refactor: Refactor BothLibraries logic

This commit introduces a new type of `HoldableObject`: The
`SecondLevelHolder`. The primary purpose of this class is
to handle cases where two (or more) `HoldableObject`s are
stored at the same time (with one default object). The
best (and currently only) example here is the `BothLibraries`
class.

13 files changed, 56 insertions, 50 deletions

diff --git a/mesonbuild/build.py b/mesonbuild/build.py
index 25c8ca1..f9de5c4 100644
--- a/mesonbuild/build.py
+++ b/mesonbuild/build.py
@@ -28,7 +28,7 @@ from . import dependencies
 from . import mlog
 from . import programs
 from .mesonlib import (
-    HoldableObject,
+    HoldableObject, SecondLevelHolder,
     File, MesonException, MachineChoice, PerMachine, OrderedSet, listify,
     extract_as_list, typeslistify, stringlistify, classify_unity_sources,
     get_filenames_templates_dict, substitute_values, has_path_sep,
@@ -1234,8 +1234,6 @@ You probably should put it in link_with instead.''')
 
     def link(self, target):
         for t in listify(target):
-            if isinstance(t, BothLibraries):
-                t = t.get_preferred_library()
             if isinstance(self, StaticLibrary) and self.need_install:
                 if isinstance(t, (CustomTarget, CustomTargetIndex)):
                     if not t.should_install():
@@ -1264,9 +1262,6 @@ You probably should put it in link_with instead.''')
 
     def link_whole(self, target):
         for t in listify(target):
-            # Always use the static library from BothLibraries, since shared libs aren't supported anyway
-            if isinstance(t, BothLibraries):
-                t = t.static
             if isinstance(t, (CustomTarget, CustomTargetIndex)):
                 if not t.is_linkable_target():
                     raise InvalidArguments(f'Custom target {t!r} is not linkable.')
@@ -2152,7 +2147,7 @@ class SharedModule(SharedLibrary):
     def get_default_install_dir(self, environment):
         return environment.get_shared_module_dir()
 
-class BothLibraries(HoldableObject):
+class BothLibraries(SecondLevelHolder):
     def __init__(self, shared: SharedLibrary, static: StaticLibrary) -> None:
         self._preferred_library = 'shared'
         self.shared = shared
@@ -2162,7 +2157,7 @@ class BothLibraries(HoldableObject):
     def __repr__(self) -> str:
         return f'<BothLibraries: static={repr(self.static)}; shared={repr(self.shared)}>'
 
-    def get_preferred_library(self) -> BuildTarget:
+    def get_default_object(self) -> BuildTarget:
         if self._preferred_library == 'shared':
             return self.shared
         elif self._preferred_library == 'static':
@@ -2174,8 +2169,6 @@ class CommandBase:
         cmd = listify(cmd)
         final_cmd = []
         for c in cmd:
-            if isinstance(c, BothLibraries):
-                c = c.get_preferred_library()
             if isinstance(c, str):
                 final_cmd.append(c)
             elif isinstance(c, File):
@@ -2274,7 +2267,6 @@ class CustomTarget(Target, CommandBase):
     def process_kwargs(self, kwargs, backend):
         self.process_kwargs_base(kwargs)
         self.sources = extract_as_list(kwargs, 'input')
-        self.sources = [x.get_preferred_library() if isinstance(x, BothLibraries) else x for x in self.sources]
         if 'output' not in kwargs:
             raise InvalidArguments('Missing keyword argument "output".')
         self.outputs = listify(kwargs['output'])
@@ -2651,8 +2643,6 @@ def get_sources_string_names(sources, backend):
     '''
     names = []
     for s in sources:
-        if isinstance(s, BothLibraries):
-            s = s.get_preferred_library()
         if isinstance(s, str):
             names.append(s)
         elif isinstance(s, (BuildTarget, CustomTarget, CustomTargetIndex, GeneratedList)):
diff --git a/mesonbuild/dependencies/base.py b/mesonbuild/dependencies/base.py
index 7240a81..e12c697 100644
--- a/mesonbuild/dependencies/base.py
+++ b/mesonbuild/dependencies/base.py
@@ -29,7 +29,7 @@ from ..interpreterbase import FeatureDeprecated
 if T.TYPE_CHECKING:
     from ..compilers.compilers import Compiler
     from ..environment import Environment
-    from ..build import BuildTarget, BothLibraries
+    from ..build import BuildTarget
     from ..mesonlib import FileOrString
 
