Function canonicalMIMEHeaderKey

textproto_reader.go:380–427

canonicalMIMEHeaderKey is like CanonicalMIMEHeaderKey but is allowed to mutate the provided byte slice before returning the string. For invalid inputs (if a contains spaces or non-token bytes), a is unchanged and a string copy is returned. ok is true if the header key contains only valid character

(a []byte)

// ReadMIMEHeader accepts header keys containing spaces, but does not
// canonicalize them.
func canonicalMIMEHeaderKey(a []byte) (_ string, ok bool) {
	if len(a) == 0 {
		return "", false
	}

	// See if a looks like a header key. If not, return it unchanged.
	noCanon := false
	for _, c := range a {
		if validHeaderFieldByte(c) {
			continue
		}
		// Don't canonicalize.
		if c == ' ' {
			// We accept invalid headers with a space before the
			// colon, but must not canonicalize them.
			// See https://go.dev/issue/34540.
			noCanon = true
			continue
		}
		return string(a), false
	}
	if noCanon {
		return string(a), true
	}

	upper := true
	for i, c := range a {
		// Canonicalize: first letter upper case
		// and upper case after each dash.
		// (Host, User-Agent, If-Modified-Since).
		// MIME headers are ASCII only, so no Unicode issues.
		if upper && 'a' <= c && c <= 'z' {
			c -= toLower
		} else if !upper && 'A' <= c && c <= 'Z' {
			c += toLower
		}
		a[i] = c
		upper = c == '-' // for next time
	}
	commonHeaderOnce.Do(initCommonHeader)
	// The compiler recognizes m[string(byteSlice)] as a special
	// case, so a copy of a's bytes into a new string does not
	// happen in this map lookup:
	if v := commonHeader[string(a)]; v != "" {
		return v, true
	}
	return string(a), true
}

// validHeaderValueByte reports whether c is a valid byte in a header
// field value. RFC 7230 says: