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Copy pathreader.go
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155 lines (139 loc) · 3.95 KB
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// Copyright (c) 2015-2021 MinIO, Inc.
//
// This file is part of MinIO Object Storage stack
//
// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU Affero General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// This program is distributed in the hope that it will be useful
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU Affero General Public License for more details.
//
// You should have received a copy of the GNU Affero General Public License
// along with this program. If not, see <http://www.gnu.org/licenses/>.
package etag
import (
"crypto/md5"
"fmt"
"hash"
"io"
)
// Tagger is the interface that wraps the basic ETag method.
type Tagger interface {
ETag() ETag
}
type wrapReader struct {
io.Reader
Tagger
}
var _ Tagger = wrapReader{} // compiler check
// ETag returns the ETag of the underlying Tagger.
func (r wrapReader) ETag() ETag {
if r.Tagger == nil {
return nil
}
return r.Tagger.ETag()
}
// Wrap returns an io.Reader that reads from the wrapped
// io.Reader and implements the Tagger interaface.
//
// If content implements Tagger then the returned Reader
// returns ETag of the content. Otherwise, it returns
// nil as ETag.
//
// Wrap provides an adapter for io.Reader implemetations
// that don't implement the Tagger interface.
// It is mainly used to provide a high-level io.Reader
// access to the ETag computed by a low-level io.Reader:
//
// content := etag.NewReader(r.Body, nil)
//
// compressedContent := Compress(content)
// encryptedContent := Encrypt(compressedContent)
//
// // Now, we need an io.Reader that can access
// // the ETag computed over the content.
// reader := etag.Wrap(encryptedContent, content)
func Wrap(wrapped, content io.Reader) io.Reader {
if t, ok := content.(Tagger); ok {
return wrapReader{
Reader: wrapped,
Tagger: t,
}
}
return wrapReader{
Reader: wrapped,
}
}
// A Reader wraps an io.Reader and computes the
// MD5 checksum of the read content as ETag.
//
// Optionally, a Reader can also verify that
// the computed ETag matches an expected value.
// Therefore, it compares both ETags once the
// underlying io.Reader returns io.EOF.
// If the computed ETag does not match the
// expected ETag then Read returns a VerifyError.
//
// Reader implements the Tagger interface.
type Reader struct {
src io.Reader
md5 hash.Hash
checksum ETag
readN int64
}
// NewReader returns a new Reader that computes the
// MD5 checksum of the content read from r as ETag.
//
// If the provided etag is not nil the returned
// Reader compares the etag with the computed
// MD5 sum once the r returns io.EOF.
func NewReader(r io.Reader, etag ETag) *Reader {
if er, ok := r.(*Reader); ok {
if er.readN == 0 && Equal(etag, er.checksum) {
return er
}
}
return &Reader{
src: r,
md5: md5.New(),
checksum: etag,
}
}
// Read reads up to len(p) bytes from the underlying
// io.Reader as specified by the io.Reader interface.
func (r *Reader) Read(p []byte) (int, error) {
n, err := r.src.Read(p)
r.readN += int64(n)
r.md5.Write(p[:n])
if err == io.EOF && len(r.checksum) != 0 {
if etag := r.ETag(); !Equal(etag, r.checksum) {
return n, VerifyError{
Expected: r.checksum,
Computed: etag,
}
}
}
return n, err
}
// ETag returns the ETag of all the content read
// so far. Reading more content changes the MD5
// checksum. Therefore, calling ETag multiple
// times may return different results.
func (r *Reader) ETag() ETag {
sum := r.md5.Sum(nil)
return ETag(sum)
}
// VerifyError is an error signaling that a
// computed ETag does not match an expected
// ETag.
type VerifyError struct {
Expected ETag
Computed ETag
}
func (v VerifyError) Error() string {
return fmt.Sprintf("etag: expected ETag %q does not match computed ETag %q", v.Expected, v.Computed)
}