config: Fix requires handling

We were previously replacing the list of "requires" if we happened to
find a set of requires not in the app config. This meant occasionally
some config macros would not be defined for baremetal apps depending on
the order the config files were discovered and parsed.

Now we always append to the list of requires, which is how it's supposed
to work.

Author: rwalton-arm

news/20210729080119.bugfix

@@ -0,0 +1 @@
+Fix handling of baremetal "requires" configuration.

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

@@ -29,6 +29,8 @@ def __setitem__(self, key: Hashable, item: Any) -> None:
             self._handle_overrides(item)
         elif key == MACROS_SECTION:
             self.data[MACROS_SECTION] = self.data.get(MACROS_SECTION, set()) | item
+        elif key == REQUIRES_SECTION:
+            self.data[REQUIRES_SECTION] = self.data.get(REQUIRES_SECTION, set()) | item
         else:
             super().__setitem__(key, item)
 
@@ -71,6 +73,7 @@ def _update_config_section(self, config_settings: List[ConfigSetting]) -> None:
 CONFIG_SECTION = "config"
 MACROS_SECTION = "macros"
 OVERRIDES_SECTION = "overrides"
+REQUIRES_SECTION = "requires"
 
 
 def _apply_override(data: dict, override: Override) -> None:

tests/build/_internal/config/test_config.py

@@ -111,6 +111,14 @@ def test_macros_are_appended_to(self):
 
         assert conf["macros"] == {"A", "B"}
 
+    def test_requires_are_appended_to(self):
+        conf = Config({"requires": {"A"}})
+
+        conf.update({"requires": {"B"}})
+        conf.update({"requires": {"C"}})
+
+        assert conf["requires"] == {"A", "B", "C"}
+
     def test_warns_and_skips_override_for_undefined_config_parameter(self, caplog):
         conf = Config()
         override_name = "this-does-not-exist"