Move storagedriver package to registry/storage/driver

This change is slightly more complex than previous package maves in that the
package name changed. To address this, we simply always reference the package
driver as storagedriver to avoid compatbility issues with existing code. While
unfortunate, this can be cleaned up over time.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

docs/storage/driver/azure/zerofillwriter.go

@@ -0,0 +1,49 @@
+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
+}