b2: Support file hiding instead of deleting them permanently

Automatically fall back to hiding files if not authorized to permanently
delete files. This allows using restic with an append-only application
key with B2.  Thus, an attacker cannot directly delete backups with the
API key used by restic.

To use this feature create an application key without the deleteFiles
capability. It is recommended to restrict the key to just one bucket.
For example using the b2 command line tool:

    b2 create-key --bucket <bucketName> <keyName> listBuckets,readFiles,writeFiles,listFiles

Suggested-by: Daniel Gröber <dxld@darkboxed.org>

internal/backend/b2/b2.go

@@ -18,6 +18,7 @@ import (
 
 	"github.com/cenkalti/backoff/v4"
 	"github.com/kurin/blazer/b2"
+	"github.com/kurin/blazer/base"
 )
 
 // b2Backend is a backend which stores its data on Backblaze B2.
@@ -28,6 +29,8 @@ type b2Backend struct {
 	listMaxItems int
 	layout.Layout
 	sem sema.Semaphore
+
+	canDelete bool
 }
 
 // Billing happens in 1000 item granlarity, but we are more interested in reducing the number of network round trips
@@ -104,6 +107,7 @@ func Open(ctx context.Context, cfg Config, rt http.RoundTripper) (restic.Backend
 		},
 		listMaxItems: defaultListMaxItems,
 		sem:          sem,
+		canDelete:    true,
 	}
 
 	return be, nil
@@ -314,11 +318,27 @@ func (be *b2Backend) Remove(ctx context.Context, h restic.Handle) error {
 	// the retry backend will also repeat the remove method up to 10 times
 	for i := 0; i < 3; i++ {
 		obj := be.bucket.Object(be.Filename(h))
-		err := obj.Delete(ctx)
-		if err == nil {
-			// keep deleting until we are sure that no leftover file versions exist
-			continue
+
+		var err error
+		if be.canDelete {
+			err = obj.Delete(ctx)
+			if err == nil {
+				// keep deleting until we are sure that no leftover file versions exist
+				continue
+			}
+
+			code, _ := base.Code(err)
+			if code == 401 { // unauthorized
+				// fallback to hide if not allowed to delete files
+				be.canDelete = false
+				debug.Log("Removing %v failed, falling back to b2_hide_file.", h)
+				continue
+			}
+		} else {
+			// hide adds a new file version hiding all older ones, thus retries are not necessary
+			err = obj.Hide(ctx)
 		}
+
 		// consider a file as removed if b2 informs us that it does not exist
 		if b2.IsNotExist(err) {
 			return nil