Created attachment 366254 [details] [review]
Proposed patch (against master, but also applies to 3.22.26)

I tried to add some custom compose sequence to my ~/.config/gtk-3.0/Compose,
e.g. the line

<Multi_key> <less> <minus> : "\20620" #LEFTWARDS ARROW

but Gtk reports the following error

GTK+ supports to output one char only: "\20620" #LEFTWARDS ARROW: <Multi_key> <less> <minus> : "\20620" #LEFTWARDS ARROW

The parsing is done in parse_compose_value() in gtk/gtkcomposetable.c
and it get's it wrong for \-escaped (octal) strings: the check

if (g_utf8_get_char (g_utf8_next_char (words[1])) > 0)
  g_warning ("GTK+ supports to output one char only: %s: %s", val, line);

is also applied when the string is escaped (where it is trivially true).

Parsing uses a lot of implicit state (e.g. for the handling of
compose_data->comment), is sometimes funny

     /* The escaped octal */
     else if (uch >= '0' && uch <= '8')     <--- '8' in an octal number?

and doesn't cover escaped hexadecimal strings like the original from libX11.

The syntax for the target string in libX11 modules/im/ximcp/imLcPrs.c is:

      STRING        ::= '"' { CHAR } '"'
      CHAR          ::= GRAPHIC_CHAR | ESCAPED_CHAR
      GRAPHIC_CHAR  ::= locale (codeset) dependent code
      ESCAPED_CHAR  ::= ('\\' | '\"' | OCTAL | HEX )
      OCTAL         ::= '\' OCTAL_CHAR [OCTAL_CHAR [OCTAL_CHAR]]
      OCTAL_CHAR    ::= (0|1|2|3|4|5|6|7)
      HEX           ::= '\' (x|X) HEX_CHAR [HEX_CHAR]]
      HEX_CHAR      ::= (0|1|2|3|4|5|6|7|8|9|A|B|C|D|E|F|a|b|c|d|e|f)

Patch attached that fixes the original problem and parses escaped hexadecimal strings, too. The problem is still present in Git master.

Of course, the suggestion in #773919 would be even better.