 
@@ -222,8 +222,8 @@ class Dependency(HoldableObject):
 
 class InternalDependency(Dependency):
     def __init__(self, version: str, incdirs: T.List[str], compile_args: T.List[str],
-                 link_args: T.List[str], libraries: T.List[T.Union['BuildTarget', 'BothLibraries']],
-                 whole_libraries: T.List[T.Union['BuildTarget', 'BothLibraries']], sources: T.List['FileOrString'],
+                 link_args: T.List[str], libraries: T.List['BuildTarget'],
+                 whole_libraries: T.List['BuildTarget'], sources: T.List['FileOrString'],
                  ext_deps: T.List[Dependency], variables: T.Dict[str, T.Any]):
         super().__init__(DependencyTypeName('internal'), {})
         self.version = version
@@ -237,11 +237,6 @@ class InternalDependency(Dependency):
         self.ext_deps = ext_deps
         self.variables = variables
 
-        # Deal with BothLibraries
-        from ..build import BothLibraries
-        self.libraries = [x.get_preferred_library() if isinstance(x, BothLibraries) else x for x in self.libraries]
-        self.whole_libraries = [x.static if isinstance(x, BothLibraries) else x for x in self.whole_libraries]
-
     def __deepcopy__(self, memo: T.Dict[int, 'InternalDependency']) -> 'InternalDependency':
         result = self.__class__.__new__(self.__class__)
         assert isinstance(result, InternalDependency)
diff --git a/mesonbuild/interpreter/interpreter.py b/mesonbuild/interpreter/interpreter.py
index d9ecd25..e4fb0d4 100644
--- a/mesonbuild/interpreter/interpreter.py
+++ b/mesonbuild/interpreter/interpreter.py
@@ -26,7 +26,7 @@ from ..programs import ExternalProgram, NonExistingExternalProgram
 from ..dependencies import Dependency
 from ..depfile import DepFile
 from ..interpreterbase import ContainerTypeInfo, InterpreterBase, KwargInfo, typed_kwargs, typed_pos_args
-from ..interpreterbase import noPosargs, noKwargs, stringArgs, permittedKwargs, noArgsFlattening, unholder_return
+from ..interpreterbase import noPosargs, noKwargs, stringArgs, permittedKwargs, noArgsFlattening, noSecondLevelHolderResolving, unholder_return
 from ..interpreterbase import InterpreterException, InvalidArguments, InvalidCode, SubdirDoneRequest
 from ..interpreterbase import Disabler, disablerIfNotFound
 from ..interpreterbase import FeatureNew, FeatureDeprecated, FeatureNewKwargs, FeatureDeprecatedKwargs
@@ -2537,8 +2537,6 @@ Try setting b_lundef to false instead.'''.format(self.coredata.options[OptionKey
             sources = [sources]
         results: T.List['SourceOutputs'] = []
         for s in sources:
-            if isinstance(s, build.BothLibraries):
-                s = s.get_preferred_library()
             if isinstance(s, str):
                 self.validate_within_subproject(self.subdir, s)
                 results.append(mesonlib.File.from_source_file(self.environment.source_dir, self.subdir, s))
@@ -2713,6 +2711,7 @@ This will become a hard error in the future.''', location=self.current_node)
 
