migration: re-define the migration registration process

We make it a bit more ergonomic (this revision was originally
prototyped in cockroachdb#57445).

Release note: None

pkg/migration/manager.go

@@ -179,12 +179,19 @@ func (m *Manager) Migrate(ctx context.Context, from, to clusterversion.ClusterVe
 			}
 		}
 
-		// TODO(irfansharif): We'll want to retrieve the right migration off of
-		// our registry of migrations, if any, and execute it.
 		// TODO(irfansharif): We'll want to be able to override which migration
 		// is retrieved here within tests. We could make the registry be a part
 		// of the manager, and all tests to provide their own.
-		_ = Registry[clusterVersion]
+
+		// Finally, run the actual migration.
+		migration, ok := registry[clusterVersion]
+		if !ok {
+			log.Infof(ctx, "no migration registered for %s, skipping", clusterVersion)
+			continue
+		}
+		if err := migration.Run(ctx, h); err != nil {
+			return err
+		}
 	}
 
 	return nil

pkg/migration/migrations.go

@@ -16,16 +16,15 @@ import (
 	"github.com/cockroachdb/cockroach/pkg/clusterversion"
 )
 
-// Registry defines the global mapping between a cluster version, and the
+// registry defines the global mapping between a cluster version and the
 // associated migration. The migration is only executed after a cluster-wide
-// bump of the version gate.
-var Registry = make(map[clusterversion.ClusterVersion]Migration)
+// bump of the corresponding version gate.
+var registry = make(map[clusterversion.ClusterVersion]Migration)
 
 func init() {
 	// TODO(irfansharif): We'll want to register individual migrations with
 	// specific internal cluster versions here.
-	//
-	//  Registry[clusterversion.ByKey(clusterversion.VersionWhatever)] = WhateverMigration
+	_ = register // register(clusterversion.WhateverMigration, WhateverMigration, "whatever migration")
 }
 
 // Migration defines a program to be executed once every node in the cluster is

pkg/migration/util.go

@@ -64,3 +64,13 @@ func fenceVersionFor(
 	fenceCV.Internal--
 	return fenceCV
 }
+
+// register is a short hand to register a given migration within the global
+// registry.
+func register(key clusterversion.Key, fn migrationFn, desc string) {
+	cv := clusterversion.ClusterVersion{Version: clusterversion.ByKey(key)}
+	if _, ok := registry[cv]; ok {
+		log.Fatalf(context.Background(), "doubly registering migration for %s", cv)
+	}
+	registry[cv] = Migration{cv: cv, fn: fn, desc: desc}
+}