config: Add support for requires config option

Support for a config option `requires` in `mbed_app.json` is added
which enables application to choose required mbed libraries. if
`requires` is mentioned in `mbed_app.json` then only those mbed
libraries and other libraries those mbed libraries depend on will be
selected. Other mbed libraries are ignored.

news/20201214165520.feature

@@ -0,0 +1 @@
+Add support for requires config option in mbed_app.json

src/mbed_tools/build/_internal/config/assemble_build_config.py

@@ -9,7 +9,8 @@
 from mbed_tools.build._internal.config.config import Config
 from mbed_tools.build._internal.config.cumulative_data import CumulativeData
 from mbed_tools.build._internal.config.source import Source
-from mbed_tools.build._internal.find_files import LabelFilter, filter_files, find_files
+from mbed_tools.lib.json_helpers import decode_json_file
+from mbed_tools.build._internal.find_files import LabelFilter, RequiresFilter, filter_files, find_files
 
 
 def assemble_config(target_attributes: dict, mbed_program_directory: Path, mbed_app_file: Optional[Path]) -> Config:
@@ -27,13 +28,23 @@ def _assemble_config_from_sources_and_lib_files(
     target_attributes: dict, mbed_lib_files: Iterable[Path], mbed_app_file: Optional[Path] = None
 ) -> Config:
     previous_cumulative_data = None
+    requires = list()
     target_source = Source.from_target(target_attributes)
     current_cumulative_data = CumulativeData.from_sources([target_source])
+    if mbed_app_file:
+        app_data = decode_json_file(mbed_app_file)
+        requires = app_data["requires"] if "requires" in app_data else []
+
     while previous_cumulative_data != current_cumulative_data:
         current_labels = current_cumulative_data.labels | current_cumulative_data.extra_labels
         filtered_files = _filter_files(
-            mbed_lib_files, current_labels, current_cumulative_data.features, current_cumulative_data.components
+            mbed_lib_files,
+            current_labels,
+            current_cumulative_data.features,
+            current_cumulative_data.components,
+            requires,
         )
+
         mbed_lib_sources = [Source.from_mbed_lib(file, current_labels) for file in filtered_files]
         all_sources = [target_source] + mbed_lib_sources
         if mbed_app_file:
@@ -48,14 +59,24 @@ def _assemble_config_from_sources_and_lib_files(
 
 
 def _filter_files(
-    files: Iterable[Path], labels: Iterable[str], features: Iterable[str], components: Iterable[str]
+    files: Iterable[Path],
+    labels: Iterable[str],
+    features: Iterable[str],
+    components: Iterable[str],
+    requires: Iterable[str],
 ) -> Iterable[Path]:
     filters = (
         LabelFilter("TARGET", labels),
         LabelFilter("FEATURE", features),
         LabelFilter("COMPONENT", components),
     )
-    return filter_files(files, filters)
+    filtered_files = filter_files(files, filters)
+
+    if not requires:
+        return filtered_files
+    else:
+        requires_filter = RequiresFilter(requires)
+        return requires_filter(filtered_files)
 
 
 def _update_target_attributes(target_attributes: dict, cumulative_data: CumulativeData) -> None:

src/mbed_tools/build/_internal/find_files.py

@@ -5,7 +5,9 @@
 """Find files in MbedOS program directory."""
 from pathlib import Path
 import fnmatch
-from typing import Callable, Iterable, Optional, List, Tuple
+from typing import Callable, Iterable, Optional, List, Tuple, Any, Set
+
+from mbed_tools.lib.json_helpers import decode_json_file
 
 
 def find_files(filename: str, directory: Path) -> List[Path]:
@@ -66,6 +68,56 @@ def filter_files(files: Iterable[Path], filters: Iterable[Callable]) -> Iterable
     return [file for file in files if all(f(file) for f in filters)]
 
 
+class RequiresFilter:
+    """Filter out mbed libraries not needed by application.
+
+     The 'requires' config option in mbed_app.json can specify list of mbed
+    libraries (mbed_lib.json) that application requires. Apply 'requires'
+    filter to remove mbed_lib.json files not required by application.
+    """
+
+    def __init__(self, requires: Iterable[str]):
+        """Initialise the filter attributes.
+
+        Args:
+            requires: List of required mbed libraries.
+        """
+        self._requires = requires
+
+    def __call__(self, files: Iterable[Path]) -> Iterable[Path]:
+        """Apply requires filter and remove mbed_lib.json files not required by application."""
+        requires_filtered_files: Set[Path] = set()
+        return self.apply_requires_filter(requires_filtered_files, files, self._requires)
+
+    @staticmethod
+    def apply_requires_filter(
+        requires_filtered_files: set, files: Iterable[Path], requires: Iterable[str]
+    ) -> Iterable[Path]:
+        """Remove mbed_lib.json files not required by application."""
+        for required_mbed_lib in requires:
+            for mbed_lib_path in files:
+                lib_contents = decode_json_file(mbed_lib_path)
+
+                if required_mbed_lib == RequiresFilter.get_mbed_lib_name(lib_contents):
+                    requires_filtered_files.add(mbed_lib_path)
+
+                    RequiresFilter.apply_requires_filter(
+                        requires_filtered_files, files, RequiresFilter.get_mbed_lib_requires(lib_contents)
+                    )
+
+        return requires_filtered_files
+
+    @staticmethod
+    def get_mbed_lib_requires(lib_contents: Any) -> Any:
+        """Get list of mbed libraries required by appplication."""
+        return lib_contents["requires"] if "requires" in lib_contents else []
+
+    @staticmethod
+    def get_mbed_lib_name(lib_contents: Any) -> Any:
+        """Get mbed library name."""
+        return lib_contents["name"]
+
+
 class LabelFilter:
     """Filter out given paths using path labelling rules.
 

tests/build/test_generate_config.py

@@ -395,3 +395,23 @@ def test_settings_from_multiple_libs_included(matching_target_and_filter, progra
 
     assert "MBED_CONF_PLATFORM_STDIO_BAUD_RATE=9600" in config_text
     assert "MBED_LFS_READ_SIZE=64" in config_text
+
+
+def test_requires_config_option(program):
+    create_mbed_app_json(program.root, requires=["bare-metal"])
+    create_mbed_lib_json(program.mbed_os.root / "bare-metal" / "mbed_lib.json", "bare-metal", requires=["platform"])
+    create_mbed_lib_json(
+        program.mbed_os.root / "platform" / "mbed_lib.json", "platform", config={"stdio-baud-rate": {"value": 9600}},
+    )
+    create_mbed_lib_json(
+        program.mbed_os.root / "storage" / "mbed_lib.json",
+        "filesystem",
+        config={"read_size": {"macro_name": "MBED_LFS_READ_SIZE", "value": 64}},
+    )
+
+    generate_config("K64F", "GCC_ARM", program)
+
+    config_text = program.files.cmake_config_file.read_text()
+
+    assert "MBED_CONF_PLATFORM_STDIO_BAUD_RATE=9600" in config_text
+    assert "MBED_LFS_READ_SIZE=64" not in config_text