     @noKwargs
     @noArgsFlattening
+    @noSecondLevelHolderResolving
     def func_set_variable(self, node, args, kwargs):
         if len(args) != 2:
             raise InvalidCode('Set_variable takes two arguments.')
diff --git a/mesonbuild/interpreter/interpreterobjects.py b/mesonbuild/interpreter/interpreterobjects.py
index 261d781..03725b9 100644
--- a/mesonbuild/interpreter/interpreterobjects.py
+++ b/mesonbuild/interpreter/interpreterobjects.py
@@ -19,7 +19,7 @@ from ..interpreterbase import (
                                FeatureCheckBase, FeatureNewKwargs, FeatureNew, FeatureDeprecated,
                                typed_pos_args, typed_kwargs, KwargInfo, stringArgs, permittedKwargs,
                                noArgsFlattening, noPosargs, noKwargs, unholder_return, TYPE_var, TYPE_kwargs, TYPE_nvar, TYPE_nkwargs,
-                               flatten, InterpreterException, InvalidArguments, InvalidCode)
+                               flatten, resolve_second_level_holders, InterpreterException, InvalidArguments, InvalidCode)
 from ..dependencies import Dependency, ExternalLibrary, InternalDependency
 from ..programs import ExternalProgram
 from ..mesonlib import HoldableObject, MesonException, OptionKey, listify, Popen_safe
@@ -754,6 +754,8 @@ class ModuleObjectHolder(ObjectHolder[ModuleObject]):
             raise InvalidCode(f'Unknown method {method_name!r} in object.')
         if not getattr(method, 'no-args-flattening', False):
             args = flatten(args)
+        if not getattr(method, 'no-second-level-holder-flattening', False):
+            args, kwargs = resolve_second_level_holders(args, kwargs)
         state = ModuleState(self.interpreter)
         # Many modules do for example self.interpreter.find_program_impl(),
         # so we have to ensure they use the current interpreter and not the one
@@ -798,7 +800,7 @@ class BuildTargetHolder(ObjectHolder[_BuildTarget]):
     @property
     def _target_object(self) -> build.BuildTarget:
         if isinstance(self.held_object, build.BothLibraries):
-            return self.held_object.get_preferred_library()
+            return self.held_object.get_default_object()
         assert isinstance(self.held_object, build.BuildTarget)
         return self.held_object
 
diff --git a/mesonbuild/interpreterbase/__init__.py b/mesonbuild/interpreterbase/__init__.py
index d776ae1..298a034 100644
--- a/mesonbuild/interpreterbase/__init__.py
+++ b/mesonbuild/interpreterbase/__init__.py
@@ -33,12 +33,14 @@ __all__ = [
     'check_stringlist',
     'default_resolve_key',
     'flatten',
+    'resolve_second_level_holders',
 
     'noPosargs',
     'builtinMethodNoKwargs',
     'noKwargs',
     'stringArgs',
     'noArgsFlattening',
+    'noSecondLevelHolderResolving',
     'unholder_return',
     'disablerIfNotFound',
     'permittedKwargs',
@@ -91,6 +93,7 @@ from .decorators import (
     noKwargs,
     stringArgs,
     noArgsFlattening,
+    noSecondLevelHolderResolving,
     unholder_return,
     disablerIfNotFound,
     permittedKwargs,
@@ -115,5 +118,5 @@ from .exceptions import (
 )
 
 from .disabler import Disabler, is_disabled
-from .helpers import check_stringlist, default_resolve_key, flatten
+from .helpers import check_stringlist, default_resolve_key, flatten, resolve_second_level_holders
 from .interpreterbase import MesonVersionString, InterpreterBase
diff --git a/mesonbuild/interpreterbase/baseobjects.py b/mesonbuild/interpreterbase/baseobjects.py
index d82aad2..e65e4cb 100644
--- a/mesonbuild/interpreterbase/baseobjects.py
+++ b/mesonbuild/interpreterbase/baseobjects.py
@@ -14,7 +14,7 @@
 
 from .. import mparser
 from .exceptions import InvalidCode
-from .helpers import flatten
+from .helpers import flatten, resolve_second_level_holders
 from ..mesonlib import HoldableObject
 
 import typing as T
@@ -57,6 +57,8 @@ class InterpreterObject:
             method = self.methods[method_name]
             if not getattr(method, 'no-args-flattening', False):
                 args = flatten(args)
+            if not getattr(method, 'no-second-level-holder-flattening', False):
+                args, kwargs = resolve_second_level_holders(args, kwargs)
             return method(args, kwargs)
         raise InvalidCode(f'Unknown method "{method_name}" in object {self} of type {type(self).__name__}.')
 
diff --git a/mesonbuild/interpreterbase/decorators.py b/mesonbuild/interpreterbase/decorators.py
index b05df5b..b91e9d5 100644
--- a/mesonbuild/interpreterbase/decorators.py
+++ b/mesonbuild/interpreterbase/decorators.py
@@ -68,6 +68,10 @@ def noArgsFlattening(f: TV_func) -> TV_func:
     setattr(f, 'no-args-flattening', True)  # noqa: B010
     return f
 
