Console: provide VT100-like end-of-line print behavior.

On the majority of VT100-like terminals, when you print a
character in the last column, the cursor sits in a kind of
"limbo", as if the cursor position were one column past the
edge of the screen. Thus when a line feed is output next,
the cursor is then at the start of the next line.

The Win32 console write doesn't do this; the cursor position
wraps to the start of the next line. If a newline is put out,
it translates to an extra newline going to the next-next line.
This spoils the behavior of programs which depend on the VT100
behavior.

In this patch, the VT100 behavior is simulated as follows. A
new flag in the dev_console structure is set when a character
is output to the last column. This flag is then observed in
order to discard a newline (or rather any character which is
mapped mapped to the DWN action).

* winsup/cygwin/fhandler.h (class dev_console): New boolean
data member, eat_newline. This indicates that if a character
is output which moves down to the next line, it should be
discarded rather than sent to the console.

* winsup/cygwin/fhandler_console.cc
(fhandler_console::write_normal): Set the eat_newline flag
if the text output ends up at column zero. Honor the
eat_newline flag when processing a DWN character, and clear
it when processing certain other control characters.

2 files changed, 23 insertions, 0 deletions

diff --git a/winsup/cygwin/fhandler.h b/winsup/cygwin/fhandler.h
index 94fe0f9db..21c71d702 100644
--- a/winsup/cygwin/fhandler.h
+++ b/winsup/cygwin/fhandler.h
@@ -1306,6 +1306,7 @@ class dev_console
   bool iso_2022_G1;
   bool alternate_charset_active;
   bool metabit;
+  bool eat_newline;
   char backspace_keycode;
 
   char my_title_buf [TITLESIZE + 1];
diff --git a/winsup/cygwin/fhandler_console.cc b/winsup/cygwin/fhandler_console.cc
index d4c077bc6..a2bd29994 100644
--- a/winsup/cygwin/fhandler_console.cc
+++ b/winsup/cygwin/fhandler_console.cc
@@ -2079,6 +2079,18 @@ do_print:
       if (len >= CONVERT_LIMIT)
 	return found + trunc_buf.len;
     }
+
+  /* If we output something and now x is at the left
+   * column, it means that we just output to the last column.
+   * In this case we can pretend we are still in the original
+   * line and swallow the next character if it is a newline,
+   * or a newline followed by a carriage return.
+   */
+  {
+    int x, y;
+    cursor_get (&x, &y);
+    con.eat_newline = (x == 0);
+  }
   /* If there's still something in the src buffer, but it's not a truncated
      multibyte sequence, then we stumbled over a control character or an
      invalid multibyte sequence.  Print it. */
@@ -2101,6 +2113,12 @@ do_print:
 	  break;
 	case DWN:
 	  cursor_get (&x, &y);
+
+	  if (con.eat_newline) {
+	    con.eat_newline = false;
+	    break;
+	  }
+
 	  if (y >= srBottom)
 	    {
 	      if (y >= con.b.srWindow.Bottom && !con.scroll_region.Top)
@@ -2115,8 +2133,10 @@ do_print:
 	  break;
 	case BAK:
 	  cursor_rel (-1, 0);
+	  con.eat_newline = false;
 	  break;
 	case IGN:
+	  con.eat_newline = false;
 	  cursor_rel (1, 0);
 	  break;
 	case CR:
@@ -2145,10 +2165,12 @@ do_print:
 	    }
 	  break;
 	case TAB:
+	  con.eat_newline = false;
 	  cursor_get (&x, &y);
 	  cursor_set (false, 8 * (x / 8 + 1), y);
 	  break;
 	case NOR:
+	  con.eat_newline = false;
 	  write_replacement_char ();
 	  break;
 	}