check: Suggest usage of `restic repair packs` for corrupted blobs

For now, the guide is only shown if the blob content does not match its
hash. The main intended usage is to handle data corruption errors when
using maximum compression in restic 0.16.0

cmd/restic/cmd_check.go

@@ -330,11 +330,28 @@ func runCheck(ctx context.Context, opts CheckOptions, gopts GlobalOptions, args
 
 		go chkr.ReadPacks(ctx, packs, p, errChan)
 
+		var salvagePacks restic.IDs
+
 		for err := range errChan {
 			errorsFound = true
 			Warnf("%v\n", err)
+			if err, ok := err.(*checker.ErrPackData); ok {
+				if strings.Contains(err.Error(), "wrong data returned, hash is") {
+					salvagePacks = append(salvagePacks, err.PackID)
+				}
+			}
 		}
 		p.Done()
+
+		if len(salvagePacks) > 0 {
+			Warnf("\nThe repository contains pack files with damaged blobs. These blobs must be removed to repair the repository. This can be done using the following commands:\n\n")
+			var strIds []string
+			for _, id := range salvagePacks {
+				strIds = append(strIds, id.String())
+			}
+			Warnf("restic repair packs %v\nrestic repair snapshots --forget\n\n", strings.Join(strIds, " "))
+			Warnf("Corrupted blobs are either caused by hardware problems or bugs in restic. Please open an issue at https://github.com/restic/restic/issues/new/choose for further troubleshooting!\n")
+		}
 	}
 
 	switch {

internal/checker/checker.go

@@ -90,6 +90,16 @@ func (err *ErrOldIndexFormat) Error() string {
 	return fmt.Sprintf("index %v has old format", err.ID)
 }
 
+// ErrPackData is returned if errors are discovered while verifying a packfile
+type ErrPackData struct {
+	PackID restic.ID
+	errs   []error
+}
+
+func (e *ErrPackData) Error() string {
+	return fmt.Sprintf("pack %v contains %v errors: %v", e.PackID, len(e.errs), e.errs)
+}
+
 func (c *Checker) LoadSnapshots(ctx context.Context) error {
 	var err error
 	c.snapshots, err = backend.MemorizeList(ctx, c.repo.Backend(), restic.SnapshotFile)
@@ -635,7 +645,7 @@ func checkPack(ctx context.Context, r restic.Repository, id restic.ID, blobs []r
 	}
 
 	if len(errs) > 0 {
-		return errors.Errorf("pack %v contains %v errors: %v", id, len(errs), errs)
+		return &ErrPackData{PackID: id, errs: errs}
 	}
 
 	return nil