Fix pressing down in the TUI (Re: [RFC 8.3 0/3] Some style fixes)

[Pedro Alves <palves@redhat.com>]

On 03/12/2019 04:44 PM, Eli Zaretskii wrote:
> 1. The first patch fixes the problem noticed by Pedro: pressing DOWN
> arrow in the command window doesn't scroll the source window.  This is
> because we don't initialize the s->nlines field, and then
> tui_vertical_source_scroll thinks we are off the chart.  This fixes
> that:
> 
> --- gdb/source-cache.c~4	2019-03-10 08:34:47.422752400 +0200
> +++ gdb/source-cache.c	2019-03-12 11:50:15.094147600 +0200
> @@ -194,6 +194,12 @@ source_cache::get_source_lines (struct s
>  	  std::ifstream input (fullname);
>  	  if (input.is_open ())
>  	    {
> +	      if (s->line_charpos == 0)
> +		{
> +		  scoped_fd desc = open_source_file (s);
> +		  if (desc.get () >= 0)
> +		    find_source_lines (s, desc.get ());

I think this should return false if open_source_file fails?  I.e., like this:

	      if (s->line_charpos == 0)
		{
		  scoped_fd desc = open_source_file (s);
		  if (desc.get () < 0)
                    return false;
		  find_source_lines (s, desc.get ());

Otherwise this LGTM.  I see get_plain_source_lines has similar
code, so my immediate thought was to move that to a helper function,
but there's a difference that makes that unviable -- get_plain_source_lines
always wants to open the source file first, while here we can avoid
it unless line_charpos is 0.

Thanks,
Pedro Alves