Handle forward-delete key in emacs and vi editors

On every modern system, the forward-delete key on PC/Mac keyboards
generates the VT220 sequence ESC [ 3 ~. Every other shell with an
editor handles this now, ksh93 seems to be the last not to.

src/cmd/ksh93/edit/emacs.c: escape():
- Handle the ^[[3 as part of normal escape processing, then read an
  extra character to check for the final '~'. If detected, insert
  an ERASECHAR key event.

src/cmd/ksh93/edit/vi.c: mvcursor():
- Replace the ^[[3~ sequence by an 'x' command. We have to
  special-case its processing, because vi mode parses numbers as
  repetition operators. The escape sequence contains a number,
  making it incompatible with normal command handling. This means
  number repetitions don't work with the forward-delete key. If
  that annoys anyone enough to fix it, a patch would be welcome.
  For now, it will do to make the forward-delete key stop
  exhibiting bizarre behaviour (beep + change case + move forward).

src/cmd/ksh93/sh.1
- Copy-edit emacs documentation for VT220-style sequences; map them
  to their actual key, otherwise it's meaningless to the reader.
- Document the new forward-delete key behaviour for emacs mode.
- Leave the forward-delete key for vi mode undocumented for now, as
  repetitions don't work, so it doesn't really match the vi canon.
  (OTOH, it doesn't work in vim, either...)

NEWS

@@ -8,6 +8,8 @@ Any uppercase BUG_* names are modernish shell bug IDs.
 - Corrected rounding of floating point values by ksh's printf %f formatting
   operator. Fix contributed by @hyenias.
 
+- The forward-delete key now works as expected in emacs and vi editing modes.
+
 2020-09-11:
 
 - The 'command' regular builtin utility (which runs a simple command, removing

src/cmd/ksh93/edit/emacs.c

@@ -1109,6 +1109,19 @@ static int escape(register Emacs_t* ep,register genchar *out,int count)
 			    case 'Y':
 				ed_ungetchar(ep->ed,cntl('E'));
 				return(-1);
+			    case '3':
+				if(ed_getchar(ep->ed,1)=='~')
+				{	/*
+					 * VT220 forward-delete key.
+					 * Since ERASECHAR and EOFCHAR are usually both mapped to ^D, we
+					 * should only issue ERASECHAR if there is something to delete,
+					 * otherwise forward-delete on empty line will terminate the shell.
+					 */
+					if(cur < eol)
+						ed_ungetchar(ep->ed,ERASECHAR);
+					return(-1);
+				}
+				/* FALLTHROUGH */
 			    default:
 				ed_ungetchar(ep->ed,i);
 			}

src/cmd/ksh93/edit/vi.c

@@ -1606,6 +1606,7 @@ static int mvcursor(register Vi_t* vp,register int motion)
 	register int tcur_virt;
 	register int incr = -1;
 	register int bound = 0;
+	int c;
 
 	switch(motion)
 	{
@@ -1632,7 +1633,14 @@ static int mvcursor(register Vi_t* vp,register int motion)
 		break;
 
 	case '[':
-		switch(motion=getcount(vp,ed_getchar(vp->ed,-1)))
+		c = ed_getchar(vp->ed,-1);
+		if(c=='3' && ed_getchar(vp->ed,-1)=='~')
+		{
+			/* VT220 forward-delete key */
+			ed_ungetchar(vp->ed,'x');
+			return(1);
+		}
+		switch(motion=getcount(vp,c))
 		{
 		    case 'A':
 #if SHOPT_EDPREDICT

src/cmd/ksh93/sh.1

@@ -4597,6 +4597,11 @@ operate from any place on the line
 Neither the `RETURN' nor the `LINE FEED' key is
 entered after edit commands except when noted.
 .PP
+The \fBM-[\fR multi-character commands below are DEC VT220 escape sequences
+generated by keys on modern PC/Mac keyboards, such as the arrow keys.
+You could type them directly but they are meant to recognize the keys
+in question, which are indicated in parentheses.
+.PP
 .PD 0
 .TP 10
 .BI ^F
@@ -4604,7 +4609,7 @@ Move cursor forward (right) one character.
 .PP
 .TP 10
 .BI M-[C
-Move cursor forward (right) one character.
+(Right arrow) Same as \fB^F\fR.
 .PP
 .TP 10
 .BI M-f
@@ -4620,7 +4625,7 @@ Move cursor backward (left) one character.
 .PP
 .TP 10
 .BI M-[D
-Move cursor backward (left) one character.
+(Left arrow) Same as \fB^B\fR.
 .PP
 .TP 10
 .BI M-b
@@ -4632,15 +4637,19 @@ Move cursor to start of line.
 .PP
 .TP 10
 .BI M-[H
-Move cursor to start of line.
+(Home) Same as \fB^A\fR.
 .PP
 .TP 10
 .BI ^E
 Move cursor to end of line.
 .PP
 .TP 10
+.BI M-[F
+(End) Same as \fB^E\fR.
+.PP
+.TP 10
 .BI M-[Y
-Move cursor to end of line.
+Same as \fB^E\fR.
 .PP
 .TP 10
 .BI ^] char
@@ -4664,9 +4673,7 @@ Interchange the cursor and mark.
 by the
 .IR stty (1)
 command, usually
-.B ^H
-or
-.BR # .)
+.B ^H .)
 Delete previous character.
 .PP
 .TP 10
@@ -4686,6 +4693,10 @@ editing features (if any).
 Delete current character.
 .PP
 .TP 10
+.BI M-[3~
+(Forward delete) Same as \fB^D\fR.
+.PP
+.TP 10
 .BI M-d
 Delete current word.
 .PP
@@ -4748,15 +4759,16 @@ Push the region from the cursor to the mark on the stack.
 .I kill
 (User defined kill character as defined
 by the stty command, usually
-.B ^G
-or
-.BR @ .)
+.B ^U .)
 Kill the entire current line.
 If two
 .I kill
 characters are entered in succession, all
 kill characters from then on cause a line feed
 (useful when using paper terminals).
+A subsequent pair of
+.I kill
+characters undoes this change.
 .PP
 .TP 10
 .BI ^Y
@@ -4805,6 +4817,7 @@ Moves back one line when not on the first line of a multi-line command.
 .PP
 .TP 10
 .BI M-[A
+(Up arrow)
 If the cursor is at the end of the line, it is equivalent to
 .B ^R
 with
@@ -4812,7 +4825,7 @@ with
 set to the contents of the current line.
 Otherwise, it is
 equivalent to
-.BR ^P.
+.BR ^P .
 .PP
 .TP 10
 .BI M-<
@@ -4832,8 +4845,9 @@ the next command line forward in time is accessed.
 .PP
 .TP 10
 .BI M-[B
+(Down arrow)
 Equivalent to
-.BR ^N.
+.BR ^N .
 .PP
 .TP 10
 .BI ^R string