+def noSecondLevelHolderResolving(f: TV_func) -> TV_func:
+    setattr(f, 'no-second-level-holder-flattening', True)  # noqa: B010
+    return f
+
 def unholder_return(f: TV_func) -> T.Callable[..., TYPE_var]:
     @wraps(f)
     def wrapped(*wrapped_args: T.Any, **wrapped_kwargs: T.Any) -> T.Any:
diff --git a/mesonbuild/interpreterbase/helpers.py b/mesonbuild/interpreterbase/helpers.py
index 2602e80..2352577 100644
--- a/mesonbuild/interpreterbase/helpers.py
+++ b/mesonbuild/interpreterbase/helpers.py
@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from .. import mparser, mlog
+from .. import mesonlib, mparser, mlog
 from .exceptions import InvalidArguments, InterpreterException
 
 import collections.abc
@@ -38,6 +38,17 @@ def flatten(args: T.Union['TYPE_var', T.List['TYPE_var']]) -> T.List['TYPE_var']
             result.append(a)
     return result
 
+def resolve_second_level_holders(args: T.List['TYPE_var'], kwargs: 'TYPE_kwargs') -> T.Tuple[T.List['TYPE_var'], 'TYPE_kwargs']:
+    def resolver(arg: 'TYPE_var') -> 'TYPE_var':
+        if isinstance(arg, list):
+            return [resolver(x) for x in arg]
+        if isinstance(arg, dict):
+            return {k: resolver(v) for k, v in arg.items()}
+        if isinstance(arg, mesonlib.SecondLevelHolder):
+            return arg.get_default_object()
+        return arg
+    return [resolver(x) for x in args], {k: resolver(v) for k, v in kwargs.items()}
+
 def check_stringlist(a: T.Any, msg: str = 'Arguments must be strings.') -> None:
     if not isinstance(a, list):
         mlog.debug('Not a list:', str(a))
diff --git a/mesonbuild/interpreterbase/interpreterbase.py b/mesonbuild/interpreterbase/interpreterbase.py
index 3ea10f4..3f5726b 100644
--- a/mesonbuild/interpreterbase/interpreterbase.py
+++ b/mesonbuild/interpreterbase/interpreterbase.py
@@ -42,7 +42,7 @@ from .exceptions import (
 
 from .decorators import FeatureNew, builtinMethodNoKwargs
 from .disabler import Disabler, is_disabled
-from .helpers import check_stringlist, default_resolve_key, flatten
+from .helpers import check_stringlist, default_resolve_key, flatten, resolve_second_level_holders
 from ._unholder import _unholder
 
 import os, copy, re
@@ -546,6 +546,8 @@ The result of this is undefined and will become a hard error in a future Meson r
             func_args = posargs
             if not getattr(func, 'no-args-flattening', False):
                 func_args = flatten(posargs)
+            if not getattr(func, 'no-second-level-holder-flattening', False):
+                func_args, kwargs = resolve_second_level_holders(func_args, kwargs)
             res = func(node, func_args, kwargs)
             return self._holderify(res)
         else:
diff --git a/mesonbuild/mesonlib/universal.py b/mesonbuild/mesonlib/universal.py
index 4b4eb6e..0ed99a5 100644
--- a/mesonbuild/mesonlib/universal.py
+++ b/mesonbuild/mesonlib/universal.py
@@ -48,6 +48,7 @@ __all__ = [
     'python_command',
     'project_meson_versions',
     'HoldableObject',
+    'SecondLevelHolder',
     'File',
     'FileMode',
     'GitException',
@@ -273,6 +274,14 @@ class HoldableObject(metaclass=abc.ABCMeta):
     ''' Dummy base class for all objects that can be
         held by an interpreter.baseobjects.ObjectHolder '''
 
+class SecondLevelHolder(HoldableObject, metaclass=abc.ABCMeta):
+    ''' A second level object holder. The primary purpose
+        of such objects is to hold multiple objects with one
+        default option. '''
+
+    @abc.abstractmethod
+    def get_default_object(self) -> HoldableObject: ...
+
 class FileMode:
     # The first triad is for owner permissions, the second for group permissions,
     # and the third for others (everyone else).
diff --git a/mesonbuild/modules/gnome.py b/mesonbuild/modules/gnome.py
index a8f0cc5..efd1f00 100644
--- a/mesonbuild/modules/gnome.py
+++ b/mesonbuild/modules/gnome.py
@@ -494,8 +494,6 @@ class GnomeModule(ExtensionModule):
             return cflags, internal_ldflags, external_ldflags, external_ldflags_nodedup, gi_includes
 
