projects: warn on "global" assign of plugs and slots

This commit adds a warning (via emit.message()) when a snapcraft.yaml
project has top-level plugs and/or slots that are used for assignment
and not per-plug/slot configuration. Thanks to lucyllewy for suggested
wording of the warning.

snapcraft/projects.py

@@ -21,10 +21,11 @@
 
 import pydantic
 from craft_archives import repo
+from craft_cli import emit
 from craft_grammar.models import GrammarSingleEntryDictList, GrammarStr, GrammarStrList
 from pydantic import PrivateAttr, conlist, constr
 
-from snapcraft import parts
+from snapcraft import parts, utils
 from snapcraft.errors import ProjectValidationError
 from snapcraft.utils import get_effective_base, get_host_architecture
 
@@ -376,6 +377,14 @@ class ContentPlug(ProjectModel):
 
 MANDATORY_ADOPTABLE_FIELDS = ("version", "summary", "description")
 
+GLOBAL_PLUGS_SLOTS_WARNING = (
+    "Implicit {lowercase} assignment in {culprits}. "
+    "{uppercase}s should be assigned to the app that they apply, "
+    "and not implicitly assigned via the global '{lowercase}s:' "
+    "stanza which is intended for configuration only."
+    "\n(Reference: https://snapcraft.io/docs/snapcraft-interfaces)"
+)
+
 
 class Project(ProjectModel):
     """Snapcraft project definition.
@@ -429,6 +438,7 @@ class Project(ProjectModel):
     @pydantic.validator("plugs")
     @classmethod
     def _validate_plugs(cls, plugs):
+        empty_plugs = []
         if plugs is not None:
             for plug_name, plug in plugs.items():
                 if (
@@ -442,8 +452,36 @@ def _validate_plugs(cls, plugs):
                 if isinstance(plug, list):
                     raise ValueError(f"Plug '{plug_name}' cannot be a list.")
 
+                if plug is None:
+                    empty_plugs.append(plug_name)
+
+        if empty_plugs:
+            culprits = utils.humanize_list(empty_plugs, "and")
+            message = GLOBAL_PLUGS_SLOTS_WARNING.format(
+                lowercase="plug", uppercase="Plug", culprits=culprits
+            )
+            emit.message(message)
+
         return plugs
 
+    @pydantic.validator("slots")
+    @classmethod
+    def _validate_slots(cls, slots):
+        empty_slots = []
+        if slots is not None:
+            for slot_name, slot in slots.items():
+                if slot is None:
+                    empty_slots.append(slot_name)
+
+        if empty_slots:
+            culprits = utils.humanize_list(empty_slots, "and")
+            message = GLOBAL_PLUGS_SLOTS_WARNING.format(
+                lowercase="slot", uppercase="Slot", culprits=culprits
+            )
+            emit.message(message)
+
+        return slots
+
     @pydantic.root_validator(pre=True)
     @classmethod
     def _validate_adoptable_fields(cls, values):

tests/spread/core22/plugs-warn/expected_output.txt

@@ -0,0 +1,5 @@
+Implicit plug assignment in 'desktop' and 'desktop-legacy'. Plugs should be assigned to the app that they apply, and not implicitly assigned via the global 'plugs:' stanza which is intended for configuration only.
+(Reference: https://snapcraft.io/docs/snapcraft-interfaces)
+Implicit slot assignment in 'network' and 'opengl'. Slots should be assigned to the app that they apply, and not implicitly assigned via the global 'slots:' stanza which is intended for configuration only.
+(Reference: https://snapcraft.io/docs/snapcraft-interfaces)
+Initializing parts lifecycle

tests/spread/core22/plugs-warn/snap/snapcraft.yaml

@@ -0,0 +1,20 @@
+name: plugs-warn
+base: core22
+version: '0.1'
+summary: Check warnings for top-level enabling of slots and plugs
+description: Check warnings for top-level enabling of slots and plugs.
+
+grade: stable
+confinement: strict
+
+plugs:
+  desktop:
+  desktop-legacy:
+
+slots:
+  network:
+  opengl:
+
+parts:
+  my-part:
+    plugin: nil

tests/spread/core22/plugs-warn/task.yaml

@@ -0,0 +1,9 @@
+summary: Check warnings for top-level enabling of slots and plugs
+
+restore: |
+  snapcraft clean --destructive-mode
+  rm -f ./*.snap
+
+execute: |
+  expected_output=`cat expected_output.txt`
+  snapcraft pull -v --destructive-mode | MATCH "$expected_output"

tests/unit/test_projects.py

@@ -563,6 +563,30 @@ def test_project_build_base_devel_grade_stable_error(self, project_yaml_data):
         with pytest.raises(errors.ProjectValidationError, match=error):
             Project.unmarshal(project_yaml_data(build_base="devel", grade="stable"))
 
+    def test_project_global_plugs_warning(self, project_yaml_data, emitter):
+        data = project_yaml_data(plugs={"desktop": None, "desktop-legacy": None})
+        Project.unmarshal(data)
+        expected_message = (
+            "Implicit plug assignment in 'desktop' and 'desktop-legacy'. "
+            "Plugs should be assigned to the app that they apply, and not "
+            "implicitly assigned via the global 'plugs:' stanza "
+            "which is intended for configuration only."
+            "\n(Reference: https://snapcraft.io/docs/snapcraft-interfaces)"
+        )
+        emitter.assert_message(expected_message)
+
+    def test_project_global_slots_warning(self, project_yaml_data, emitter):
+        data = project_yaml_data(slots={"home": None, "removable-media": None})
+        Project.unmarshal(data)
+        expected_message = (
+            "Implicit slot assignment in 'home' and 'removable-media'. "
+            "Slots should be assigned to the app that they apply, and not "
+            "implicitly assigned via the global 'slots:' stanza "
+            "which is intended for configuration only."
+            "\n(Reference: https://snapcraft.io/docs/snapcraft-interfaces)"
+        )
+        emitter.assert_message(expected_message)
+
 
 class TestHookValidation:
     """Validate hooks."""