Avoid unnecessary reopening of HTTP streams in GetObject() (#1908)

Sometimes consumers of Seek() might use it for purposes other than
chaging the current read/write offset. For example, using whence =
io.SeekCurrent to get the current seek position. However, minio-go
invalidates the underlying HTTP stream unconditionally when Seek() is
called, resulting in massive performance degradation in these scenarios.

This commit avoids these issues by only reopening the stream if the seek
position is actually changed.

api-get-object.go

@@ -550,6 +550,8 @@ func (o *Object) Seek(offset int64, whence int) (n int64, err error) {
 		}
 	}
 
+	newOffset := o.currOffset
+
 	// Switch through whence.
 	switch whence {
 	default:
@@ -558,12 +560,12 @@ func (o *Object) Seek(offset int64, whence int) (n int64, err error) {
 		if o.objectInfo.Size > -1 && offset > o.objectInfo.Size {
 			return 0, io.EOF
 		}
-		o.currOffset = offset
+		newOffset = offset
 	case 1:
 		if o.objectInfo.Size > -1 && o.currOffset+offset > o.objectInfo.Size {
 			return 0, io.EOF
 		}
-		o.currOffset += offset
+		newOffset += offset
 	case 2:
 		// If we don't know the object size return an error for io.SeekEnd
 		if o.objectInfo.Size < 0 {
@@ -579,16 +581,17 @@ func (o *Object) Seek(offset int64, whence int) (n int64, err error) {
 		if o.objectInfo.Size+offset < 0 {
 			return 0, errInvalidArgument(fmt.Sprintf("Seeking at negative offset not allowed for %d", whence))
 		}
-		o.currOffset = o.objectInfo.Size + offset
+		newOffset = o.objectInfo.Size + offset
 	}
 	// Reset the saved error since we successfully seeked, let the Read
 	// and ReadAt decide.
 	if o.prevErr == io.EOF {
 		o.prevErr = nil
 	}
 
-	// Ask lower level to fetch again from source
-	o.seekData = true
+	// Ask lower level to fetch again from source when necessary
+	o.seekData = (newOffset != o.currOffset) || o.seekData
+	o.currOffset = newOffset
 
 	// Return the effective offset.
 	return o.currOffset, nil