Simplify data verification with HashReader. (#5071)

Verify() was being called by caller after the data
has been successfully read after io.EOF. This disconnection
opens a race under concurrent access to such an object.
Verification is not necessary outside of Read() call,
we can simply just do checksum verification right inside
Read() call at io.EOF.

This approach simplifies the usage.

cmd/gateway-b2.go

@@ -30,6 +30,7 @@ import (
 
 	b2 "github.com/minio/blazer/base"
 	"github.com/minio/minio-go/pkg/policy"
+	h2 "github.com/minio/minio/pkg/hash"
 )
 
 // Supported bucket types by B2 backend.
@@ -371,13 +372,13 @@ const (
 // being uploaded. Note that the content length is the size of the file plus 40
 // of the original size of the reader.
 //
-// newB2Reader implements a B2 compatible reader by wrapping the HashReader into
+// newB2Reader implements a B2 compatible reader by wrapping the hash.Reader into
 // a new io.Reader which will emit out the sha1 hex digits at io.EOF.
 // It also means that your overall content size is now original size + 40 bytes.
-// Additionally this reader also verifies Hash encapsulated inside HashReader
+// Additionally this reader also verifies Hash encapsulated inside hash.Reader
 // at io.EOF if the verification failed we return an error and do not send
 // the content to server.
-func newB2Reader(r *HashReader, size int64) *B2Reader {
+func newB2Reader(r *h2.Reader, size int64) *B2Reader {
 	return &B2Reader{
 		r:        r,
 		size:     size,
@@ -385,13 +386,13 @@ func newB2Reader(r *HashReader, size int64) *B2Reader {
 	}
 }
 
-// B2Reader - is a Reader wraps the HashReader which will emit out the sha1
+// B2Reader - is a Reader wraps the hash.Reader which will emit out the sha1
 // hex digits at io.EOF. It also means that your overall content size is
 // now original size + 40 bytes. Additionally this reader also verifies
-// Hash encapsulated inside HashReader at io.EOF if the verification
+// Hash encapsulated inside hash.Reader at io.EOF if the verification
 // failed we return an error and do not send the content to server.
 type B2Reader struct {
-	r        *HashReader
+	r        *h2.Reader
 	size     int64
 	sha1Hash hash.Hash
 
@@ -408,10 +409,6 @@ func (nb *B2Reader) Read(p []byte) (int, error) {
 	// Read into hash to update the on going checksum.
 	n, err := io.TeeReader(nb.r, nb.sha1Hash).Read(p)
 	if err == io.EOF {
-		// Verify checksum at io.EOF
-		if err = nb.r.Verify(); err != nil {
-			return n, err
-		}
 		// Stream is not corrupted on this end
 		// now fill in the last 40 bytes of sha1 hex
 		// so that the server can verify the stream on
@@ -424,7 +421,7 @@ func (nb *B2Reader) Read(p []byte) (int, error) {
 }
 
 // PutObject uploads the single upload to B2 backend by using *b2_upload_file* API, uploads upto 5GiB.
-func (l *b2Objects) PutObject(bucket string, object string, data *HashReader, metadata map[string]string) (ObjectInfo, error) {
+func (l *b2Objects) PutObject(bucket string, object string, data *h2.Reader, metadata map[string]string) (ObjectInfo, error) {
 	var objInfo ObjectInfo
 	bkt, err := l.Bucket(bucket)
 	if err != nil {
@@ -432,7 +429,6 @@ func (l *b2Objects) PutObject(bucket string, object string, data *HashReader, me
 	}
 	contentType := metadata["content-type"]
 	delete(metadata, "content-type")
-	delete(metadata, "etag")
 
 	var u *b2.URL
 	u, err = bkt.GetUploadURL(l.ctx)
@@ -550,7 +546,7 @@ func (l *b2Objects) CopyObjectPart(srcBucket string, srcObject string, destBucke
 }
 
 // PutObjectPart puts a part of object in bucket, uses B2's LargeFile upload API.
-func (l *b2Objects) PutObjectPart(bucket string, object string, uploadID string, partID int, data *HashReader) (pi PartInfo, err error) {
+func (l *b2Objects) PutObjectPart(bucket string, object string, uploadID string, partID int, data *h2.Reader) (pi PartInfo, err error) {
 	bkt, err := l.Bucket(bucket)
 	if err != nil {
 		return pi, err