golang/go · error

error reading preprocessed profile: %w

Error message

error reading preprocessed profile: %w

What it means

In FromSerialized, the very first scanner.Scan() returned false (no token) AND scanner.Err() is non-nil — i.e. an I/O or scanner error occurred while reading the header line, distinct from a clean empty file (1276) or a wrong header (1277).

Source

Thrown at src/cmd/internal/pgo/deserialize.go:40

		// Empty file.
		return false, nil
	} else if err != nil {
		return false, fmt.Errorf("error reading profile header: %w", err)
	}

	return string(hdr) == serializationHeader, nil
}

// FromSerialized parses a profile from serialization output of Profile.WriteTo.
func FromSerialized(r io.Reader) (*Profile, error) {
	d := emptyProfile()

	scanner := bufio.NewScanner(r)
	scanner.Split(bufio.ScanLines)

	if !scanner.Scan() {
		if err := scanner.Err(); err != nil {
			return nil, fmt.Errorf("error reading preprocessed profile: %w", err)
		}
		return nil, fmt.Errorf("preprocessed profile missing header")
	}
	if gotHdr := scanner.Text() + "\n"; gotHdr != serializationHeader {
		return nil, fmt.Errorf("preprocessed profile malformed header; got %q want %q", gotHdr, serializationHeader)
	}

	for scanner.Scan() {
		readStr := scanner.Text()

		callerName := readStr

		if !scanner.Scan() {
			if err := scanner.Err(); err != nil {
				return nil, fmt.Errorf("error reading preprocessed profile: %w", err)
			}
			return nil, fmt.Errorf("preprocessed profile entry missing callee")
		}

View on GitHub (pinned to b6b368adc5)

Solutions

1. Regenerate the profile with Profile.WriteTo to ensure it is well-formed.
2. If lines may be long, supply a scanner with a larger buffer.
3. Verify the file is complete and not truncated.

// Confirm non-empty and well-formed before FromSerialized.
// fi, err := os.Stat(path)
// if err != nil { return err }
// if fi.Size() == 0 { return errors.New("profile empty") }

// prof, err := pgo.FromSerialized(r)
// if err != nil && strings.Contains(err.Error(), "error reading preprocessed profile") {
//     // I/O error on header; regenerate the profile
//     return err
// }

When it happens

Trigger: bufio.Scanner returns an error on the first Scan call against the profile stream — e.g. a line longer than the scanner buffer, or an underlying read error.

Common situations: A profile with an extremely long header line exceeding the default scanner token limit, or a corrupted/partially-flushed profile file.

Related errors

• error reading profile header: %w
• error opening target for caching: %w
• error adding target to cache: %w
• preprocessed profile missing header
• preprocessed profile malformed header; got %q want %q

AI-assisted analysis of golang/go@b6b368adc5 (2026-08-12). Data as JSON: /api/errors/6fa7558f6575fbfb. Report an issue: GitHub.