mem: calculate md5 content hash for uploads

The mem backend is primarily used for testing. This ensures that the
upload hash calculation gets appropriate test coverage.

internal/backend/mem/mem_backend.go

@@ -3,6 +3,8 @@ package mem
 import (
 	"bytes"
 	"context"
+	"crypto/md5"
+	"encoding/base64"
 	"hash"
 	"io"
 	"io/ioutil"
@@ -87,6 +89,16 @@ func (be *MemoryBackend) Save(ctx context.Context, h restic.Handle, rd restic.Re
 		return errors.Errorf("wrote %d bytes instead of the expected %d bytes", len(buf), rd.Length())
 	}
 
+	beHash := be.Hasher()
+	// must never fail according to interface
+	_, _ = beHash.Write(buf)
+	if !bytes.Equal(beHash.Sum(nil), rd.Hash()) {
+		return errors.Errorf("invalid file hash or content, got %s expected %s",
+			base64.RawStdEncoding.EncodeToString(beHash.Sum(nil)),
+			base64.RawStdEncoding.EncodeToString(rd.Hash()),
+		)
+	}
+
 	be.data[h] = buf
 	debug.Log("saved %v bytes at %v", len(buf), h)
 
@@ -217,7 +229,7 @@ func (be *MemoryBackend) Location() string {
 
 // Hasher may return a hash function for calculating a content hash for the backend
 func (be *MemoryBackend) Hasher() hash.Hash {
-	return nil
+	return md5.New()
 }
 
 // Delete removes all data in the backend.