commonHeaderOnce.Do(initCommonHeader)
	for h := range commonHeader {
		if h != CanonicalMIMEHeaderKey(h) {
			t.Errorf("Non-canonical header %q in commonHeader", h)
		}
	}
	b := []byte("content-Length")
	want := "Content-Length"
	n := testing.AllocsPerRun(200, func() {
		if x, _ := canonicalMIMEHeaderKey(b); x != want {
			t.Fatalf("canonicalMIMEHeaderKey(%q) = %q; want %q", b, x, want)
		}
	})
	if n > 0 {

// returned without modifications.
func CanonicalMIMEHeaderKey(s string) string {
	// Quick check for canonical encoding.
	upper := true
	for i := 0; i < len(s); i++ {
		c := s[i]
		if !validHeaderFieldByte(c) {
			return s
		}
		if upper && 'a' <= c && c <= 'z' {
			s, _ = canonicalMIMEHeaderKey([]byte(s))
			return s
		}
		if !upper && 'A' <= c && c <= 'Z' {

		if kv == "" {
			return m, err
		}

		// Key ends at first colon.
		k, v, ok := strings.Cut(kv, ":")
		if !ok {
			return m, errors.New("malformed header line: " + kv)
		}
		key := textproto.CanonicalMIMEHeaderKey(k)

		// Permit empty key, because that is what we did in the past.
		if key == "" {
			continue
		}

		// Skip initial spaces in value.
		value := strings.TrimLeft(v, " \t")

	// users to provide RFC 3339 compliant dates.
	retDate, err = amztime.ISO8601Parse(dateStr)
	if err != nil {
		return rmode, r, ErrInvalidRetentionDate
	}
	_, replReq := h[textproto.CanonicalMIMEHeaderKey(xhttp.MinIOSourceReplicationRequest)]

	t, err := UTCNowNTP()
	if err != nil {
		lockLogIf(context.Background(), err)
		return rmode, r, ErrPastObjectLockRetainDate
	}