backend: Relax requirement for new files

Before, all backend implementations were required to return an error if
the file that is to be written already exists in the backend. For most
backends, that means making a request (e.g. via HTTP) and returning an
error when the file already exists.

This is not accurate, the file could have been created between the HTTP
request testing for it, and when writing starts. In addition, apart from
the `config` file in the repo, all other file names have pseudo-random
names with a very very low probability of a collision. And even if a
file name is written again, the way the restic repo is structured this
just means that the same content is placed there again. Which is not a
problem, just not very efficient.

So, this commit relaxes the requirement to return an error when the file
in the backend already exists, which allows reducing the number of API
requests and thereby the latency for remote backends.

internal/backend/azure/azure.go

@@ -135,16 +135,6 @@ func (be *Backend) Save(ctx context.Context, h restic.Handle, rd io.Reader) (err
 
 	debug.Log("Save %v at %v", h, objName)
 
-	// Check key does not already exist
-	found, err := be.container.GetBlobReference(objName).Exists()
-	if err != nil {
-		return errors.Wrap(err, "GetBlobReference().Exists()")
-	}
-	if found {
-		debug.Log("%v already exists", h)
-		return errors.New("key already exists")
-	}
-
 	be.sem.GetToken()
 
 	// wrap the reader so that net/http client cannot close the reader, return

internal/backend/b2/b2.go

@@ -196,12 +196,6 @@ func (be *b2Backend) Save(ctx context.Context, h restic.Handle, rd io.Reader) er
 	debug.Log("Save %v, name %v", h, name)
 	obj := be.bucket.Object(name)
 
-	_, err := obj.Attrs(ctx)
-	if err == nil {
-		debug.Log("  %v already exists", h)
-		return errors.New("key already exists")
-	}
-
 	w := obj.NewWriter(ctx)
 	n, err := io.Copy(w, rd)
 	debug.Log("  saved %d bytes, err %v", n, err)

internal/backend/gs/gs.go

@@ -218,13 +218,6 @@ func (be *Backend) Save(ctx context.Context, h restic.Handle, rd io.Reader) (err
 
 	be.sem.GetToken()
 
-	// Check key does not already exist
-	if _, err := be.service.Objects.Get(be.bucketName, objName).Do(); err == nil {
-		debug.Log("%v already exists", h)
-		be.sem.ReleaseToken()
-		return errors.New("key already exists")
-	}
-
 	debug.Log("InsertObject(%v, %v)", be.bucketName, objName)
 
 	// Set chunk size to zero to disable resumable uploads.

internal/backend/s3/s3.go

@@ -235,13 +235,6 @@ func (be *Backend) Save(ctx context.Context, h restic.Handle, rd io.Reader) (err
 	be.sem.GetToken()
 	defer be.sem.ReleaseToken()
 
-	// Check key does not already exist
-	_, err = be.client.StatObject(be.cfg.Bucket, objName, minio.StatObjectOptions{})
-	if err == nil {
-		debug.Log("%v already exists", h)
-		return errors.New("key already exists")
-	}
-
 	var size int64 = -1
 
 	type lenner interface {

internal/backend/swift/swift.go

@@ -165,19 +165,6 @@ func (be *beSwift) Save(ctx context.Context, h restic.Handle, rd io.Reader) (err
 	be.sem.GetToken()
 	defer be.sem.ReleaseToken()
 
-	// Check key does not already exist
-	switch _, _, err = be.conn.Object(be.container, objName); err {
-	case nil:
-		debug.Log("%v already exists", h)
-		return errors.New("key already exists")
-
-	case swift.ObjectNotFound:
-		// Ok, that's what we want
-
-	default:
-		return errors.Wrap(err, "conn.Object")
-	}
-
 	encoding := "binary/octet-stream"
 
 	debug.Log("PutObject(%v, %v, %v)", be.container, objName, encoding)

internal/backend/test/tests.go

@@ -793,14 +793,10 @@ func (s *Suite) TestBackend(t *testing.T) {
 
 		// test adding the first file again
 		ts := testStrings[0]
-
-		// create blob
 		h := restic.Handle{Type: tpe, Name: ts.id}
-		err := b.Save(context.TODO(), h, strings.NewReader(ts.data))
-		test.Assert(t, err != nil, "backend has allowed overwrite of existing blob: expected error for %v, got %v", h, err)
 
 		// remove and recreate
-		err = s.delayedRemove(t, b, h)
+		err := s.delayedRemove(t, b, h)
 		test.OK(t, err)
 
 		// test that the blob is gone