rest: rework error reporting and report too short files

internal/backend/rest/rest.go

@@ -30,6 +30,20 @@ type Backend struct {
 	layout.Layout
 }
 
+// restError is returned whenever the server returns a non-successful HTTP status.
+type restError struct {
+	backend.Handle
+	StatusCode int
+	Status     string
+}
+
+func (e *restError) Error() string {
+	if e.StatusCode == http.StatusNotFound && e.Handle.Type.String() != "invalid" {
+		return fmt.Sprintf("%v does not exist", e.Handle)
+	}
+	return fmt.Sprintf("unexpected HTTP response (%v): %v", e.StatusCode, e.Status)
+}
+
 func NewFactory() location.Factory {
 	return location.NewHTTPBackendFactory("rest", ParseConfig, StripPassword, Create, Open)
 }
@@ -96,7 +110,7 @@ func Create(ctx context.Context, cfg Config, rt http.RoundTripper) (*Backend, er
 	}
 
 	if resp.StatusCode != http.StatusOK {
-		return nil, fmt.Errorf("server response unexpected: %v (%v)", resp.Status, resp.StatusCode)
+		return nil, &restError{backend.Handle{}, resp.StatusCode, resp.Status}
 	}
 
 	return be, nil
@@ -150,26 +164,31 @@ func (b *Backend) Save(ctx context.Context, h backend.Handle, rd backend.RewindR
 	}
 
 	if resp.StatusCode != http.StatusOK {
-		return errors.Errorf("server response unexpected: %v (%v)", resp.Status, resp.StatusCode)
+		return &restError{h, resp.StatusCode, resp.Status}
 	}
 
 	return nil
 }
 
-// notExistError is returned whenever the requested file does not exist on the
-// server.
-type notExistError struct {
-	backend.Handle
-}
-
-func (e *notExistError) Error() string {
-	return fmt.Sprintf("%v does not exist", e.Handle)
-}
-
 // IsNotExist returns true if the error was caused by a non-existing file.
 func (b *Backend) IsNotExist(err error) bool {
-	var e *notExistError
-	return errors.As(err, &e)
+	var e *restError
+	return errors.As(err, &e) && e.StatusCode == http.StatusNotFound
+}
+
+func (b *Backend) IsPermanentError(err error) bool {
+	if b.IsNotExist(err) {
+		return true
+	}
+
+	var rerr *restError
+	if errors.As(err, &rerr) {
+		if rerr.StatusCode == http.StatusRequestedRangeNotSatisfiable || rerr.StatusCode == http.StatusUnauthorized || rerr.StatusCode == http.StatusForbidden {
+			return true
+		}
+	}
+
+	return false
 }
 
 // Load runs fn with a reader that yields the contents of the file at h at the
@@ -221,14 +240,13 @@ func (b *Backend) openReader(ctx context.Context, h backend.Handle, length int,
 		return nil, errors.Wrap(err, "client.Do")
 	}
 
-	if resp.StatusCode == http.StatusNotFound {
-		_ = drainAndClose(resp)
-		return nil, &notExistError{h}
-	}
-
 	if resp.StatusCode != http.StatusOK && resp.StatusCode != http.StatusPartialContent {
 		_ = drainAndClose(resp)
-		return nil, errors.Errorf("unexpected HTTP response (%v): %v", resp.StatusCode, resp.Status)
+		return nil, &restError{h, resp.StatusCode, resp.Status}
+	}
+
+	if length > 0 && resp.ContentLength != int64(length) {
+		return nil, &restError{h, http.StatusRequestedRangeNotSatisfiable, "partial out of bounds read"}
 	}
 
 	return resp.Body, nil
@@ -251,12 +269,8 @@ func (b *Backend) Stat(ctx context.Context, h backend.Handle) (backend.FileInfo,
 		return backend.FileInfo{}, err
 	}
 
-	if resp.StatusCode == http.StatusNotFound {
-		return backend.FileInfo{}, &notExistError{h}
-	}
-
 	if resp.StatusCode != http.StatusOK {
-		return backend.FileInfo{}, errors.Errorf("unexpected HTTP response (%v): %v", resp.StatusCode, resp.Status)
+		return backend.FileInfo{}, &restError{h, resp.StatusCode, resp.Status}
 	}
 
 	if resp.ContentLength < 0 {
@@ -288,12 +302,8 @@ func (b *Backend) Remove(ctx context.Context, h backend.Handle) error {
 		return err
 	}
 
-	if resp.StatusCode == http.StatusNotFound {
-		return &notExistError{h}
-	}
-
 	if resp.StatusCode != http.StatusOK {
-		return errors.Errorf("blob not removed, server response: %v (%v)", resp.Status, resp.StatusCode)
+		return &restError{h, resp.StatusCode, resp.Status}
 	}
 
 	return nil
@@ -330,7 +340,7 @@ func (b *Backend) List(ctx context.Context, t backend.FileType, fn func(backend.
 
 	if resp.StatusCode != http.StatusOK {
 		_ = drainAndClose(resp)
-		return errors.Errorf("List failed, server response: %v (%v)", resp.Status, resp.StatusCode)
+		return &restError{backend.Handle{Type: t}, resp.StatusCode, resp.Status}
 	}
 
 	if resp.Header.Get("Content-Type") == ContentTypeV2 {