erasure: simplify XL backend operations (#4649) (#4758)

This change provides new implementations of the XL backend operations:
 - create file
 - read   file
 - heal   file
Further this change adds table based tests for all three operations.

This affects also the bitrot algorithm integration. Algorithms are now
integrated in an idiomatic way (like crypto.Hash).
Fixes #4696
Fixes #4649
Fixes #4359

cmd/storage-interface.go

@@ -16,7 +16,11 @@
 
 package cmd
 
-import "github.com/minio/minio/pkg/disk"
+import (
+	"io"
+
+	"github.com/minio/minio/pkg/disk"
+)
 
 // StorageAPI interface.
 type StorageAPI interface {
@@ -37,8 +41,7 @@ type StorageAPI interface {
 	// File operations.
 	ListDir(volume, dirPath string) ([]string, error)
 	ReadFile(volume string, path string, offset int64, buf []byte) (n int64, err error)
-	ReadFileWithVerify(volume string, path string, offset int64, buf []byte,
-		algo HashAlgo, expectedHash string) (n int64, err error)
+	ReadFileWithVerify(volume string, path string, offset int64, buf []byte, verifier *BitrotVerifier) (n int64, err error)
 	PrepareFile(volume string, path string, len int64) (err error)
 	AppendFile(volume string, path string, buf []byte) (err error)
 	RenameFile(srcVolume, srcPath, dstVolume, dstPath string) error
@@ -48,3 +51,47 @@ type StorageAPI interface {
 	// Read all.
 	ReadAll(volume string, path string) (buf []byte, err error)
 }
+
+// storageReader is an io.Reader view of a disk
+type storageReader struct {
+	storage      StorageAPI
+	volume, path string
+	offset       int64
+}
+
+func (r *storageReader) Read(p []byte) (n int, err error) {
+	nn, err := r.storage.ReadFile(r.volume, r.path, r.offset, p)
+	r.offset += nn
+	n = int(nn)
+
+	if err == io.ErrUnexpectedEOF && nn > 0 {
+		err = io.EOF
+	}
+	return
+}
+
+// storageWriter is a io.Writer view of a disk.
+type storageWriter struct {
+	storage      StorageAPI
+	volume, path string
+}
+
+func (w *storageWriter) Write(p []byte) (n int, err error) {
+	err = w.storage.AppendFile(w.volume, w.path, p)
+	if err == nil {
+		n = len(p)
+	}
+	return
+}
+
+// StorageWriter returns a new io.Writer which appends data to the file
+// at the given disk, volume and path.
+func StorageWriter(storage StorageAPI, volume, path string) io.Writer {
+	return &storageWriter{storage, volume, path}
+}
+
+// StorageReader returns a new io.Reader which reads data to the file
+// at the given disk, volume, path and offset.
+func StorageReader(storage StorageAPI, volume, path string, offset int64) io.Reader {
+	return &storageReader{storage, volume, path, offset}
+}