distribution/registry/storage/driver/azure/zerofillwriter.go

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package azure
import (
"bytes"
"io"
)
type blockBlobWriter interface {
GetSize(container, blob string) (int64, error)
WriteBlobAt(container, blob string, offset int64, chunk io.Reader) (int64, error)
}
// zeroFillWriter enables writing to an offset outside a block blob's size
// by offering the chunk to the underlying writer as a contiguous data with
// the gap in between filled with NUL (zero) bytes.
type zeroFillWriter struct {
blockBlobWriter
}
func newZeroFillWriter(b blockBlobWriter) zeroFillWriter {
w := zeroFillWriter{}
w.blockBlobWriter = b
return w
}
// Write writes the given chunk to the specified existing blob even though
// offset is out of blob's size. The gaps are filled with zeros. Returned
// written number count does not include zeros written.
func (z *zeroFillWriter) Write(container, blob string, offset int64, chunk io.Reader) (int64, error) {
size, err := z.blockBlobWriter.GetSize(container, blob)
if err != nil {
return 0, err
}
var reader io.Reader
var zeroPadding int64
if offset <= size {
reader = chunk
} else {
zeroPadding = offset - size
offset = size // adjust offset to be the append index
zeros := bytes.NewReader(make([]byte, zeroPadding))
reader = io.MultiReader(zeros, chunk)
}
nn, err := z.blockBlobWriter.WriteBlobAt(container, blob, offset, reader)
nn -= zeroPadding
return nn, err
}