read -S now correctly handles nested double quotes

Prior to this bugfix, the following set of commands would
fail to print two double quotes:

IFS=',' read -S a b c <<<'foo,"""title"" data",bar'
echo $b

This fix is from ksh93v- 2013-10-10-alpha, although it has
been revised to use stakputc to put the required double quote
into the buffer for consistency with the ksh93u+ codebase.

src/cmd/ksh93/bltins/read.c:
 - When handling nested double quotes, put the required double
   quote in read's buffer with stakputc.

src/cmd/ksh93/tests/builtins.sh:
 - Add the regression test for `read -S` from ksh93v-.

NEWS

@@ -3,6 +3,10 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2020-06-14:
+- 'read -S' is now able to correctly handle strings with double quotes
+  nested inside of double quotes.
+
 2020-06-13:
 
 - Fixed a timezone name determination bug on FreeBSD that caused the

src/cmd/ksh93/bltins/read.c

@@ -566,13 +566,21 @@ int sh_readline(register Shell_t *shp,char **names, volatile int fd, int flags,s
 #endif /*SHOPT_MULTIBYTE */
 		    case S_QUOTE:
 			c = shp->ifstable[*cp++];
-			inquote = !inquote;
+			if(inquote && c==S_QUOTE)
+				c = -1;
+			else
+				inquote = !inquote;
 			if(val)
 			{
 				stakputs(val);
 				use_stak = 1;
 				*val = 0;
 			}
+			if(c==-1)
+			{
+				stakputc('"');
+				c = shp->ifstable[*cp++];
+			}
 			continue;
 		    case S_ESC:
 			/* process escape character */

src/cmd/ksh93/tests/builtins.sh

@@ -692,5 +692,10 @@ EOF
 PATH=/dev/null
 whence -q export) || err_exit '`builtin -d` deletes special builtins'
 
+# ======
+# `read -S` should handle double quotes correctly
+IFS=',' read -S a b c <<<'foo,"""title"" data",bar'
+[[ $b == '"title" data' ]] || err_exit '"" inside "" not handled correctly with read -S'
+
 # ======
 exit $((Errors<125?Errors:125))