Make ~ expand to home directory after unsetting HOME

There was an issue with tilde expansion if the HOME var is unset.

	$ unset HOME
	$ echo ~
	martijn

Only the username is returned. Users are more likely to expect the
current user's home directory as configured in the OS.

POSIXly, the expansion of ~ is based on the value of HOME. If HOME
is unset, the results are unspecified. After unsetting HOME, in
bash, ~ returns the user's home directory as specified by the OS,
whereas in all other shells, ~ expands to the empty string. Only
ksh93 returns the username. The behaviour of bash is more useful.

Discussion:
https://github.com/ksh93/ksh/pull/225#issuecomment-799074107

src/cmd/ksh93/sh/macro.c,
src/cmd/ksh93/tests/tilde.sh:
- sh_tilde(): Backport fix by Mike Gilbert from ksh2020.
  See:	https://github.com/att/ast/issues/1391
	https://github.com/att/ast/pull/1396
	https://github.com/att/ast/commit/070d365d
- Add test.

src/cmd/ksh93/COMPATIBILITY:
- Note this change.

NEWS

@@ -3,6 +3,11 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2021-03-15:
+
+- If the HOME variable is unset, the bare tilde ~ now expands to the current
+  user's system-configured home directory instead of merely the username.
+
 2021-03-13:
 
 - Fixed a file descriptor leak that occurred when ksh used /dev/fd for

src/cmd/ksh93/COMPATIBILITY

@@ -104,6 +104,9 @@ For more details, see the NEWS file and for complete details, see the git log.
 	For example, the following no longer produces an unexpected error:
 	    $ ksh -o nounset -c 'float n; [[ -v n ]] && echo $n'
 
+20.	If the HOME variable is unset, the bare tilde ~ now expands to the
+	current user's system home directory instead of merely the username.
+
 ____________________________________________________________________________
 
 		KSH-93 VS. KSH-88

src/cmd/ksh93/include/version.h

@@ -20,7 +20,7 @@
 
 #define SH_RELEASE_FORK	"93u+m"		/* only change if you develop a new ksh93 fork */
 #define SH_RELEASE_SVER	"1.0.0-alpha"	/* semantic version number: https://semver.org */
-#define SH_RELEASE_DATE	"2021-03-13"	/* must be in this format for $((.sh.version)) */
+#define SH_RELEASE_DATE	"2021-03-15"	/* must be in this format for $((.sh.version)) */
 #define SH_RELEASE_CPYR	"(c) 2020-2021 Contributors to ksh " SH_RELEASE_FORK
 
 /* Scripts sometimes field-split ${.sh.version}, so don't change amount of whitespace. */

src/cmd/ksh93/sh/macro.c

@@ -2692,7 +2692,7 @@ static char *sh_tilde(Shell_t *shp,register const char *string)
 {
 	register char		*cp;
 	register int		c;
-	register struct passwd	*pw;
+	register struct passwd	*pw = NIL(struct passwd*);
 	register Namval_t *np=0;
 	unsigned int save;
 	static Dt_t *logins_tree;
@@ -2700,9 +2700,13 @@ static char *sh_tilde(Shell_t *shp,register const char *string)
 		return(NIL(char*));
 	if((c = *string)==0)
 	{
-		if(!(cp=nv_getval(sh_scoped(shp,HOME))))
-			cp = getlogin();
-		return(cp);
+		static char	*username;
+		if(cp = nv_getval(sh_scoped(shp, HOME)))
+			return(cp);
+		/* Fallback for unset HOME: get username and treat ~ like ~username */
+		if(!username && !((pw = getpwuid(getuid())) && (username = sh_strdup(pw->pw_name))))
+			return(NIL(char*));
+		string = username;
 	}
 	if((c=='-' || c=='+') && string[1]==0)
 	{
@@ -2741,7 +2745,7 @@ static char *sh_tilde(Shell_t *shp,register const char *string)
 #endif /* _WINIX */
 	if(logins_tree && (np=nv_search(string,logins_tree,0)))
 		return(nv_getval(np));
-	if(!(pw = getpwnam(string)))
+	if(!pw && !(pw = getpwnam(string)))
 		return(NIL(char*));
 #if _WINIX
 skip:

src/cmd/ksh93/tests/tilde.sh

@@ -87,4 +87,12 @@ chmod +x $tmp/tilde
 nl=$'\n'
 [[ $($tmp/tilde foo) == "$PWD$nl$PWD" ]] 2> /dev/null  || err_exit 'tilde fails inside a script run by name'
 
+# ======
+# After unsetting HOME, ~ should expand to the current user's OS-configured home directory.
+unset HOME
+got=~
+[[ $got == /* && -d $got ]] || err_exit "expansion of bare tilde breaks after unsetting HOME (value: $(printf %q "$got"))"
+HOME=$tmp
+
+# ======
 exit $((Errors<125?Errors:125))