Errors now conform the pkg/errors.Cause() interface (#7)

fields.go

@@ -128,6 +128,13 @@ func (c *fields) Is(target error) bool {
 	return ok
 }
 
+// Cause returns the wrapped error which was the original
+// cause of the issue. We only support this because some code
+// depends on github.com/pkg/errors.Cause() returning the cause
+// of the error.
+// deprecated use error.Is() or error.As() instead
+func (c *fields) Cause() error { return c.wrapped }
+
 func (c *fields) Error() string {
 	if c.msg == NoMsg {
 		return c.wrapped.Error()

fields_test.go

@@ -248,3 +248,9 @@ func TestFieldsStack(t *testing.T) {
 	assert.Equal(t, "value1", m["key1"])
 	assert.Equal(t, io.EOF.Error(), err.Error())
 }
+
+// Ensure errors.Fields returns an error that is compatible with `github.com/pkf/errors.Cause()`
+func TestFieldsCause(t *testing.T) {
+	err := errors.Fields{"key1": "value1"}.Wrap(io.EOF, "message")
+	assert.Equal(t, io.EOF, pkgErrorCause(err))
+}

stack.go

@@ -32,6 +32,13 @@ func (w *stack) Is(target error) bool {
 	return ok
 }
 
+// Cause returns the wrapped error which was the original
+// cause of the issue. We only support this because some code
+// depends on github.com/pkg/errors.Cause() returning the cause
+// of the error.
+// deprecated use error.Is() or error.As() instead
+func (w *stack) Cause() error { return w.error }
+
 func (w *stack) HasFields() map[string]any {
 	if child, ok := w.error.(HasFields); ok {
 		return child.HasFields()

stack_test.go

@@ -144,3 +144,27 @@ func TestFormatStack(t *testing.T) {
 		})
 	}
 }
+
+// Ensure errors.Stack() returns an that works with `github.com/pkf/errors.Cause()`
+func TestStackCause(t *testing.T) {
+	err := errors.Stack(io.EOF)
+	assert.Equal(t, io.EOF, pkgErrorCause(err))
+}
+
+// pkgErrorCause is identical to github.com/pkg/errors.Cause()
+// Much of our existing code uses that function and depends
+// on our errors conforming to the old style Causer interface.
+func pkgErrorCause(err error) error {
+	type causer interface {
+		Cause() error
+	}
+
+	for err != nil {
+		cause, ok := err.(causer)
+		if !ok {
+			break
+		}
+		err = cause.Cause()
+	}
+	return err
+}

wrap.go

@@ -58,6 +58,13 @@ func (e *wrappedError) Is(target error) bool {
 	return ok
 }
 
+// Cause returns the wrapped error which was the original
+// cause of the issue. We only support this because some code
+// depends on github.com/pkg/errors.Cause() returning the cause
+// of the error.
+// deprecated use error.Is() or error.As() instead
+func (e *wrappedError) Cause() error { return e.wrapped }
+
 func (e *wrappedError) Error() string {
 	if e.msg == NoMsg {
 		return e.wrapped.Error()

wrap_test.go

@@ -133,3 +133,9 @@ func TestCause(t *testing.T) {
 	assert.Equal(t, "error: wrap 1: wrap 2: the cause", err.Error())
 	assert.Equal(t, "the cause", cause.Error())
 }
+
+// Ensure errors.wrappedError returns an error that is compatible with `github.com/pkf/errors.Cause()`
+func TestWrappedCause(t *testing.T) {
+	err := errors.Wrap(io.EOF, "message")
+	assert.Equal(t, io.EOF, pkgErrorCause(err))
+}