dutchcoders/transfer.sh · error

Could not encode metadata

Error message

Could not encode metadata

What it means

postHandler builds a metadata struct (content type, length, download limits, deletion token, encryption flag) and encodes it as JSON into a buffer before storing it as `<filename>.metadata`. If json.Encoder.Encode returns an error, the server responds with 500 and this message. In practice this is nearly unreachable: the metadata struct contains only JSON-serializable types, so the encoder essentially never fails.

Source

Thrown at server/handlers.go:522

				if err != nil {
					s.logger.Printf("%s", err.Error())
					http.Error(w, "Could not perform prescan", http.StatusInternalServerError)
					return
				}

				if status != clamavScanStatusOK {
					s.logger.Printf("prescan positive: %s", status)
					http.Error(w, "Clamav prescan found a virus", http.StatusPreconditionFailed)
					return
				}
			}

			metadata := metadataForRequest(contentType, contentLength, s.randomTokenLength, r)

			buffer := &bytes.Buffer{}
			if err := json.NewEncoder(buffer).Encode(metadata); err != nil {
				s.logger.Printf("%s", err.Error())
				http.Error(w, "Could not encode metadata", http.StatusInternalServerError)

				return
			} else if err := s.storage.Put(r.Context(), token, fmt.Sprintf("%s.metadata", filename), buffer, "text/json", uint64(buffer.Len())); err != nil {
				s.logger.Printf("%s", err.Error())
				http.Error(w, "Could not save metadata", http.StatusInternalServerError)

				return
			}

			s.logger.Printf("Uploading %s %s %d %s", token, filename, contentLength, contentType)

			reader, err := attachEncryptionReader(file, r.Header.Get("X-Encrypt-Password"))
			if err != nil {
				http.Error(w, "Could not crypt file", http.StatusInternalServerError)
				return
			}

			if err = s.storage.Put(r.Context(), token, filename, reader, contentType, uint64(contentLength)); err != nil {

View on GitHub (pinned to c37bfd9579)

Solutions

1. Inspect the server log printed just before the response; it contains the underlying encoding error.
2. If you modified the metadata struct, remove or fix the unserializable field (add a MarshalJSON or drop the field).
3. Upgrade to a stock release of the server where this path is effectively dead code.
4. Retry the upload — no client-side state is corrupted, nothing was written to storage.

Example fix

// before: unserializable field added to metadata
//   Files []os.File
// after: store only serializable data
//   FileNames []string

resp, err := http.Post(url, mime, body)
if err == nil && resp.StatusCode == http.StatusInternalServerError &&
    strings.Contains(readBody(resp), "Could not encode metadata") {
    return fmt.Errorf("server-side metadata encoding bug; report to operator")
}

When it happens

Trigger: json.NewEncoder(buffer).Encode(metadata) returns an error — theoretically only if a value in the metadata struct becomes unserializable (e.g. a custom type with a broken MarshalJSON); not triggerable by any client request input.

Common situations: Seen only when the code has been modified to put unsupported types (channels, funcs, invalid UTF-8 handled by custom marshalers) into the metadata struct; stock deployments never produce this.

Related errors

• could not encode metadata
• Clamav prescan found a virus
• Could not save metadata
• Could not crypt file
• maxDownloads expired

AI-assisted analysis of dutchcoders/transfer.sh@c37bfd9579 (2026-09-05). Data as JSON: /api/errors/2ef94b4d091c9274. Report an issue: GitHub.