Add new ReadFileWithVerify storage-layer API (#4349)

This is an enhancement to the XL/distributed-XL mode. FS mode is
unaffected.

The ReadFileWithVerify storage-layer call is similar to ReadFile with
the additional functionality of performing bit-rot checking. It
accepts additional parameters for a hashing algorithm to use and the
expected hex-encoded hash string.

This patch provides significant performance improvement because:

1. combines the step of reading the file (during
erasure-decoding/reconstruction) with bit-rot verification;

2. limits the number of file-reads; and

3. avoids transferring the file over the network for bit-rot
verification.

ReadFile API is implemented as ReadFileWithVerify with empty hashing
arguments.

Credits to AB and Harsha for the algorithmic improvement.

Fixes #4236.

cmd/storage-errors.go

@@ -16,7 +16,10 @@
 
 package cmd
 
-import "errors"
+import (
+	"errors"
+	"fmt"
+)
 
 // errUnexpected - unexpected error, requires manual intervention.
 var errUnexpected = errors.New("Unexpected error, please report this issue at https://github.com/minio/minio/issues")
@@ -68,3 +71,21 @@ var errVolumeAccessDenied = errors.New("volume access denied")
 
 // errVolumeAccessDenied - cannot access file, insufficient permissions.
 var errFileAccessDenied = errors.New("file access denied")
+
+// errBitrotHashAlgoInvalid - the algo for bit-rot hash
+// verification is empty or invalid.
+var errBitrotHashAlgoInvalid = errors.New("bit-rot hash algorithm is invalid")
+
+// hashMisMatchError - represents a bit-rot hash verification failure
+// error.
+type hashMismatchError struct {
+	expected string
+	computed string
+}
+
+// error method for the hashMismatchError
+func (h hashMismatchError) Error() string {
+	return fmt.Sprintf(
+		"Bitrot verification mismatch - expected %v, received %v",
+		h.expected, h.computed)
+}