fix (extras/crypto): add guarding for Sha256SumWriter/Sha256SumReader (…

…#169)

Add some mutexes in Sha256SumWriter and Sha256SumReader to ensure
we don't get panics

extras/crypto/sha256sum.go

@@ -10,6 +10,7 @@ import (
 	"fmt"
 	"hash"
 	"io"
+	"sync"
 
 	wrapping "github.com/hashicorp/go-kms-wrapping/v2"
 )
@@ -44,6 +45,7 @@ func Sha256Sum(ctx context.Context, r io.Reader, opt ...wrapping.Option) ([]byte
 // Sha256SumWriter provides multi-writer which will be used to write to a
 // hash and produce a sum.  It implements io.WriterCloser and io.StringWriter.
 type Sha256SumWriter struct {
+	l    sync.Mutex
 	hash hash.Hash
 	tee  io.Writer
 	w    io.Writer
@@ -69,6 +71,8 @@ func NewSha256SumWriter(ctx context.Context, w io.Writer) (*Sha256SumWriter, err
 // func.
 func (w *Sha256SumWriter) Write(b []byte) (int, error) {
 	const op = "crypto.(Sha256SumWriter).Write"
+	w.l.Lock()
+	defer w.l.Unlock()
 	n, err := w.tee.Write(b)
 	if err != nil {
 		return n, fmt.Errorf("%s: %w", op, err)
@@ -106,6 +110,8 @@ func (w *Sha256SumWriter) Sum(_ context.Context, opt ...wrapping.Option) ([]byte
 	if err != nil {
 		return nil, fmt.Errorf("%s: %w", op, err)
 	}
+	w.l.Lock()
+	defer w.l.Unlock()
 	h := w.hash.Sum(nil)
 	switch {
 	case opts.WithHexEncoding:
@@ -119,6 +125,7 @@ func (w *Sha256SumWriter) Sum(_ context.Context, opt ...wrapping.Option) ([]byte
 // Sha256SumReader provides an io.Reader which can be used to calculate a sum
 // while reading a file. It implements io.ReaderCloser.
 type Sha256SumReader struct {
+	l    sync.Mutex
 	hash hash.Hash
 	tee  io.Reader
 	r    io.Reader
@@ -142,6 +149,8 @@ func NewSha256SumReader(_ context.Context, r io.Reader) (*Sha256SumReader, error
 
 func (r *Sha256SumReader) Read(b []byte) (int, error) {
 	const op = "crypto.(Sha256SumReader).Read"
+	r.l.Lock()
+	defer r.l.Unlock()
 	n, err := r.tee.Read(b)
 	if err != nil {
 		return n, fmt.Errorf("%s: %w", op, err)
@@ -170,6 +179,8 @@ func (r *Sha256SumReader) Sum(_ context.Context, opt ...wrapping.Option) ([]byte
 	if err != nil {
 		return nil, fmt.Errorf("%s: %w", op, err)
 	}
+	r.l.Lock()
+	defer r.l.Unlock()
 	h := r.hash.Sum(nil)
 	switch {
 	case opts.WithHexEncoding: