module_cmd: allow loading already-loaded modules

lib/spack/spack/test/util/module_cmd.py

@@ -38,6 +38,22 @@ def mock_module(*args, **kwargs):
     assert "test_module:test_module_2" in os.environ["LOADEDMODULES"]
 
 
+@pytest.mark.not_on_windows("Module files are not supported on Windows")
+def test_load_module_already_loaded(monkeypatch, working_env):
+    """Test that loading an already-loaded module succeeds without reloading it."""
+
+    os.environ["LOADEDMODULES"] = "test_module"
+
+    def mock_module(*args, **kwargs):
+        if args[0] == "show":
+            return ""
+        pytest.fail("load should not be called for an already-loaded module")
+
+    monkeypatch.setattr(spack.util.module_cmd, "module", mock_module)
+
+    spack.util.module_cmd.load_module("test_module")
+
+
 @pytest.mark.not_on_windows("Module files are not supported on Windows")
 def test_load_module_failure(monkeypatch, working_env):
     """Test that load_module raises an exception when a module load fails."""

lib/spack/spack/util/module_cmd.py

@@ -106,6 +106,12 @@ def load_module(mod):
     # Store the LOADEDMODULES before trying to load the new module
     loaded_modules_before = os.environ.get("LOADEDMODULES", "")
 
+    # Loading a module that is already loaded is a successful no-op for
+    # module systems such as Lmod.  In that case LOADEDMODULES is unchanged,
+    # so treating an unchanged value below as a failure would be incorrect.
+    if mod in loaded_modules_before.split(":"):
+        return
+
     # Load the module now that there are no conflicts
     # Some module systems use stdout and some use stderr
     module("load", mod)