allow packages to request no submodules be updated (#40409)

* allow packages to request no submodules be updated when self.submodules is a
  callable function

* Extend the test added in Allow more fine-grained control over what submodules are
  updated: part 2 #27293 to include this case

* Update the type signature for the submodules arg of version() in directives.py

---------

Co-authored-by: tjfulle <tjfulle@users.noreply.github.com>

3 files changed, 39 insertions, 5 deletions

diff --git a/lib/spack/spack/directives.py b/lib/spack/spack/directives.py
index 2524562428..feb0412d84 100644
--- a/lib/spack/spack/directives.py
+++ b/lib/spack/spack/directives.py
@@ -34,7 +34,7 @@ import collections.abc
 import functools
 import os.path
 import re
-from typing import Any, Callable, List, Optional, Set, Tuple, Union
+from typing import TYPE_CHECKING, Any, Callable, List, Optional, Set, Tuple, Union
 
 import llnl.util.lang
 import llnl.util.tty.color
@@ -57,6 +57,9 @@ from spack.version import (
     VersionLookupError,
 )
 
+if TYPE_CHECKING:
+    import spack.package_base
+
 __all__ = [
     "DirectiveError",
     "DirectiveMeta",
@@ -349,6 +352,7 @@ class DirectiveMeta(type):
         return _decorator
 
 
+SubmoduleCallback = Callable[["spack.package_base.PackageBase"], Union[str, List[str], bool]]
 directive = DirectiveMeta.directive
 
 
@@ -380,7 +384,7 @@ def version(
     tag: Optional[str] = None,
     branch: Optional[str] = None,
     get_full_repo: Optional[bool] = None,
-    submodules: Optional[bool] = None,
+    submodules: Union[SubmoduleCallback, Optional[bool]] = None,
     submodules_delete: Optional[bool] = None,
     # other version control
     svn: Optional[str] = None,
diff --git a/lib/spack/spack/fetch_strategy.py b/lib/spack/spack/fetch_strategy.py
index 9393a7c78b..c803b304c2 100644
--- a/lib/spack/spack/fetch_strategy.py
+++ b/lib/spack/spack/fetch_strategy.py
@@ -929,9 +929,12 @@ class GitFetchStrategy(VCSFetchStrategy):
         git_commands = []
         submodules = self.submodules
         if callable(submodules):
-            submodules = list(submodules(self.package))
-            git_commands.append(["submodule", "init", "--"] + submodules)
-            git_commands.append(["submodule", "update", "--recursive"])
+            submodules = submodules(self.package)
+            if submodules:
+                if isinstance(submodules, str):
+                    submodules = [submodules]
+                git_commands.append(["submodule", "init", "--"] + submodules)
+                git_commands.append(["submodule", "update", "--recursive"])
         elif submodules:
             git_commands.append(["submodule", "update", "--init", "--recursive"])
 
diff --git a/lib/spack/spack/test/git_fetch.py b/lib/spack/spack/test/git_fetch.py
index a68eccb2a5..52b164e422 100644
--- a/lib/spack/spack/test/git_fetch.py
+++ b/lib/spack/spack/test/git_fetch.py
@@ -363,3 +363,30 @@ def test_gitsubmodules_delete(
         assert not os.path.isdir(file_path)
         file_path = os.path.join(s.package.stage.source_path, "third_party/submodule1")
         assert not os.path.isdir(file_path)
+
+
+@pytest.mark.disable_clean_stage_check
+def test_gitsubmodules_falsey(
+    mock_git_repository, default_mock_concretization, mutable_mock_repo, monkeypatch
+):
+    """
+    Test GitFetchStrategy behavior when callable submodules returns Falsey
+    """
+
+    def submodules_callback(package):
+        return False
+
+    type_of_test = "tag-branch"
+    t = mock_git_repository.checks[type_of_test]
+
+    # Construct the package under test
+    s = default_mock_concretization("git-test")
+    args = copy.copy(t.args)
+    args["submodules"] = submodules_callback
+    monkeypatch.setitem(s.package.versions, Version("git"), args)
+    s.package.do_stage()
+    with working_dir(s.package.stage.source_path):
+        file_path = os.path.join(s.package.stage.source_path, "third_party/submodule0/r0_file_0")
+        assert not os.path.isfile(file_path)
+        file_path = os.path.join(s.package.stage.source_path, "third_party/submodule1/r0_file_1")
+        assert not os.path.isfile(file_path)