navidrome/core/artwork/tee_reader.go

package artwork

import (
	"bytes"
	"io"
)

// teeReader mirrors bytes read from src into buf, and on Close invokes onComplete with the captured
// bytes only if the stream was fully consumed (EOF) and stayed within maxBytes. Partial reads and
// oversized streams are skipped, so the callback only ever receives a complete, bounded payload.
type teeReader struct {
	src        io.ReadCloser
	buf        bytes.Buffer
	maxBytes   int
	onComplete func(data []byte)
	eof        bool
	over       bool
}

func newTeeReader(src io.ReadCloser, maxBytes int, onComplete func(data []byte)) *teeReader {
	return &teeReader{src: src, maxBytes: maxBytes, onComplete: onComplete}
}

func (t *teeReader) Read(p []byte) (int, error) {
	n, err := t.src.Read(p)
	if n > 0 && !t.over {
		if t.buf.Len()+n > t.maxBytes {
			t.over = true
			t.buf.Reset()
		} else {
			t.buf.Write(p[:n])
		}
	}
	if err == io.EOF {
		t.eof = true
	}
	return n, err
}

func (t *teeReader) Close() error {
	err := t.src.Close()
	if t.eof && !t.over && t.onComplete != nil {
		cb := t.onComplete
		t.onComplete = nil // fire at most once, even on double Close
		cb(t.buf.Bytes())
	}
	return err
}