dependencies: allow get_variable to expose files from subprojects

There are somewhat common, reasonable and legitimate use cases for a
dependency to provide data files installed to /usr which are used as
command inputs. When getting a dependency from a subproject, however,
the attempt to directly construct an input file from a subproject
results in a sandbox violation. This means not all dependencies can be
wrapped as a subproject.

One example is wayland-protocols XML files which get scanned and used to
produce C source files.

Teach Meson to recognize when a string path is the result of fetching a
dep.get_variable(), and special case this to be exempt from subproject
violations.

A requirement of this is that the file must be installed by
install_data() or install_subdir() because otherwise it is not actually
representative of what a pkg-config dependency would provide.

4 files changed, 49 insertions, 4 deletions

diff --git a/mesonbuild/interpreter/interpreter.py b/mesonbuild/interpreter/interpreter.py
index b31b7a8..5bac8ba 100644
--- a/mesonbuild/interpreter/interpreter.py
+++ b/mesonbuild/interpreter/interpreter.py
@@ -420,6 +420,7 @@ class Interpreter(InterpreterBase, HoldableObject):
             bool: P_OBJ.BooleanHolder,
             str: P_OBJ.StringHolder,
             P_OBJ.MesonVersionString: P_OBJ.MesonVersionStringHolder,
+            P_OBJ.DependencyVariableString: P_OBJ.DependencyVariableStringHolder,
 
             # Meson types
             mesonlib.File: OBJ.FileHolder,
@@ -2716,7 +2717,12 @@ external dependencies (including libraries) must go to "dependencies".''')
     @typed_pos_args('join_paths', varargs=str, min_varargs=1)
     @noKwargs
     def func_join_paths(self, node: mparser.BaseNode, args: T.Tuple[T.List[str]], kwargs: 'TYPE_kwargs') -> str:
-        return os.path.join(*args[0]).replace('\\', '/')
+        parts = args[0]
+        ret = os.path.join(*parts).replace('\\', '/')
+        if isinstance(parts[0], P_OBJ.DependencyVariableString):
+            return P_OBJ.DependencyVariableString(ret)
+        else:
+            return ret
 
     def run(self) -> None:
         super().run()
@@ -2759,6 +2765,26 @@ Try setting b_lundef to false instead.'''.format(self.coredata.options[OptionKey
     # declare_dependency).
     def validate_within_subproject(self, subdir, fname):
         srcdir = Path(self.environment.source_dir)
+        builddir = Path(self.environment.build_dir)
+        if isinstance(fname, P_OBJ.DependencyVariableString):
+            def validate_installable_file(fpath: Path) -> bool:
+                installablefiles: T.Set[Path] = set()
+                for d in self.build.data:
+                    for s in d.sources:
+                        installablefiles.add(Path(s.absolute_path(srcdir, builddir)))
+                installabledirs = [str(Path(srcdir, s.source_subdir)) for s in self.build.install_dirs]
+                if fpath in installablefiles:
+                    return True
+                for d in installabledirs:
+                    if str(fpath).startswith(d):
+                        return True
+                return False
+
+            norm = Path(fname)
+            # variables built from a dep.get_variable are allowed to refer to
+            # subproject files, as long as they are scheduled to be installed.
+            if norm.is_absolute() and '..' not in norm.parts and validate_installable_file(norm):
+                return
         norm = Path(srcdir, subdir, fname).resolve()
         if os.path.isdir(norm):
             inputtype = 'directory'
diff --git a/mesonbuild/interpreter/interpreterobjects.py b/mesonbuild/interpreter/interpreterobjects.py
index 39085e5..4aa5548 100644
--- a/mesonbuild/interpreter/interpreterobjects.py
+++ b/mesonbuild/interpreter/interpreterobjects.py
@@ -21,6 +21,7 @@ from ..interpreterbase import (
                                typed_pos_args, typed_kwargs, typed_operator,
                                noArgsFlattening, noPosargs, noKwargs, unholder_return,
                                flatten, resolve_second_level_holders, InterpreterException, InvalidArguments, InvalidCode)
+from ..interpreter.primitives import DependencyVariableString
 from ..interpreter.type_checking import NoneType, ENV_SEPARATOR_KW
 from ..dependencies import Dependency, ExternalLibrary, InternalDependency
 from ..programs import ExternalProgram
@@ -483,14 +484,14 @@ class DependencyHolder(ObjectHolder[Dependency]):
         default_varname = args[0]
         if default_varname is not None:
             FeatureNew('Positional argument to dependency.get_variable()', '0.58.0').use(self.subproject, self.current_node)
-        return self.held_object.get_variable(
+        return DependencyVariableString(self.held_object.get_variable(
             cmake=kwargs['cmake'] or default_varname,
             pkgconfig=kwargs['pkgconfig'] or default_varname,
             configtool=kwargs['configtool'] or default_varname,
             internal=kwargs['internal'] or default_varname,
             default_value=kwargs['default_value'],
             pkgconfig_define=kwargs['pkgconfig_define'],
-        )
+        ))
 
     @FeatureNew('dependency.include_type', '0.52.0')
     @noPosargs
diff --git a/mesonbuild/interpreter/primitives/__init__.py b/mesonbuild/interpreter/primitives/__init__.py
index b4fe621..1874d0d 100644
--- a/mesonbuild/interpreter/primitives/__init__.py
+++ b/mesonbuild/interpreter/primitives/__init__.py
@@ -10,6 +10,8 @@ __all__ = [
     'StringHolder',
     'MesonVersionString',
     'MesonVersionStringHolder',
+    'DependencyVariableString',
+    'DependencyVariableStringHolder',
 ]
 
 from .array import ArrayHolder
@@ -17,4 +19,8 @@ from .boolean import BooleanHolder
 from .dict import DictHolder
 from .integer import IntegerHolder
 from .range import RangeHolder
-from .string import StringHolder, MesonVersionString, MesonVersionStringHolder
+from .string import (
+    StringHolder,
+    MesonVersionString, MesonVersionStringHolder,
+    DependencyVariableString, DependencyVariableStringHolder
+)
diff --git a/mesonbuild/interpreter/primitives/string.py b/mesonbuild/interpreter/primitives/string.py
index 9129303..bf213ef 100644
--- a/mesonbuild/interpreter/primitives/string.py
+++ b/mesonbuild/interpreter/primitives/string.py
@@ -179,3 +179,15 @@ class MesonVersionStringHolder(StringHolder):
     def version_compare_method(self, args: T.Tuple[str], kwargs: TYPE_kwargs) -> bool:
         self.interpreter.tmp_meson_version = args[0]
         return version_compare(self.held_object, args[0])
+
+# These special subclasses of string exist to cover the case where a dependency
+# exports a string variable interchangeable with a system dependency. This
+# matters because a dependency can only have string-type get_variable() return
+# values. If at any time dependencies start supporting additional variable
+# types, this class could be deprecated.
+class DependencyVariableString(str):
+    pass
+
+class DependencyVariableStringHolder(StringHolder):
+    def op_div(self, other: str) -> DependencyVariableString:
+        return DependencyVariableString(super().op_div(other))