Repository: go-gitea/gitea
File: services/attachment/attachment.go (Go)

Code before:
	"code.gitea.io/gitea/services/context/upload"

	"github.com/google/uuid"
)

// NewAttachment creates a new attachment object, but do not verify.
func NewAttachment(ctx context.Context, attach *repo_model.Attachment, file io.Reader, size int64) (*repo_model.Attachment, error) {
	if attach.RepoID == 0 {
		return nil, fmt.Errorf("attachment %s should belong to a repository", attach.Name)
	}

	err := db.WithTx(ctx, func(ctx context.Context) error {
		attach.UUID = uuid.New().String()
		size, err := storage.Attachments.Save(attach.RelativePath(), file, size)
		if err != nil {
			return fmt.Errorf("Create: %w", err)
		}
		attach.Size = size

		return db.Insert(ctx, attach)
	})

	return attach, err
}

type ErrAttachmentSizeExceed struct {
	MaxSize int64
	Size    int64
}

func (e *ErrAttachmentSizeExceed) Error() string {
	return fmt.Sprintf("attachment size %d exceed limit %d", e.Size, e.MaxSize)
}

func IsErrAttachmentSizeExceed(err error) bool {
	_, ok := err.(*ErrAttachmentSizeExceed)
	return ok
}

// UploadAttachment upload new attachment into storage and update database
func UploadAttachment(ctx context.Context, file io.Reader, allowedTypes string, maxFileSize, fileSize int64, attach *repo_model.Attachment) (*repo_model.Attachment, error) {
	buf := make([]byte, 1024)
	n, _ := util.ReadAtMost(file, buf)
	buf = buf[:n]

	if err := upload.Verify(buf, attach.Name, allowedTypes); err != nil {
		return nil, err
	}

	if maxFileSize >= 0 && fileSize > (maxFileSize) {
		return nil, &ErrAttachmentSizeExceed{MaxSize: maxFileSize, Size: fileSize}

Review comment (suggestion):
If you'd like to introduce a new "business error", it needs to be able to unwrap to some base errors, like other Gitea errors.

Code after:
	"code.gitea.io/gitea/modules/setting"
	"code.gitea.io/gitea/modules/storage"
	"code.gitea.io/gitea/modules/util"
	"code.gitea.io/gitea/services/context/upload"

	"github.com/google/uuid"
)

// NewAttachment creates a new attachment object, but do not verify.
func NewAttachment(ctx context.Context, attach *repo_model.Attachment, file io.Reader, size int64) (*repo_model.Attachment, error) {
	if attach.RepoID == 0 {
		return nil, fmt.Errorf("attachment %s should belong to a repository", attach.Name)
	}

	err := db.WithTx(ctx, func(ctx context.Context) error {
		attach.UUID = uuid.New().String()
		size, err := storage.Attachments.Save(attach.RelativePath(), file, size)
		if err != nil {
			return fmt.Errorf("Attachments.Save: %w", err)
		}
		attach.Size = size
		return db.Insert(ctx, attach)
	})

	return attach, err
}

type UploaderFile struct {
	rd         io.ReadCloser
	size       int64
	respWriter http.ResponseWriter
}

func NewLimitedUploaderKnownSize(r io.Reader, size int64) *UploaderFile {
	return &UploaderFile{rd: io.NopCloser(r), size: size}
}

func NewLimitedUploaderMaxBytesReader(r io.ReadCloser, w http.ResponseWriter) *UploaderFile {
	return &UploaderFile{rd: r, size: -1, respWriter: w}
}

func UploadAttachmentGeneralSizeLimit(ctx context.Context, file *UploaderFile, allowedTypes string, attach *repo_model.Attachment) (*repo_model.Attachment, error) {
	return uploadAttachment(ctx, file, allowedTypes, setting.Attachment.MaxSize<<20, attach)
}

func uploadAttachment(ctx context.Context, file *UploaderFile, allowedTypes string, maxFileSize int64, attach *repo_model.Attachment) (*repo_model.Attachment, error) {
	src := file.rd
	if file.size < 0 {
		src = http.MaxBytesReader(file.respWriter, src, maxFileSize)
	}
	buf := make([]byte, 1024)