Add support for custom error messages

Add a function to each defined error to set a custom message.

Signed-off-by: Derek McGowan <derek@mcg.dev>

errors.go

@@ -69,6 +69,10 @@ func (errUnknown) Error() string { return "unknown" }
 
 func (errUnknown) Unknown() {}
 
+func (e errUnknown) WithMessage(msg string) error {
+	return customMessage{e, msg}
+}
+
 // unknown maps to Moby's "ErrUnknown"
 type unknown interface {
 	Unknown()
@@ -86,6 +90,10 @@ func (errInvalidArgument) Error() string { return "invalid argument" }
 
 func (errInvalidArgument) InvalidParameter() {}
 
+func (e errInvalidArgument) WithMessage(msg string) error {
+	return customMessage{e, msg}
+}
+
 // invalidParameter maps to Moby's "ErrInvalidParameter"
 type invalidParameter interface {
 	InvalidParameter()
@@ -113,6 +121,10 @@ func (errNotFound) Error() string { return "not found" }
 
 func (errNotFound) NotFound() {}
 
+func (e errNotFound) WithMessage(msg string) error {
+	return customMessage{e, msg}
+}
+
 // notFound maps to Moby's "ErrNotFound"
 type notFound interface {
 	NotFound()
@@ -127,6 +139,10 @@ type errAlreadyExists struct{}
 
 func (errAlreadyExists) Error() string { return "already exists" }
 
+func (e errAlreadyExists) WithMessage(msg string) error {
+	return customMessage{e, msg}
+}
+
 // IsAlreadyExists returns true if the error is due to an already existing
 // metadata item
 func IsAlreadyExists(err error) bool {
@@ -137,6 +153,10 @@ type errPermissionDenied struct{}
 
 func (errPermissionDenied) Error() string { return "permission denied" }
 
+func (e errPermissionDenied) WithMessage(msg string) error {
+	return customMessage{e, msg}
+}
+
 // forbidden maps to Moby's "ErrForbidden"
 type forbidden interface {
 	Forbidden()
@@ -152,6 +172,10 @@ type errResourceExhausted struct{}
 
 func (errResourceExhausted) Error() string { return "resource exhausted" }
 
+func (e errResourceExhausted) WithMessage(msg string) error {
+	return customMessage{e, msg}
+}
+
 // IsResourceExhausted returns true if the error is due to
 // a lack of resources or too many attempts.
 func IsResourceExhausted(err error) bool {
@@ -162,6 +186,10 @@ type errFailedPrecondition struct{}
 
 func (e errFailedPrecondition) Error() string { return "failed precondition" }
 
+func (e errFailedPrecondition) WithMessage(msg string) error {
+	return customMessage{e, msg}
+}
+
 // IsFailedPrecondition returns true if an operation could not proceed due to
 // the lack of a particular condition
 func IsFailedPrecondition(err error) bool {
@@ -174,6 +202,10 @@ func (errConflict) Error() string { return "conflict" }
 
 func (errConflict) Conflict() {}
 
+func (e errConflict) WithMessage(msg string) error {
+	return customMessage{e, msg}
+}
+
 // conflict maps to Moby's "ErrConflict"
 type conflict interface {
 	Conflict()
@@ -191,6 +223,10 @@ func (errNotModified) Error() string { return "not modified" }
 
 func (errNotModified) NotModified() {}
 
+func (e errNotModified) WithMessage(msg string) error {
+	return customMessage{e, msg}
+}
+
 // notModified maps to Moby's "ErrNotModified"
 type notModified interface {
 	NotModified()
@@ -206,6 +242,10 @@ type errAborted struct{}
 
 func (errAborted) Error() string { return "aborted" }
 
+func (e errAborted) WithMessage(msg string) error {
+	return customMessage{e, msg}
+}
+
 // IsAborted returns true if an operation was aborted.
 func IsAborted(err error) bool {
 	return errors.Is(err, errAborted{})
@@ -215,6 +255,10 @@ type errOutOfRange struct{}
 
