Fix more spurious comsub execution in tab completion (re: 7a2d3564)

Comsubs were either executed or caused a syntax error when attempting
to complete them within single quotes. Since single quotes do not
expand anything, no such completion should take place.

$ '`/de<TAB>-ksh: /dev/: cannot execute [Is a directory]

$ '$(/de<TAB>-ksh: syntax error: `end of file' unexpected

src/cmd/ksh93/edit/completion.c:
- find_begin():
  - Remove recursive handling for '`' comsubs from 7a2d3564; it is
    sufficient to set the return pointer to the current location cp
    (the character following '`') if we're not in single quotes.
  - For '$' and '`', if we are within single quotes, set type '\''
    and set the return pointer bp to the location of the '$' or
    '`'.
- ed_expand(): If find_begin() sets type '\'' and the current begin
  character is $ or `, refuse to attempt completion; return -1 to
  cause a terminal beep.

Related:
https://github.com/ksh93/ksh/issues/268
https://github.com/ksh93/ksh/issues/462#issuecomment-1038482307

NEWS

@@ -3,6 +3,12 @@ For full details, see the git log at: https://github.com/ksh93/ksh/tree/1.0
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2022-06-05:
+
+- Fixed a bug where tab completion would spuriously execute a command
+  substitution after entering: '`something<TAB> or generate a spurious
+  'end of file unexpected' syntax error after entering: '$(something<TAB>.
+
 2022-06-04:
 
 - Added a new --functrace long-form shell option which causes the -x/--xtrace

src/cmd/ksh93/edit/completion.c

@@ -135,7 +135,11 @@ static char *find_begin(char outbuff[], char *last, int endchar, int *type)
 			break;
 		    case '$':
 			if(inquote == '\'')
+			{
+				*type = '\'';
+				bp = xp;
 				break;
+			}
 			c = *(unsigned char*)cp;
 			if(mode!='*' && (isaletter(c) || c=='{'))
 			{
@@ -176,11 +180,12 @@ static char *find_begin(char outbuff[], char *last, int endchar, int *type)
 			break;
 		    case '`':
 			if(inquote=='\'')
-				break;
+			{
-			*type = mode;
+				*type = '\'';
-			xp = find_begin(cp,last,'`',type);
-			if(*(cp=xp)!='`')
 				bp = xp;
+			}
+			else
+				bp = cp;
 			break;
 		    case '=':
 			if(!inquote)
@@ -287,9 +292,15 @@ int ed_expand(Edit_t *ep, char outbuff[],int *cur,int *eol,int mode, int count)
 		c =  *(unsigned char*)out;
 		var = mode;
 		begin = out = find_begin(outbuff,last,0,&var);
-		/* addstar set to zero if * should not be added */
+		if(var=='\'' && (*begin=='$' || *begin=='`'))
-		if(var=='$')
 		{
+			/* avoid spurious expansion or comsub execution within '...' */
+			rval = -1;
+			goto done;
+		}
+		else if(var=='$')
+		{
+			/* expand ${!varname@} to complete variable name(s) */
 			stakputs("${!");
 			stakwrite(out,last-out);
 			stakputs("@}");
@@ -297,6 +308,7 @@ int ed_expand(Edit_t *ep, char outbuff[],int *cur,int *eol,int mode, int count)
 		}
 		else
 		{
+			/* addstar set to zero if * should not be added */
 			addstar = '*';
 			while(out < last)
 			{

src/cmd/ksh93/include/version.h

@@ -21,7 +21,7 @@
 
 #define SH_RELEASE_FORK	"93u+m"		/* only change if you develop a new ksh93 fork */
 #define SH_RELEASE_SVER	"1.0.0-beta.2"	/* semantic version number: https://semver.org */
-#define SH_RELEASE_DATE	"2022-06-04"	/* must be in this format for $((.sh.version)) */
+#define SH_RELEASE_DATE	"2022-06-05"	/* must be in this format for $((.sh.version)) */
 #define SH_RELEASE_CPYR	"(c) 2020-2022 Contributors to ksh " SH_RELEASE_FORK
 
 /* Scripts sometimes field-split ${.sh.version}, so don't change amount of whitespace. */

src/cmd/ksh93/tests/pty.sh

@@ -831,6 +831,7 @@ r ^:test-1: test \$\{.sh.level\}\r\n$
 ((SHOPT_VSH || SHOPT_ESH)) && tst $LINENO <<"!"
 L tab completion executes command substitutions
 # https://github.com/ksh93/ksh/issues/268
+# https://github.com/ksh93/ksh/issues/462#issuecomment-1038482307
 
 d 15
 p :test-1:
@@ -839,6 +840,20 @@ r ^:test-1: \$\(echo true\)\r\n$
 p :test-2:
 w `echo true`\t
 r ^:test-2: `echo true`\r\n$
+p :test-3:
+w '`/dev\t
+r ^:test-3: '`/dev[[:blank:]]*\r\n$
+# escape from PS2 prompt with Ctrl+C
+r ^> $
+c \cC
+p :test-4:
+w '$(/dev\t
+r ^:test-4: '\$\(/dev[[:blank:]]*\r\n$
+r ^> $
+c \cC
+p :test-5:
+w $'`/dev\t
+r ^:test-5: \$'`/dev[[:blank:]]*\r\n$
 !
 
 ((SHOPT_ESH)) && VISUAL=emacs tst $LINENO <<"!"