vault: Relax backend instantiation configuration constraints

For the migration context case, it's ok to relax the constraints on the configuration not specifying the cache, scheduler or storage key entries (because they are not used).

It's not the case for the actual runtime case (which requires them). So assertions in the code using those backends has been added to fail as soon as possible.

[1] Successful migration check run with this patch

swh@vault-rpc-697776867c-mslf2:~$ swh db --config-file=$SWH_CONFIG_FILENAME version vault

module: vault:postgresql
current code version: 4
version: 4

[2] Success runtime (rpc) context run with this patch

swh@vault-rpc-697776867c-mslf2:~$ ./entrypoint.sh
Starting the swh-vault API server
Connection in use: ('0.0.0.0', 5005)
connection to ('0.0.0.0', 5005) failed: [Errno 98] Address already in use

Note: ^ failure to connect since the pod has already the main process attached to it (expectedly), run to entertain there is no failure regarding backend instantiation with the changes.

[3] Current failure with latest version (without the patch)

-> https://gitlab.softwareheritage.org/-/snippets/1648

Refs. swh/infra/sysadm-environment#5562 (closed)

swh/vault/backend.py

-# Copyright (C) 2017-2024  The Software Heritage developers
+# Copyright (C) 2017-2025  The Software Heritage developers
 # See the AUTHORS file at the top-level directory of this distribution
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
@@ -108,9 +108,25 @@ class VaultBackend(VaultDB):
 
     def __init__(self, **config):
         super().__init__(**config)
-        self.cache = VaultCache(**config["cache"])
-        self.scheduler = get_scheduler(**config["scheduler"])
-        self.storage = get_storage(**config["storage"])
+
+        # For the migration context case, it's ok to relax the constraint on the
+        # configuration not specifying the cache, scheduler or storage keys, they are
+        # not used. For the actual runtime case, this is a hard requirement. Assertions
+        # where needed has been added, so that should fail asap if not provided.
+        config_cache = config.get("cache")
+        config_scheduler = config.get("scheduler")
+        config_storage = config.get("storage")
+
+        if config_cache and config_scheduler and config_storage:
+            # At runtime (for the rpc server), we must have those instances configured
+            self.cache = VaultCache(**config_cache)
+            self.scheduler = get_scheduler(**config_scheduler)
+            self.storage = get_storage(**config_storage)
+        else:
+            # For the migration/check backend context, unused so relax constraint
+            self.cache = None
+            self.scheduler = None
+            self.storage = None
 
     @db_transaction()
     def progress(
@@ -154,6 +170,10 @@ class VaultBackend(VaultDB):
     ):
         """Create and send a cooking task"""
         cooker_class = get_cooker_cls(bundle_type, swhid.object_type)
+        # Check early as possible the storage instance is configured properly
+        assert (
+            self.storage is not None
+        ), "<storage> configuration entry must be provided."
         cooker = cooker_class(swhid, backend=self, storage=self.storage)
 
         if not cooker.check_exists():
@@ -182,6 +202,8 @@ class VaultBackend(VaultDB):
         )
 
     def put_bundle(self, bundle_type: str, swhid: CoreSWHID, bundle) -> bool:
+        # Check early as possible the cache instance is configured properly
+        assert self.cache is not None, "<cache> configuration entry must be provided."
         self.cache.add(bundle_type, swhid, bundle)
         return True
 
@@ -240,6 +262,11 @@ class VaultBackend(VaultDB):
         # psycopg2 >= 2.7 (only available on postgresql servers)
         from psycopg2.extras import execute_values
 
+        # Check early as possible the scheduler instance is configured properly
+        assert (
+            self.scheduler is not None
+        ), "Scheduler configuration entry must be provided."
+
         for bundle_type, _ in batch:
             if bundle_type not in COOKER_TYPES:
                 raise NotFoundExc(f"{bundle_type} is an unknown type.")