 func (errOutOfRange) Error() string { return "out of range" }
 
+func (e errOutOfRange) WithMessage(msg string) error {
+	return customMessage{e, msg}
+}
+
 // IsOutOfRange returns true if an operation could not proceed due
 // to data being out of the expected range.
 func IsOutOfRange(err error) bool {
@@ -227,6 +271,10 @@ func (errNotImplemented) Error() string { return "not implemented" }
 
 func (errNotImplemented) NotImplemented() {}
 
+func (e errNotImplemented) WithMessage(msg string) error {
+	return customMessage{e, msg}
+}
+
 // notImplemented maps to Moby's "ErrNotImplemented"
 type notImplemented interface {
 	NotImplemented()
@@ -243,6 +291,10 @@ func (errInternal) Error() string { return "internal" }
 
 func (errInternal) System() {}
 
+func (e errInternal) WithMessage(msg string) error {
+	return customMessage{e, msg}
+}
+
 // system maps to Moby's "ErrSystem"
 type system interface {
 	System()
@@ -259,6 +311,10 @@ func (errUnavailable) Error() string { return "unavailable" }
 
 func (errUnavailable) Unavailable() {}
 
+func (e errUnavailable) WithMessage(msg string) error {
+	return customMessage{e, msg}
+}
+
 // unavailable maps to Moby's "ErrUnavailable"
 type unavailable interface {
 	Unavailable()
@@ -275,6 +331,10 @@ func (errDataLoss) Error() string { return "data loss" }
 
 func (errDataLoss) DataLoss() {}
 
+func (e errDataLoss) WithMessage(msg string) error {
+	return customMessage{e, msg}
+}
+
 // dataLoss maps to Moby's "ErrDataLoss"
 type dataLoss interface {
 	DataLoss()
@@ -291,6 +351,10 @@ func (errUnauthorized) Error() string { return "unauthorized" }
 
 func (errUnauthorized) Unauthorized() {}
 
+func (e errUnauthorized) WithMessage(msg string) error {
+	return customMessage{e, msg}
+}
+
 // unauthorized maps to Moby's "ErrUnauthorized"
 type unauthorized interface {
 	Unauthorized()
@@ -307,6 +371,8 @@ func isInterface[T any](err error) bool {
 		switch x := err.(type) {
 		case T:
 			return true
+		case customMessage:
+			err = x.err
 		case interface{ Unwrap() error }:
 			err = x.Unwrap()
 			if err == nil {
@@ -324,3 +390,22 @@ func isInterface[T any](err error) bool {
 		}
 	}
 }
+
+// customMessage is used to provide a defined error with a custom message.
+// The message is not wrapped but can be compared by the `Is(error) bool` interface.
+type customMessage struct {
+	err error
+	msg string
+}
+
+func (c customMessage) Is(err error) bool {
+	return c.err == err
+}
+
+func (c customMessage) As(target any) bool {
+	return errors.As(c.err, target)
+}
+
+func (c customMessage) Error() string {
+	return c.msg
+}

errors_test.go

@@ -19,6 +19,7 @@ package errdefs
 import (
 	"context"
 	"errors"
+	"reflect"
 	"testing"
 )
 