     def _unwrap_gir_target(self, girtarget, state):
-        if isinstance(girtarget, build.BothLibraries):
-            girtarget = girtarget.get_preferred_library()
         if not isinstance(girtarget, (build.Executable, build.SharedLibrary,
                                       build.StaticLibrary)):
             raise MesonException(f'Gir target must be an executable or library but is "{girtarget}" of type {type(girtarget).__name__}')
diff --git a/mesonbuild/modules/pkgconfig.py b/mesonbuild/modules/pkgconfig.py
index f42c1e6..c6eaedc 100644
--- a/mesonbuild/modules/pkgconfig.py
+++ b/mesonbuild/modules/pkgconfig.py
@@ -22,9 +22,7 @@ from .. import mesonlib
 from .. import mlog
 from . import ModuleReturnValue
 from . import ExtensionModule
-from ..interpreterbase import permittedKwargs, FeatureNew, FeatureNewKwargs, TYPE_var, TYPE_kwargs
-
-import typing as T
+from ..interpreterbase import permittedKwargs, FeatureNew, FeatureNewKwargs
 
 already_warned_objs = set()
 
@@ -112,7 +110,6 @@ class DependenciesHelper:
 
     def _process_libs(self, libs, public: bool):
         libs = mesonlib.listify(libs)
-        libs = [x.get_preferred_library() if isinstance(x, build.BothLibraries) else x for x in libs]
         processed_libs = []
         processed_reqs = []
         processed_cflags = []
@@ -454,19 +451,6 @@ class PkgConfigModule(ExtensionModule):
             if cflags and not dataonly:
                 ofile.write('Cflags: {}\n'.format(' '.join(cflags)))
 
-
-    @staticmethod
-    def _handle_both_libraries(args: T.List[TYPE_var], kwargs: TYPE_kwargs) -> T.Tuple[T.List[TYPE_var], TYPE_kwargs]:
-        def _do_extract(arg: TYPE_var) -> TYPE_var:
-            if isinstance(arg, list):
-                return [_do_extract(x) for x in arg]
-            elif isinstance(arg, dict):
-                return {k: _do_extract(v) for k, v in arg.items()}
-            elif isinstance(arg, build.BothLibraries):
-                return arg.get_preferred_library()
-            return arg
-        return [_do_extract(x) for x in args], {k: _do_extract(v) for k, v in kwargs.items()}
-
     @FeatureNewKwargs('pkgconfig.generate', '0.59.0', ['unescaped_variables', 'unescaped_uninstalled_variables'])
     @FeatureNewKwargs('pkgconfig.generate', '0.54.0', ['uninstalled_variables'])
     @FeatureNewKwargs('pkgconfig.generate', '0.42.0', ['extra_cflags'])
@@ -484,7 +468,6 @@ class PkgConfigModule(ExtensionModule):
         default_name = None
         mainlib = None
         default_subdirs = ['.']
-        args, kwargs = PkgConfigModule._handle_both_libraries(args, kwargs)
         if not args and 'version' not in kwargs:
             FeatureNew.single_use('pkgconfig.generate implicit version keyword', '0.46.0', state.subproject)
         elif len(args) == 1:
diff --git a/test cases/common/178 bothlibraries/meson.build b/test cases/common/178 bothlibraries/meson.build
index 9df4cd1..f3191cc 100644
--- a/test cases/common/178 bothlibraries/meson.build
+++ b/test cases/common/178 bothlibraries/meson.build
@@ -34,8 +34,16 @@ exe_static2 = executable('prog-static2', 'main.c',
                          link_with : both_libs2.get_static_lib())
 exe_both2 = executable('prog-both2', 'main.c', link_with : both_libs2)
 
+# Test {set,get}_variable
+set_variable('both_libs2', both_libs)
+both_libs3 = get_variable('both_libs')
+
 # Ensure that calling the build target methods also works
 assert(both_libs.name() == 'mylib')
+assert(both_libs2.name() == 'mylib')
+assert(both_libs3.name() == 'mylib')
+assert(both_libs2.get_shared_lib().name() == 'mylib')
+assert(both_libs3.get_static_lib().name() == 'mylib')
 
 test('runtest-shared-2', exe_shared2)
 test('runtest-static-2', exe_static2)