make Err wrap one or zero error.

staging/src/k8s.io/apiserver/pkg/admission/plugin/cel/compile.go

@@ -163,12 +163,12 @@ type variableDeclEnvs map[OptionalVariableDeclarations]*environment.EnvSet
 // CompileCELExpression returns a compiled CEL expression.
 // perCallLimit was added for testing purpose only. Callers should always use const PerCallLimit from k8s.io/apiserver/pkg/apis/cel/config.go as input.
 func (c compiler) CompileCELExpression(expressionAccessor ExpressionAccessor, options OptionalVariableDeclarations, envType environment.Type) CompilationResult {
-	resultError := func(errorString string, errType apiservercel.ErrorType, errors ...error) CompilationResult {
+	resultError := func(errorString string, errType apiservercel.ErrorType, cause error) CompilationResult {
 		return CompilationResult{
 			Error: &apiservercel.Error{
 				Type:   errType,
 				Detail: errorString,
-				Errors: errors,
+				Cause:  cause,
 			},
 			ExpressionAccessor: expressionAccessor,
 		}
@@ -176,7 +176,7 @@ func (c compiler) CompileCELExpression(expressionAccessor ExpressionAccessor, op
 
 	env, err := c.varEnvs[options].Env(envType)
 	if err != nil {
-		return resultError(fmt.Sprintf("unexpected error loading CEL environment: %v", err), apiservercel.ErrorTypeInternal)
+		return resultError(fmt.Sprintf("unexpected error loading CEL environment: %v", err), apiservercel.ErrorTypeInternal, nil)
 	}
 
 	ast, issues := env.Compile(expressionAccessor.GetExpression())
@@ -199,19 +199,19 @@ func (c compiler) CompileCELExpression(expressionAccessor ExpressionAccessor, op
 			reason = fmt.Sprintf("must evaluate to one of %v", returnTypes)
 		}
 
-		return resultError(reason, apiservercel.ErrorTypeInvalid)
+		return resultError(reason, apiservercel.ErrorTypeInvalid, nil)
 	}
 
 	_, err = cel.AstToCheckedExpr(ast)
 	if err != nil {
 		// should be impossible since env.Compile returned no issues
-		return resultError("unexpected compilation error: "+err.Error(), apiservercel.ErrorTypeInternal)
+		return resultError("unexpected compilation error: "+err.Error(), apiservercel.ErrorTypeInternal, nil)
 	}
 	prog, err := env.Program(ast,
 		cel.InterruptCheckFrequency(celconfig.CheckFrequency),
 	)
 	if err != nil {
-		return resultError("program instantiation failed: "+err.Error(), apiservercel.ErrorTypeInternal)
+		return resultError("program instantiation failed: "+err.Error(), apiservercel.ErrorTypeInternal, nil)
 	}
 	return CompilationResult{
 		Program:            prog,

staging/src/k8s.io/apiserver/pkg/admission/plugin/cel/filter.go

@@ -192,7 +192,7 @@ func (f *filter) ForInput(ctx context.Context, versionedAttr *admission.Versione
 			evaluation.Error = &cel.Error{
 				Type:   cel.ErrorTypeInvalid,
 				Detail: fmt.Sprintf("compilation error: %v", compilationResult.Error),
-				Errors: []error{compilationResult.Error},
+				Cause:  compilationResult.Error,
 			}
 			continue
 		}
@@ -212,7 +212,7 @@ func (f *filter) ForInput(ctx context.Context, versionedAttr *admission.Versione
 				return nil, -1, &cel.Error{
 					Type:   cel.ErrorTypeInvalid,
 					Detail: fmt.Sprintf("validation failed due to running out of cost budget, no further validation rules will be run"),
-					Errors: []error{cel.ErrOutOfBudget},
+					Cause:  cel.ErrOutOfBudget,
 				}
 			}
 			remainingBudget -= compositionCost
@@ -230,14 +230,14 @@ func (f *filter) ForInput(ctx context.Context, versionedAttr *admission.Versione
 				return nil, -1, &cel.Error{
 					Type:   cel.ErrorTypeInvalid,
 					Detail: fmt.Sprintf("runtime cost could not be calculated for expression: %v, no further expression will be run", compilationResult.ExpressionAccessor.GetExpression()),
-					Errors: []error{cel.ErrOutOfBudget},
+					Cause:  cel.ErrOutOfBudget,
 				}
 			} else {
 				if *rtCost > math.MaxInt64 || int64(*rtCost) > remainingBudget {
 					return nil, -1, &cel.Error{
 						Type:   cel.ErrorTypeInvalid,
 						Detail: fmt.Sprintf("validation failed due to running out of cost budget, no further validation rules will be run"),
-						Errors: []error{cel.ErrOutOfBudget},
+						Cause:  cel.ErrOutOfBudget,
 					}
 				}
 				remainingBudget -= int64(*rtCost)

staging/src/k8s.io/apiserver/pkg/admission/plugin/policy/validating/validator_test.go

@@ -53,7 +53,7 @@ func (f *fakeCelFilter) ForInput(ctx context.Context, versionedAttr *admission.V
 		return nil, -1, &apiservercel.Error{
 			Type:   apiservercel.ErrorTypeInvalid,
 			Detail: fmt.Sprintf("validation failed due to running out of cost budget, no further validation rules will be run"),
-			Errors: []error{apiservercel.ErrOutOfBudget},
+			Cause:  apiservercel.ErrOutOfBudget,
 		}
 	}
 	if f.throwError {

staging/src/k8s.io/apiserver/pkg/cel/errors.go

@@ -53,9 +53,9 @@ type Error struct {
 	Type   ErrorType
 	Detail string
 
-	// Errors are optional wrapped errors that can be useful to
+	// Cause is an optional wrapped errors that can be useful to
 	// programmatically retrieve detailed errors.
-	Errors []error
+	Cause error
 }
 
 var _ error = &Error{}
@@ -77,9 +77,9 @@ func (v *Error) Is(err error) bool {
 	return false
 }
 
-// Unwrap returns wrapped errors.
+// Unwrap returns the wrapped Cause.
-func (v *Error) Unwrap() []error {
+func (v *Error) Unwrap() error {
-	return v.Errors
+	return v.Cause
 }
 
 // ErrorType is a machine-readable value providing more detail about why

staging/src/k8s.io/apiserver/pkg/cel/errors_test.go

@@ -27,7 +27,7 @@ func TestOutOfBudgetError(t *testing.T) {
 	err := &Error{
 		Type:   ErrorTypeInvalid,
 		Detail: "expression out of budget",
-		Errors: []error{ErrOutOfBudget},
+		Cause:  ErrOutOfBudget,
 	}
 	if !errors.Is(err, ErrOutOfBudget) {
 		t.Errorf("unexpected %v is not %v", err, ErrOutOfBudget)
@@ -45,7 +45,7 @@ func TestCompilationError(t *testing.T) {
 	err := &Error{
 		Type:   ErrorTypeInvalid,
 		Detail: "fake compilation failed",
-		Errors: []error{NewCompilationError(issues)},
+		Cause:  NewCompilationError(issues),
 	}
 	if !errors.Is(err, ErrCompilation) {
 		t.Errorf("unexpected %v is not %v", err, ErrCompilation)