Subject: Re: bug#33403: [Geiser-users] Data length limit in Guile/Geiser/Scheme evaluation

From: Mark H Weaver <[email-masked]>
Date: Fri, 16 Nov 2018 02:13:40 -0500

Mark H Weaver <[email-masked]> writes:

> If, after pasting this, you type another close quote, 5 close parens,
> and then repaste the last two lines, it will print the garbled input and
> return to a prompt.

Actually, instead of pasting the last two lines as-is, I replaced
"(length classification)" with "classification", so that instead of
printing the length, it prints the actual s-exp.  Then you can see what
happened to that final string literal.

> Anyway, to make a long story short, after some debugging, I found that
> precisely the same truncation of the first line happens when using 'cat'
> from GNU coreutils.  Simply type 'cat' and paste the same text, and
> you'll see that in the output, only the first 4095 bytes of the first
> line were retained.
>
> So, I'm not sure where the problem is, but it's not a problem in Guile.

This is a documented limitation in Linux's terminal handling when in
canonical mode.  See the termios(3) man page, which includes this text:

   Canonical and noncanonical mode
       
       The setting of the ICANON canon flag in c_lflag determines
       whether the terminal is operating in canonical mode (ICANON set)
       or noncanonical mode (ICANON unset).  By default, ICANON is set.

       In canonical mode:

       * Input is made available line by line.  An input line is
         available when one of the line delimiters is typed (NL, EOL,
         EOL2; or EOF at the start of line).  Except in the case of EOF,
         the line delimiter is included in the buffer returned by
         read(2).

       * Line editing is enabled (ERASE, KILL; and if the IEXTEN flag is
         set: WERASE, REPRINT, LNEXT).  A read(2) returns at most one
         line of input; if the read(2) requested fewer bytes than are
         available in the current line of input, then only as many bytes
         as requested are read, and the remaining characters will be
         available for a future read(2).

       * The maximum line length is 4096 chars (including the
         terminating newline character); lines longer than 4096 chars
         are truncated.  After 4095 characters, input processing (e.g.,
         ISIG and ECHO* processing) continues, but any input data after
         4095 characters up to (but not including) any terminating
         newline is discarded.  This ensures that the terminal can
         always receive more input until at least one line can be read.

Note that last item above.

       Mark