env = os.path.abspath(env_name_or_dir)
+        short_name = os.path.basename(spack_env)
 
     else:
-        tty.die("No such environment: '%s'" % env)
+        tty.die("No such environment: '%s'" % env_name_or_dir)
+
+    env_prompt = '[%s]' % short_name
 
     if spack_env == os.environ.get('SPACK_ENV'):
-        tty.debug("Environment %s is already active" % args.activate_env)
+        tty.debug("Environment is already active")
         return
 
     # Activate new environment
diff --git a/lib/spack/spack/test/cmd/env.py b/lib/spack/spack/test/cmd/env.py
index 5c57035527..553d80c562 100644
--- a/lib/spack/spack/test/cmd/env.py
+++ b/lib/spack/spack/test/cmd/env.py
@@ -2681,3 +2681,15 @@ spack:
     with spack.environment.Environment(str(tmpdir)):
         assert str(spack.store.root) == '/tmp/store'
     assert str(spack.store.root) == current_store_root
+
+
+def test_activate_temp(monkeypatch, tmpdir):
+    """Tests whether `spack env activate --temp` creates an environment in a
+    temporary directory"""
+    env_dir = lambda: str(tmpdir)
+    monkeypatch.setattr(spack.cmd.env, "create_temp_env_directory", env_dir)
+    shell = env('activate', '--temp', '--sh')
+    active_env_var = next(line for line in shell.splitlines()
+                          if ev.spack_env_var in line)
+    assert str(tmpdir) in active_env_var
+    assert ev.is_env_dir(str(tmpdir))