dependencies: extract code to get all leaf dependencies

Extract to a separate function the code that resolves dependencies
for compiler methods.  We will reuse it for add_project_dependencies().

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

mesonbuild/dependencies/__init__.py

@@ -18,7 +18,7 @@ from .hdf5 import hdf5_factory
 from .base import Dependency, InternalDependency, ExternalDependency, NotFoundDependency
 from .base import (
         ExternalLibrary, DependencyException, DependencyMethods,
-        BuiltinDependency, SystemDependency)
+        BuiltinDependency, SystemDependency, get_leaf_external_dependencies)
 from .cmake import CMakeDependency
 from .configtool import ConfigToolDependency
 from .dub import DubDependency
@@ -65,6 +65,7 @@ __all__ = [
 
     'find_external_dependency',
     'get_dep_identifier',
+    'get_leaf_external_dependencies',
 ]
 
 """Dependency representations and discovery logic.

mesonbuild/dependencies/base.py

@@ -23,7 +23,7 @@ import itertools
 import typing as T
 from enum import Enum
 
-from .. import mlog
+from .. import mlog, mesonlib
 from ..compilers import clib_langs
 from ..mesonlib import LibType, MachineChoice, MesonException, HoldableObject
 from ..mesonlib import version_compare_many
@@ -461,6 +461,22 @@ class ExternalLibrary(ExternalDependency):
         return new
 
 
+def get_leaf_external_dependencies(deps: T.List[Dependency]) -> T.List[Dependency]:
+    if not deps:
+        # Ensure that we always return a new instance
+        return deps.copy()
+    final_deps = []
+    while deps:
+        next_deps = []
+        for d in mesonlib.listify(deps):
+            if not isinstance(d, Dependency) or d.is_built():
+                raise DependencyException('Dependencies must be external dependencies')
+            final_deps.append(d)
+            next_deps.extend(d.ext_deps)
+        deps = next_deps
+    return final_deps
+
+
 def sort_libpaths(libpaths: T.List[str], refpaths: T.List[str]) -> T.List[str]:
     """Sort <libpaths> according to <refpaths>
 

mesonbuild/interpreter/compiler.py

@@ -239,20 +239,7 @@ class CompilerHolder(ObjectHolder['Compiler']):
         return args
 
     def _determine_dependencies(self, deps: T.List['dependencies.Dependency'], endl: str = ':') -> T.Tuple[T.List['dependencies.Dependency'], str]:
-        if deps:
-            final_deps = []
-            while deps:
-                next_deps = []
-                for d in mesonlib.listify(deps):
-                    if not isinstance(d, dependencies.Dependency) or d.is_built():
-                        raise InterpreterException('Dependencies must be external dependencies')
-                    final_deps.append(d)
-                    next_deps.extend(d.ext_deps)
-                deps = next_deps
-            deps = final_deps
-        else:
-            # Ensure that we always return a new instance
-            deps = deps.copy()
+        deps = dependencies.get_leaf_external_dependencies(deps)
         return deps, self._dep_msg(deps, endl)
 
     @typed_pos_args('compiler.alignment', str)