在 go 中實現 io.WriterAt 的緩沖區

Question

我正在使用 aws-sdk 從 s3 存儲桶下載文件。 S3 下載功能需要實現 io.WriterAt 但是 bytes.Buffer 沒有實現它。 現在我正在創建一個實現 io.WriterAt 的文件，但我想要內存中的一些東西。

Answer 1

對於涉及 AWS 開發工具包的案例，請使用aws.WriteAtBuffer將 S3 對象下載到內存中。

requestInput := s3.GetObjectInput{
    Bucket: aws.String(bucket),
    Key:    aws.String(key),
}

buf := aws.NewWriteAtBuffer([]byte{})
downloader.Download(buf, &requestInput)

fmt.Printf("Downloaded %v bytes", len(buf.Bytes()))

Answer 2

使用 io.Writer 偽造 AWS WriterAt

這不是對原始問題的直接回答，而是我在這里登陸后實際使用的解決方案。 這是一個類似的用例，我認為可以幫助其他人。

AWS 文檔定義了合同，如果您將downloader.Concurrency設置為 1，您將獲得保證的順序寫入。

downloader.Download(FakeWriterAt{w}, s3.GetObjectInput{
    Bucket: aws.String(bucket),
    Key:    aws.String(key),
})
downloader.Concurrency = 1

因此，您可以使用io.Writer並將其包裝以實現io.WriterAt ，從而丟棄您不再需要的offset ：

type FakeWriterAt struct {
    w io.Writer
}

func (fw FakeWriterAt) WriteAt(p []byte, offset int64) (n int, err error) {
    // ignore 'offset' because we forced sequential downloads
    return fw.w.Write(p)
}

Answer 3

我不知道在標准庫中有什么方法可以做到這一點，但是您可以編寫自己的緩沖區。

真的不會那么難...

編輯：我無法停止思考這個問題，我最終把整件事都搞砸了，享受:)

package main

import (
    "errors"
    "fmt"
)

func main() {
    buff := NewWriteBuffer(0, 10)

    buff.WriteAt([]byte("abc"), 5)
    fmt.Printf("%#v\n", buff)
}

// WriteBuffer is a simple type that implements io.WriterAt on an in-memory buffer.
// The zero value of this type is an empty buffer ready to use.
type WriteBuffer struct {
    d []byte
    m int
}

// NewWriteBuffer creates and returns a new WriteBuffer with the given initial size and
// maximum. If maximum is <= 0 it is unlimited.
func NewWriteBuffer(size, max int) *WriteBuffer {
    if max < size && max >= 0 {
        max = size
    }
    return &WriteBuffer{make([]byte, size), max}
}

// SetMax sets the maximum capacity of the WriteBuffer. If the provided maximum is lower
// than the current capacity but greater than 0 it is set to the current capacity, if
// less than or equal to zero it is unlimited..
func (wb *WriteBuffer) SetMax(max int) {
    if max < len(wb.d) && max >= 0 {
        max = len(wb.d)
    }
    wb.m = max
}

// Bytes returns the WriteBuffer's underlying data. This value will remain valid so long
// as no other methods are called on the WriteBuffer.
func (wb *WriteBuffer) Bytes() []byte {
    return wb.d
}

// Shape returns the current WriteBuffer size and its maximum if one was provided.
func (wb *WriteBuffer) Shape() (int, int) {
    return len(wb.d), wb.m
}

func (wb *WriteBuffer) WriteAt(dat []byte, off int64) (int, error) {
    // Range/sanity checks.
    if int(off) < 0 {
        return 0, errors.New("Offset out of range (too small).")
    }
    if int(off)+len(dat) >= wb.m && wb.m > 0 {
        return 0, errors.New("Offset+data length out of range (too large).")
    }

    // Check fast path extension
    if int(off) == len(wb.d) {
        wb.d = append(wb.d, dat...)
        return len(dat), nil
    }

    // Check slower path extension
    if int(off)+len(dat) >= len(wb.d) {
        nd := make([]byte, int(off)+len(dat))
        copy(nd, wb.d)
        wb.d = nd
    }

    // Once no extension is needed just copy bytes into place.
    copy(wb.d[int(off):], dat)
    return len(dat), nil
}

Answer 4

我正在尋找一種直接從 S3 對象獲取io.ReadCloser的簡單方法。 無需緩沖響應，也無需降低並發性。

import "github.com/aws/aws-sdk-go/service/s3"

[...]

obj, err := c.s3.GetObject(&s3.GetObjectInput{
    Bucket: aws.String("my-bucket"),
    Key:    aws.String("path/to/the/object"),
})
if err != nil {
    return nil, err
}

// obj.Body is a ReadCloser 
return obj.Body, nil

Answer 5

使用 aws-sdk-go-v2，代碼庫提供的示例顯示：

// Example:
// pre-allocate in memory buffer, where headObject type is *s3.HeadObjectOutput
buf := make([]byte, int(headObject.ContentLength))
// wrap with aws.WriteAtBuffer
w := s3manager.NewWriteAtBuffer(buf)
// download file into the memory
numBytesDownloaded, err := downloader.Download(ctx, w, &s3.GetObjectInput{
    Bucket: aws.String(bucket),
    Key:    aws.String(item),
})

然后使用 w.Bytes() 作為結果。

導入“github.com/aws/aws-sdk-go-v2/feature/s3/manager”和其他需要的組件