fix(BA-6015): reject empty model definition_path and treat blanks as missing in merge

Pydantic now rejects empty strings at the DTO boundary via min_length=1, and
_merge_mounts uses a truthy check so any empty string that bypasses the DTO
no longer clobbers a resolved lower-priority model_definition_path.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: seedspirit

src/ai/backend/common/dto/manager/v2/deployment/request.py

@@ -266,10 +266,12 @@ class ModelMountConfigInput(BaseRequestModel):
     mount_destination: str = Field(description="Mount destination path inside container")
     definition_path: str | None = Field(
         default=None,
+        min_length=1,
         description=(
             "Optional path to the model definition file within the model vfolder. "
             "When omitted, the server auto-detects `model-definition.yaml` or "
-            "`model-definition.yml`."
+            "`model-definition.yml`. Empty string is rejected; omit the field to "
+            "trigger auto-detection."
         ),
     )
     subpath: str | None = Field(

src/ai/backend/manager/data/deployment/types.py

@@ -668,13 +668,11 @@ def _merge_mounts(
     return MountMetadata(
         model_vfolder_id=upper.model_vfolder_id,
         model_definition_path=upper.model_definition_path
-        if upper.model_definition_path is not None
+        if upper.model_definition_path
         else lower.model_definition_path,
         model_mount_destination=upper.model_mount_destination,
         extra_mounts=list(upper.extra_mounts),
-        vfolder_subpath=upper.vfolder_subpath
-        if upper.vfolder_subpath is not None
-        else lower.vfolder_subpath,
+        vfolder_subpath=upper.vfolder_subpath if upper.vfolder_subpath else lower.vfolder_subpath,
     )
 
 

tests/unit/common/dto/manager/v2/deployment/test_request.py

@@ -70,6 +70,25 @@ def _make_create_revision_input_dto(**kwargs: object) -> CreateRevisionInput:
     return CreateRevisionInput(**defaults)
 
 
+class TestModelMountConfigInput:
+    """Tests for ModelMountConfigInput model."""
+
+    def test_definition_path_defaults_to_none(self) -> None:
+        config = ModelMountConfigInput(
+            vfolder_id=VFolderUUID(uuid.uuid4()),
+            mount_destination="/models",
+        )
+        assert config.definition_path is None
+
+    def test_empty_definition_path_raises_validation_error(self) -> None:
+        with pytest.raises((BackendAISchemaValidationFailed, ValidationError)):
+            ModelMountConfigInput.model_validate({
+                "vfolder_id": str(uuid.uuid4()),
+                "mount_destination": "/models",
+                "definition_path": "",
+            })
+
+
 class TestExtraVFolderMountInput:
     """Tests for ExtraVFolderMountInput model."""
 

tests/unit/manager/data/deployment/test_revision_draft_merge.py

@@ -160,3 +160,12 @@ def test_mount_definition_path_preserves_lower_value_when_higher_omits_it(self)
 
         assert merged.mounts is not None
         assert merged.mounts.model_definition_path == "model-definition.yml"
+
+    def test_mount_definition_path_treats_empty_string_as_missing(self) -> None:
+        base = RevisionDraft(mounts=_mounts("model-definition.yml"))
+        override = RevisionDraft(mounts=_mounts(""))
+
+        merged = base.merge(override)
+
+        assert merged.mounts is not None
+        assert merged.mounts.model_definition_path == "model-definition.yml"