Show warning on missing backend deps (#1911)

Author: r4victor

src/dstack/_internal/server/services/backends/__init__.py

@@ -1,6 +1,6 @@
 import asyncio
 import heapq
-from typing import Callable, Coroutine, Dict, List, Optional, Tuple, Type, Union
+from typing import Callable, Coroutine, Dict, Iterable, List, Optional, Tuple, Type, Union
 from uuid import UUID
 
 from sqlalchemy import delete, update
@@ -229,7 +229,7 @@ async def get_config_info(
 async def delete_backends(
     session: AsyncSession,
     project: ProjectModel,
-    backends_types: List[BackendType],
+    backends_types: Iterable[BackendType],
 ):
     if BackendType.DSTACK in backends_types:
         raise ServerClientError("Cannot delete dstack backend")

src/dstack/_internal/server/services/config.py

@@ -7,6 +7,7 @@
 from typing_extensions import Annotated
 
 from dstack._internal.core.errors import (
+    BackendNotAvailable,
     ResourceNotExistsError,
     ServerClientError,
 )
@@ -527,18 +528,23 @@ async def _apply_project_config(
             project = await projects_services.get_project_model_by_name_or_error(
                 session=session, project_name=project_config.name
             )
-        backends_to_delete = backends_services.list_available_backend_types()
+        backends_to_delete = set(backends_services.list_available_backend_types())
         for backend_config in project_config.backends or []:
             config_info = config_to_internal_config(backend_config)
             backend_type = BackendType(config_info.type)
+            backends_to_delete.difference_update([backend_type])
             try:
-                backends_to_delete.remove(backend_type)
-            except ValueError:
+                current_config_info = await backends_services.get_config_info(
+                    project=project,
+                    backend_type=backend_type,
+                )
+            except BackendNotAvailable:
+                logger.warning(
+                    "Backend %s not available and won't be configured."
+                    " Check that backend dependencies are installed.",
+                    backend_type.value,
+                )
                 continue
-            current_config_info = await backends_services.get_config_info(
-                project=project,
-                backend_type=backend_type,
-            )
             if config_info == current_config_info:
                 continue
             backend_exists = any(backend_type == b.type for b in project.backends)