Add configuration comparison capability contract

integrations/ckconfig/family.go

@@ -112,9 +112,20 @@ func captureSuite[I, S any](suite Suite[I, S]) *preparedSuite[I, S] {
 		for _, issue := range scope.issues {
 			prepared.issues = append(prepared.issues, fmt.Errorf("configuration scope %q: %w", scope.Name, issue))
 		}
-		if len(scope.Operations) > 0 && scope.Acquire == nil {
+		requiresAcquire, requiresCompare := false, false
+		for _, operation := range scope.Operations {
+			if operation == Diff {
+				requiresCompare = true
+			} else {
+				requiresAcquire = true
+			}
+		}
+		if requiresAcquire && scope.Acquire == nil {
 			prepared.issues = append(prepared.issues, fmt.Errorf("configuration scope %q declares operations without an acquisition callback", scope.Name))
 		}
+		if requiresCompare && scope.Compare == nil {
+			prepared.issues = append(prepared.issues, fmt.Errorf("configuration scope %q declares diff without a comparison callback", scope.Name))
+		}
 		prepared.scopes = append(prepared.scopes, &preparedScope[I, S]{scope: scope})
 	}
 	sort.SliceStable(prepared.scopes, func(i, j int) bool {
@@ -156,7 +167,7 @@ func captureSuite[I, S any](suite Suite[I, S]) *preparedSuite[I, S] {
 		}
 	}
 
-	for _, operation := range []Operation{Validate, Show, Sources, Explain} {
+	for _, operation := range []Operation{Validate, Show, Sources, Explain, Diff} {
 		for _, scope := range prepared.scopes {
 			if containsOperation(scope.scope.Operations, operation) {
 				prepared.operations = append(prepared.operations, operation)
@@ -185,14 +196,14 @@ func normalizeOperations(operations []Operation, issues *[]error, scope string)
 	seen := make(map[Operation]struct{}, len(operations))
 	for _, operation := range operations {
 		switch operation {
-		case Validate, Show, Sources, Explain:
+		case Validate, Show, Sources, Explain, Diff:
 		default:
 			*issues = append(*issues, fmt.Errorf("configuration scope %q declares unknown operation %q", scope, operation))
 		}
 		seen[operation] = struct{}{}
 	}
 	ordered := make([]Operation, 0, len(seen))
-	for _, operation := range []Operation{Validate, Show, Sources, Explain} {
+	for _, operation := range []Operation{Validate, Show, Sources, Explain, Diff} {
 		if _, exists := seen[operation]; exists {
 			ordered = append(ordered, operation)
 		}