@@ -44,6 +45,119 @@ func TestInvalidArgument(t *testing.T) {
 	}
 }
 
+func TestErrorEquivalence(t *testing.T) {
+	var e1 error = ErrAborted
+	var e2 error = ErrUnknown
+	if e1 == e2 {
+		t.Fatal("should not equal the same error")
+	}
+	if errors.Is(e1, e2) {
+		t.Fatal("errors.Is should not return true")
+	}
+
+	var e3 error = errAborted{}
+	if e1 != e3 {
+		t.Fatal("new instance should be equivalent")
+	}
+	if !errors.Is(e1, e3) {
+		t.Fatal("errors.Is should be true")
+	}
+	if !errors.Is(e3, e1) {
+		t.Fatal("errors.Is should be true")
+	}
+	var aborted errAborted
+	if !errors.As(e1, &aborted) {
+		t.Fatal("errors.As should be true")
+	}
+
+	var e4 = ErrAborted.WithMessage("custom message")
+	if e1 == e4 {
+		t.Fatal("should not equal the same error")
+	}
+
+	if !errors.Is(e4, e1) {
+		t.Fatal("errors.Is should be true, e1 is in the tree of e4")
+	}
+
+	if errors.Is(e1, e4) {
+		t.Fatal("errors.Is should be false, e1 is not a custom message")
+	}
+
+	if !errors.As(e4, &aborted) {
+		t.Fatal("errors.As should be true")
+	}
+
+	var custom customMessage
+	if !errors.As(e4, &custom) {
+		t.Fatal("errors.As should be true")
+	}
+	if custom.msg != "custom message" {
+		t.Fatalf("unexpected custom message: %q", custom.msg)
+	}
+	if custom.err != e1 {
+		t.Fatalf("unexpected custom message error: %v", custom.err)
+	}
+}
+
+func TestWithMessage(t *testing.T) {
+	testErrors := []error{ErrUnknown,
+		ErrInvalidArgument,
+		ErrNotFound,
+		ErrAlreadyExists,
+		ErrPermissionDenied,
+		ErrResourceExhausted,
+		ErrFailedPrecondition,
+		ErrConflict,
+		ErrNotModified,
+		ErrAborted,
+		ErrOutOfRange,
+		ErrNotImplemented,
+		ErrInternal,
+		ErrUnavailable,
+		ErrDataLoss,
+		ErrUnauthenticated,
+	}
+	for _, err := range testErrors {
+		e1 := err
+		t.Run(err.Error(), func(t *testing.T) {
+			wm, ok := e1.(interface{ WithMessage(string) error })
+			if !ok {
+				t.Fatal("WithMessage not supported")
+			}
+			e2 := wm.WithMessage("custom message")
+
+			if e1 == e2 {
+				t.Fatal("should not equal the same error")
+			}
+
+			if !errors.Is(e2, e1) {
+				t.Fatal("errors.Is should return true")
+			}
+
+			if errors.Is(e1, e2) {
+				t.Fatal("errors.Is should be false, e1 is not a custom message")
+			}
+
+			var raw = reflect.New(reflect.TypeOf(e1)).Interface()
+			if !errors.As(e2, raw) {
+				t.Fatal("errors.As should be true")
+			}
+
+			var custom customMessage
+			if !errors.As(e2, &custom) {
+				t.Fatal("errors.As should be true")
+			}
+			if custom.msg != "custom message" {
+				t.Fatalf("unexpected custom message: %q", custom.msg)
+			}
+			if custom.err != e1 {
+				t.Fatalf("unexpected custom message error: %v", custom.err)
+			}
+
+		})
+	}
+}
+
 type customInvalidArgument struct{}
 
 func (*customInvalidArgument) Error() string {

resolve.go

@@ -66,6 +66,8 @@ func firstError(err error) error {
 			return err
 		}
 		switch e := err.(type) {
+		case customMessage:
+			err = e.err
 		case unknown:
 			return ErrUnknown
 		case invalidParameter:

resolve_test.go

@@ -61,6 +61,9 @@ func TestResolve(t *testing.T) {
 		{errors.Join(testUnavailable{}, ErrPermissionDenied), ErrUnavailable},
 		{errors.Join(errors.New("untyped join")), ErrUnknown},
 		{errors.Join(errors.New("untyped1"), errors.New("untyped2")), ErrUnknown},
+		{ErrNotFound.WithMessage("something else"), ErrNotFound},
+		{wrap(ErrNotFound.WithMessage("something else")), ErrNotFound},
+		{errors.Join(ErrNotFound.WithMessage("something else"), ErrPermissionDenied), ErrNotFound},
 	} {
 		name := fmt.Sprintf("%d-%s", i, errorString(tc.resolved))
 		tc := tc