Go で割り当てなしで reader の len をしりた〜い

ひとくちメモシリーズ

n, err := io.Copy(io.Discard, reader) がよさそう

discard は ReadFrom() を実装していて、何もしないで n だけとる

https://cs.opensource.google/go/go/+/refs/tags/go1.24.5:src/io/io.go;l=662-676

func (discard) ReadFrom(r Reader) (n int64, err error) {
    bufp := blackHolePool.Get().(*[]byte)
    readSize := 0
    for {
        readSize, err = r.Read(*bufp)
        n += int64(readSize)
        if err != nil {
            blackHolePool.Put(bufp)
            if err == EOF {
                return n, nil
            }
            return
        }
    }
}

io.Copy() は dst が io.ReadFrom() を実装してると buf の alloc なしでそれを使う

https://cs.opensource.google/go/go/+/refs/tags/go1.24.5:src/io/io.go;l=413-416

// copyBuffer is the actual implementation of Copy and CopyBuffer.
// if buf is nil, one is allocated.
func copyBuffer(dst Writer, src Reader, buf []byte) (written int64, err error) {
    // If the reader has a WriteTo method, use it to do the copy.
    // Avoids an allocation and a copy.
    if wt, ok := src.(WriterTo); ok {
        return wt.WriteTo(dst)
    }
    // Similarly, if the writer has a ReadFrom method, use it to do the copy.
    if rf, ok := dst.(ReaderFrom); ok {
        return rf.ReadFrom(src)
    }
    ...

ので、 n, err := io.Copy(io.Discard, reader) で負担少なくいい感じに n を取得できて happy という趣