feat(metadata): deprecate non-SPDX license values

docs/release-notes/snapcraft-9-0.rst

@@ -147,6 +147,16 @@ files had to end in ``.7zip``.
 Additionally, 7zip files are now documented in the :ref:`source-type <PartSpec.source_type>`
 key in the project file reference.
 
+Deprecation of non-SPDX licenses
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Previously, the ``license`` key could contain any value. Snapcraft now warns when the
+value is not a valid `SPDX license expression <https://spdx.org/licenses/>`__ or
+``proprietary``.
+
+Non-SPDX values are still accepted for compatibility, but are deprecated and may be
+rejected in a future release.
+
 Backwards-incompatible changes
 ------------------------------
 

snapcraft/models/project.py

@@ -32,6 +32,7 @@
     VersionStr,
 )
 from craft_application.models.constraints import (
+    LicenseStr,
     SingleEntryDict,
     SingleEntryList,
     UniqueList,
@@ -42,7 +43,14 @@
     Grammar,
 )
 from craft_platforms import DebianArchitecture
-from pydantic import ConfigDict, PrivateAttr, StringConstraints, error_wrappers
+from pydantic import (
+    ConfigDict,
+    PrivateAttr,
+    StringConstraints,
+    TypeAdapter,
+    ValidationError,
+    error_wrappers,
+)
 from pydantic.json_schema import (
     SkipJsonSchema,  # noqa: TC002 (typing-only-third-party-import) # pydantic needs to import types at runtime for validation
 )
@@ -2277,6 +2285,21 @@ def get_partitions(self) -> list[str] | None:
         """
         return _get_partitions_from_components(self.components)
 
+    @pydantic.field_validator("license", mode="before")
+    @classmethod
+    def _warn_deprecated_license(cls, lic: str | None) -> str | None:
+        if lic is None:
+            return None
+
+        try:
+            TypeAdapter(LicenseStr).validate_python(lic)
+        except ValidationError:
+            emit.warning(
+                "Non-SPDX licenses are deprecated. Use SPDX license strings or 'proprietary' instead."
+            )
+
+        return lic
+
 
 def _custom_error(error_msg: str):
     def _validator(v: Any, next_: Any, ctx: pydantic.ValidationInfo):

tests/unit/models/test_projects.py

@@ -19,11 +19,13 @@
 from collections.abc import Callable
 from contextlib import nullcontext
 from typing import Any, cast
+from unittest.mock import call
 
 import pydantic
 import pytest
 from craft_application.errors import CraftValidationError
 from craft_application.models import VersionStr
+from craft_cli.pytest_plugin import RecordingEmitter
 from craft_platforms import DebianArchitecture
 
 import snapcraft.models
@@ -982,6 +984,34 @@ def test_snapcraftctl_old_bases(self, key, base, project_yaml_data):
 
         Project.unmarshal(project_yaml_data(base=base, parts=parts_data))
 
+    @pytest.mark.parametrize(
+        ("lic", "should_warn"),
+        [
+            ("MIT", False),
+            ("proprietary", False),
+            (None, False),
+            ("DemonicContract", True),
+        ],
+    )
+    def test_non_spdx_deprecation(
+        self,
+        lic: str | None,
+        should_warn: bool,
+        project_yaml_data: Callable[..., Any],
+        emitter: RecordingEmitter,
+    ) -> None:
+        proj = Project.unmarshal(project_yaml_data(license=lic))
+
+        assert should_warn == (
+            call(
+                "warning",
+                "Non-SPDX licenses are deprecated. Use SPDX license strings or 'proprietary' instead.",
+            )
+            in emitter.interactions
+        )
+        # License should always remain unchanged
+        assert proj.license == lic
+
 
 class TestHookValidation:
     """Validate hooks."""