feat: optional S3-compatible storage for uploaded images, instead of the local upload directory

internal/infrastructure/storage/s3Reader.go

@@ -0,0 +1,83 @@
+package storage
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"io"
+)
+
+type rangeOpener func(offset int64) (io.ReadCloser, context.CancelFunc, error)
+
+type s3Reader struct {
+	size       int64
+	position   int64
+	bodyOffset int64
+	body       io.ReadCloser
+	cancel     context.CancelFunc
+	reopen     rangeOpener
+}
+
+func newS3Reader(size int64, body io.ReadCloser, cancel context.CancelFunc, reopen rangeOpener) *s3Reader {
+	return &s3Reader{size: size, body: body, cancel: cancel, reopen: reopen}
+}
+
+func (r *s3Reader) Read(p []byte) (int, error) {
+	if r.position >= r.size {
+		return 0, io.EOF
+	}
+	if r.body == nil || r.bodyOffset != r.position {
+		if err := r.openAtPosition(); err != nil {
+			return 0, err
+		}
+	}
+	n, err := r.body.Read(p)
+	r.position += int64(n)
+	r.bodyOffset = r.position
+	return n, err
+}
+
+func (r *s3Reader) openAtPosition() error {
+	r.release()
+	body, cancel, err := r.reopen(r.position)
+	if err != nil {
+		return err
+	}
+	r.body, r.cancel, r.bodyOffset = body, cancel, r.position
+	return nil
+}
+
+func (r *s3Reader) Seek(offset int64, whence int) (int64, error) {
+	var target int64
+	switch whence {
+	case io.SeekStart:
+		target = offset
+	case io.SeekCurrent:
+		target = r.position + offset
+	case io.SeekEnd:
+		target = r.size + offset
+	default:
+		return 0, errors.New("invalid seek whence")
+	}
+	if target < 0 {
+		return 0, fmt.Errorf("negative seek position %d", target)
+	}
+	r.position = target
+	return target, nil
+}
+
+func (r *s3Reader) Close() error {
+	r.release()
+	return nil
+}
+
+func (r *s3Reader) release() {
+	if r.body != nil {
+		_ = r.body.Close()
+		r.body = nil
+	}
+	if r.cancel != nil {
+		r.cancel()
+		r.cancel = nil
+	}
+}