upgrade_repo_v2: Use atomic replace for supported backends

internal/backend/azure/azure.go

@@ -125,6 +125,11 @@ func (be *Backend) Hasher() hash.Hash {
 	return md5.New()
 }
 
+// HasAtomicReplace returns whether Save() can atomically replace files
+func (be *Backend) HasAtomicReplace() bool {
+	return true
+}
+
 // Path returns the path in the bucket that is used for this backend.
 func (be *Backend) Path() string {
 	return be.prefix

internal/backend/b2/b2.go

@@ -147,6 +147,11 @@ func (be *b2Backend) Hasher() hash.Hash {
 	return nil
 }
 
+// HasAtomicReplace returns whether Save() can atomically replace files
+func (be *b2Backend) HasAtomicReplace() bool {
+	return true
+}
+
 // IsNotExist returns true if the error is caused by a non-existing file.
 func (be *b2Backend) IsNotExist(err error) bool {
 	return b2.IsNotExist(errors.Cause(err))

internal/backend/dryrun/dry_backend.go

@@ -67,6 +67,10 @@ func (be *Backend) Hasher() hash.Hash {
 	return be.b.Hasher()
 }
 
+func (be *Backend) HasAtomicReplace() bool {
+	return be.b.HasAtomicReplace()
+}
+
 func (be *Backend) IsNotExist(err error) bool {
 	return be.b.IsNotExist(err)
 }

internal/backend/gs/gs.go

@@ -201,6 +201,11 @@ func (be *Backend) Hasher() hash.Hash {
 	return md5.New()
 }
 
+// HasAtomicReplace returns whether Save() can atomically replace files
+func (be *Backend) HasAtomicReplace() bool {
+	return true
+}
+
 // Path returns the path in the bucket that is used for this backend.
 func (be *Backend) Path() string {
 	return be.prefix

internal/backend/local/local.go

@@ -102,6 +102,11 @@ func (b *Local) Hasher() hash.Hash {
 	return nil
 }
 
+// HasAtomicReplace returns whether Save() can atomically replace files
+func (b *Local) HasAtomicReplace() bool {
+	return true
+}
+
 // IsNotExist returns true if the error is caused by a non existing file.
 func (b *Local) IsNotExist(err error) bool {
 	return errors.Is(err, os.ErrNotExist)

internal/backend/mem/mem_backend.go

@@ -268,6 +268,11 @@ func (be *MemoryBackend) Hasher() hash.Hash {
 	return md5.New()
 }
 
+// HasAtomicReplace returns whether Save() can atomically replace files
+func (be *MemoryBackend) HasAtomicReplace() bool {
+	return false
+}
+
 // Delete removes all data in the backend.
 func (be *MemoryBackend) Delete(ctx context.Context) error {
 	be.m.Lock()

internal/backend/rest/rest.go

@@ -121,6 +121,12 @@ func (b *Backend) Hasher() hash.Hash {
 	return nil
 }
 
+// HasAtomicReplace returns whether Save() can atomically replace files
+func (b *Backend) HasAtomicReplace() bool {
+	// rest-server prevents overwriting
+	return false
+}
+
 // Save stores data in the backend at the handle.
 func (b *Backend) Save(ctx context.Context, h restic.Handle, rd restic.RewindReader) error {
 	if err := h.Valid(); err != nil {

internal/backend/s3/s3.go

@@ -269,6 +269,11 @@ func (be *Backend) Hasher() hash.Hash {
 	return nil
 }
 
+// HasAtomicReplace returns whether Save() can atomically replace files
+func (be *Backend) HasAtomicReplace() bool {
+	return true
+}
+
 // Path returns the path in the bucket that is used for this backend.
 func (be *Backend) Path() string {
 	return be.cfg.Prefix

internal/backend/sftp/sftp.go

@@ -267,6 +267,12 @@ func (r *SFTP) Hasher() hash.Hash {
 	return nil
 }
 
+// HasAtomicReplace returns whether Save() can atomically replace files
+func (r *SFTP) HasAtomicReplace() bool {
+	// we use sftp's 'Rename()' in 'Save()' which does not allow overwriting
+	return false
+}
+
 // Join joins the given paths and cleans them afterwards. This always uses
 // forward slashes, which is required by sftp.
 func Join(parts ...string) string {

internal/backend/swift/swift.go

@@ -129,6 +129,11 @@ func (be *beSwift) Hasher() hash.Hash {
 	return md5.New()
 }
 
+// HasAtomicReplace returns whether Save() can atomically replace files
+func (be *beSwift) HasAtomicReplace() bool {
+	return true
+}
+
 // Load runs fn with a reader that yields the contents of the file at h at the
 // given offset.
 func (be *beSwift) Load(ctx context.Context, h restic.Handle, length int, offset int64, fn func(rd io.Reader) error) error {