Fix completion following $'foo\'bar'

On an interactive shell in emacs or vi, type a command with a $'…'
quoted string that contains a backslash-escaped single quote, like:

    $ true $'foo\'bar' ▁

Then begin to type the name of a file present in the current
working directory and press tab. Nothing happens as completion
fails to work.

The completion code does not recognise $'…' strings. Instead, it
parses them as '…' strings in which there are no backslash escapes,
so it considers the last ' to open a second quoted string which is
not terminated.

Plus, when replacing a $'…' string with a (backslash-escaped)
completed string, the initial '$' is not replaced:

    $ $'/etc/hosts<Tab>
    $ $/etc/hosts

src/cmd/ksh93/edit/completion.c:
- find_begin():
  - Learn how to recognise $'…' strings. A new local dollarquote
    flag variable is used to distinguish them from regular '…'
    strings. The difference is that backslash escapes (and only
    those) should be recognised as in "…".
  - Set a special type -1 for $'…' as the caller will need a way
    to distinguish those from '…'.
- ed_expand(): When replacing a quoted string, remove an extra
  initial character (being the $ in $') if the type set by
  find_begin() is -1.

Resolves: https://github.com/ksh93/ksh/issues/462

NEWS

@@ -9,6 +9,9 @@ Any uppercase BUG_* names are modernish shell bug IDs.
   substitution after entering: '`something<TAB> or generate a spurious
   'end of file unexpected' syntax error after entering: '$(something<TAB>.
 
+- Fixed bug where tab completion stopped working for arguments following a
+  quoted string with an escaped backslash in the form $'foo\'bar'.
+
 2022-06-04:
 
 - Added a new --functrace long-form shell option which causes the -x/--xtrace

src/cmd/ksh93/edit/completion.c

@@ -105,12 +105,17 @@ static char *overlaid(register char *str,register const char *newstr,int nocase)
 
 /*
  * returns pointer to beginning of expansion and sets type of expansion
+ *
+ * Detects variable expansions, command substitutions, and three quoting styles:
+ * 1. '...'	inquote=='\'', dollarquote==0; no special characters
+ * 2. $'...'	inquote=='\'', dollarquote==1; skips \.
+ * 3. "..."	inquote=='"',  dollarquote==0; skips \., $..., ${...}, $(...), `...`
  */
 static char *find_begin(char outbuff[], char *last, int endchar, int *type)
 {
 	register char	*cp=outbuff, *bp, *xp;
-	register int 	c,inquote = 0, inassign=0;
-	int		mode=*type;
+	char		inquote = 0, dollarquote = 0, inassign = 0;
+	int		mode=*type, c;
 	bp = outbuff;
 	*type = 0;
 	mbinit();
@@ -127,10 +132,10 @@ static char *find_begin(char outbuff[], char *last, int endchar, int *type)
 				break;
 			}
 			if(inquote==c)
-				inquote = 0;
+				inquote = dollarquote = 0;
 			break;
 		    case '\\':
-			if(inquote != '\'')
+			if(inquote != '\'' || dollarquote)
 				mbchar(cp);
 			break;
 		    case '$':
@@ -177,6 +182,8 @@ static char *find_begin(char outbuff[], char *last, int endchar, int *type)
 				if(*(cp=xp)!=')')
 					bp = xp;
 			}
+			else if(c=='\'' && !inquote)
+				dollarquote = 1;
 			break;
 		    case '`':
 			if(inquote=='\'')
@@ -214,7 +221,11 @@ static char *find_begin(char outbuff[], char *last, int endchar, int *type)
 		}
 	}
 	if(inquote && *bp==inquote)
-		*type = *bp++;
+	{
+		/* set special type -1 for $'...' */
+		*type = dollarquote ? -1 : inquote;
+		bp++;
+	}
 	return(bp);
 }
 
@@ -360,7 +371,11 @@ int ed_expand(Edit_t *ep, char outbuff[],int *cur,int *eol,int mode, int count)
 			com = sh_argbuild(&narg,comptr,0);
 			/* special handling for leading quotes */
 			if(begin>outbuff && (begin[-1]=='"' || begin[-1]=='\''))
-			begin--;
+			{
+				begin--;
+				if(var == -1)		/* $'...' */
+					begin--;	/* also remove initial dollar */
+			}
 		}
 		sh_offstate(SH_COMPLETE);
                 /* allow a search to be aborted */

src/cmd/ksh93/tests/pty.sh

@@ -821,11 +821,12 @@ r \thist -lnN 1\r\n$
 ((SHOPT_VSH || SHOPT_ESH)) && tst $LINENO <<"!"
 L tab completion while expanding ${.sh.*} variables
 # https://github.com/att/ast/issues/1461
+# also tests $'...' string: https://github.com/ksh93/ksh/issues/462
 
 d 15
 p :test-1:
-w test \$\{.sh.level\t
-r ^:test-1: test \$\{.sh.level\}\r\n$
+w test \$'foo\\'bar' \$\{.sh.level\t
+r ^:test-1: test \$'foo\\'bar' \$\{.sh.level\}\r\n$
 !
 
 ((SHOPT_VSH || SHOPT_ESH)) && tst $LINENO <<"!"