mgr/smb: replace cross check if-block with singledispatch

The previous code relied on a cascading block of if-isintance statements
that was dense and somwehat error prone as I found out during an
experiment to add a new top level resource type. Refactor the cross
check function to use singledispatch:
  https://docs.python.org/3.9/library/functools.html#functools.singledispatch

Now instead of correctly adding check function(s) and updating the
if-block, only new check functions using the register decorator is
needed.

Note that making this checking more generic is difficult as each
different resource type really has different cross checking needs.

Signed-off-by: John Mulligan <[email protected]>

Author: phlogistonjohn

src/pybind/mgr/smb/staging.py

@@ -1,13 +1,14 @@
 from typing import (
+    Any,
     Collection,
     Dict,
     Iterator,
     List,
     Optional,
     Set,
-    Union,
 )
 
+import functools
 import logging
 import operator
 
@@ -41,9 +42,6 @@
 from .results import ErrorResult, Result, ResultGroup
 from .utils import checked
 
-ClusterRef = Union[resources.Cluster, resources.RemovedCluster]
-ShareRef = Union[resources.Share, resources.RemovedShare]
-
 log = logging.getLogger(__name__)
 
 
@@ -163,38 +161,22 @@ def ug_refs(cluster: resources.Cluster) -> Collection[str]:
     }
 
 
+@functools.singledispatch
 def cross_check_resource(
     resource: SMBResource,
     staging: Staging,
     *,
     path_resolver: PathResolver,
     earmark_resolver: EarmarkResolver,
 ) -> None:
-    if isinstance(resource, (resources.Cluster, resources.RemovedCluster)):
-        _check_cluster(resource, staging)
-    elif isinstance(resource, (resources.Share, resources.RemovedShare)):
-        _check_share(
-            resource,
-            staging,
-            path_resolver,
-            earmark_resolver,
-        )
-    elif isinstance(resource, resources.JoinAuth):
-        _check_join_auths(resource, staging)
-    elif isinstance(resource, resources.UsersAndGroups):
-        _check_users_and_groups(resource, staging)
-    else:
-        raise TypeError('not a valid smb resource')
-
+    raise TypeError(f'not a valid smb resource: {type(resource)}')
 
-def _check_cluster(cluster: ClusterRef, staging: Staging) -> None:
-    """Check that the cluster resource can be updated."""
-    if cluster.intent == Intent.PRESENT:
-        return _check_cluster_present(cluster, staging)
-    return _check_cluster_removed(cluster, staging)
 
-
-def _check_cluster_removed(cluster: ClusterRef, staging: Staging) -> None:
+@cross_check_resource.register
+def _check_removed_cluster_resource(
+    cluster: resources.RemovedCluster, staging: Staging, **_: Any
+) -> None:
+    assert cluster.intent == Intent.REMOVED
     share_ids = ShareEntry.ids(staging)
     clusters_used = {cid for cid, _ in share_ids}
     if cluster.cluster_id in clusters_used:
@@ -211,8 +193,11 @@ def _check_cluster_removed(cluster: ClusterRef, staging: Staging) -> None:
         )
 
 
-def _check_cluster_present(cluster: ClusterRef, staging: Staging) -> None:
-    assert isinstance(cluster, resources.Cluster)
+@cross_check_resource.register
+def _check_cluster_resource(
+    cluster: resources.Cluster, staging: Staging, **_: Any
+) -> None:
+    assert cluster.intent == Intent.PRESENT
     cluster.validate()
     if not staging.is_new(cluster):
         _check_cluster_modifications(cluster, staging)
@@ -291,15 +276,22 @@ def _check_cluster_modifications(
         raise ErrorResult(cluster, msg, status={'hint': hint})
 
 
-def _check_share(
-    share: ShareRef,
+@cross_check_resource.register
+def _check_removed_share_resource(
+    share: resources.RemovedShare, staging: Staging, **_: Any
+) -> None:
+    assert share.intent == Intent.REMOVED
+
+
+@cross_check_resource.register
+def _check_share_resource(
+    share: resources.Share,
     staging: Staging,
-    resolver: PathResolver,
+    path_resolver: PathResolver,
     earmark_resolver: EarmarkResolver,
 ) -> None:
     """Check that the share resource can be updated."""
-    if share.intent == Intent.REMOVED:
-        return
+    assert share.intent == Intent.PRESENT
     assert isinstance(share, resources.Share)
     share.validate()
     if share.cluster_id not in ClusterEntry.ids(staging):
@@ -310,7 +302,7 @@ def _check_share(
         )
     assert share.cephfs is not None
     try:
-        volpath = resolver.resolve_exists(
+        volpath = path_resolver.resolve_exists(
             share.cephfs.volume,
             share.cephfs.subvolumegroup,
             share.cephfs.subvolume,
@@ -411,8 +403,9 @@ def _share_name_in_use(
     return found_curr[0].get()['share_id']
 
 
-def _check_join_auths(
-    join_auth: resources.JoinAuth, staging: Staging
+@cross_check_resource.register
+def _check_join_auth_resource(
+    join_auth: resources.JoinAuth, staging: Staging, **_: Any
 ) -> None:
     """Check that the JoinAuth resource can be updated."""
     if join_auth.intent == Intent.PRESENT:
@@ -455,8 +448,9 @@ def _check_join_auths_present(
             )
 
 
-def _check_users_and_groups(
-    users_and_groups: resources.UsersAndGroups, staging: Staging
+@cross_check_resource.register
+def _check_users_and_groups_resource(
+    users_and_groups: resources.UsersAndGroups, staging: Staging, **_: Any
 ) -> None:
     """Check that the UsersAndGroups resource can be updated."""
     if users_and_groups.intent == Intent.PRESENT: