Go function to intercept escape sequences to set the terminal title.

interceptsl.go

package main

// interceptsl intercepts escape sequences to set the terminal title.
func interceptsl(ctx context.Context, w io.Writer, r io.Reader, title chan<- string) error {
	state := 0
	ibuf := make([]byte, 256)
	tbuf := make([]byte, 256)
	obuf := make([]byte, 256) // will use up to cap(ibuf) + cap(tbuf) + 4 in certain situations

	for {
		select {
		case <-ctx.Done():
			return ctx.Err()
		default:
		}

		// note: Go will handle EAGAIN, EINTR, and polling internally while reading files
		// note: for a pty, VMIN=0 and VTIME=1 seems to work well (EAGAIN will be returned when nothing is available)
		// note: for writes, we don't need to check the count since Go io.Writers are supposed to always do full writes

		n, err := r.Read(ibuf)
		if err != nil {
			return err
		} else if n == 0 {
			continue
		}

		// fast path when no escape sequences in the buffer
		if state == 0 {
			for _, c := range ibuf[:n] {
				if c == 0x1B {
					goto slowpath
				}
			}
			_, _ = w.Write(ibuf[:n])
			continue
		}

	slowpath:
		obuf = obuf[:0]
		for _, c := range ibuf[:n] {
			switch state {
			case 0: // normal output
				switch c {
				default:
					state = 0
					obuf = append(obuf, c)
				case 0x1B:
					state = 1
				}
			case 1: // at \x1B
				switch c {
				default:
					state = 0
					obuf = append(obuf, 0x1B, c)
				case ']':
					state = 2
				}
			case 2: // at \x1B]
				switch c {
				default:
					state = 0
					obuf = append(obuf, 0x1B, ']', c)
				case '0':
					state = 3
				}
			case 3: // at \x1B]0
				switch c {
				default:
					state = 0
					obuf = append(obuf, 0x1B, ']', '0', c)
				case ';':
					state = 4
					tbuf = tbuf[:0]
				}
			case 4: // in \x1B]0;
				switch c {
				default:
					// next title char
					if len(tbuf) < cap(tbuf) {
						tbuf = append(tbuf, c)
						break // out of the switch
					}
					fallthrough
				case 0x07:
					// end of title || overflow
					select {
					case title <- string(tbuf):
					default:
					}
					if len(tbuf) == cap(tbuf) {
						state = 5
					} else {
						state = 0
					}
					tbuf = tbuf[:0]
				case 0x1B:
					// start of a new escape sequence (this shouldn't happen)
					state = 1
				}
			case 5: // in title
				switch c {
				default:
					// overflowing title character
				case 0x07:
					// end of the overflowing title
					state = 0
				case 0x1B:
					// start of a new escape sequence (this shouldn't happen)
					state = 1
				}
			}
		}
		_, _ = w.Write(obuf)
	}
}