Function parseRegistryWarningHeader

docker/docker_client.go:653–693

parseRegistryWarningHeader parses a Warning: header per RFC 7234, limited to the warning values allowed by opencontainers/distribution-spec. It returns the warning string if the header has the expected format, or "" otherwise.

(header string)

// values allowed by opencontainers/distribution-spec.
// It returns the warning string if the header has the expected format, or "" otherwise.
func parseRegistryWarningHeader(header string) string {
	const expectedPrefix = `299 - "`
	const expectedSuffix = `"`

	// warning-value = warn-code SP warn-agent SP warn-text	[ SP warn-date ]
	// distribution-spec requires warn-code=299, warn-agent="-", warn-date missing
	header, ok := strings.CutPrefix(header, expectedPrefix)
	if !ok {
		return ""
	}
	header, ok = strings.CutSuffix(header, expectedSuffix)
	if !ok {
		return ""
	}

	// ”Recipients that process the value of a quoted-string MUST handle a quoted-pair
	// as if it were replaced by the octet following the backslash.”, so let’s do that…
	res := strings.Builder{}
	afterBackslash := false
	for _, c := range []byte(header) { // []byte because escaping is defined in terms of bytes, not Unicode code points
		switch {
		case c == 0x7F || (c < ' ' && c != '\t'):
			return "" // Control characters are forbidden
		case afterBackslash:
			res.WriteByte(c)
			afterBackslash = false
		case c == '"':
			// This terminates the warn-text and warn-date, forbidden by distribution-spec, follows,
			// or completely invalid input.
			return ""
		case c == '\\':
			afterBackslash = true
		default:
			res.WriteByte(c)
		}
	}
	if afterBackslash {
		return ""
	}
	return res.String()
}

// we're using the challenges from the /v2/ ping response and not the one from the destination
// URL in this request because: