Fix 'command -p' by fixing initialisation of default PATH variable

'command -p' was broken for non-interactive shells as the variable
used to store the default system PATH, std_path, was not
initialised correctly. For instance:
	$ ksh -c 'command -p ls'
	ksh: ls: not found
This fix by Siteshwar Vashisht is backported from ksh2020.
Ref.:
https://github.com/att/ast/issues/426
https://github.com/att/ast/pull/448

src/cmd/ksh93/sh/path.c:
- Correctly initialise std_path (the default PATH) when ksh is
  started as a non-interactive shell.

src/cmd/ksh93/sh.1:
- Fix vague explanation of 'command -p'.

src/cmd/ksh93/tests/path.sh:
- Add regression test.

(cherry picked from commit a76439d60b70c18cf44d84c1962fcd8df84c947c)

NEWS

@@ -4,6 +4,13 @@ For full details, see the git log at:
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2020-05-19:
+
+- Fix 'command -p'. The -p option causes the operating system's standard
+  utilities path (as output by 'getconf PATH') to be searched instead of $PATH.
+  Before this fix, this was broken on non-interactive shells as the internal
+  variable holding the default PATH value was not correctly initialised.
+
 2020-05-16:
 
 - Fix 'test -t 1', '[ -t 1 ]', '[[ -t 1 ]]' in command substitutions.

src/cmd/ksh93/include/version.h

@@ -17,4 +17,4 @@
 *                  David Korn <dgk@research.att.com>                   *
 *                                                                      *
 ***********************************************************************/
-#define SH_RELEASE	"93u+m 2020-05-16"
+#define SH_RELEASE	"93u+m 2020-05-19"

src/cmd/ksh93/sh.1

@@ -5806,7 +5806,8 @@ with the arguments given by
 The
 .B \-p
 option causes
-a default path to be searched
+the operating system's standard utilities path
+(as output by \f3getconf PATH\fP) to be searched
 rather than the one defined by the value of
 .SM
 .BR PATH .

src/cmd/ksh93/sh/path.c

@@ -56,11 +56,14 @@ static void		funload(Shell_t*,int,const char*);
 static void		exscript(Shell_t*,char*, char*[], char**);
 static int		path_chkpaths(Shell_t*,Pathcomp_t*,Pathcomp_t*,Pathcomp_t*,int);
 static void		path_checkdup(Shell_t *shp,register Pathcomp_t*);
+static			Pathcomp_t *defpath_init(Shell_t *shp);
 
-static const char	*std_path;
+static const char	*std_path = NULL;
 
-static int onstdpath(const char *name)
+static int onstdpath(Shell_t *shp, const char *name)
 {
+	if(!std_path)
+		defpath_init(shp);
 	register const char *cp = std_path, *sp;
 	if(cp)
 		while(*cp)
@@ -379,7 +382,7 @@ static void path_checkdup(Shell_t *shp,register Pathcomp_t *pp)
 	pp->mtime = statb.st_mtime;
 	pp->ino = statb.st_ino;
 	pp->dev = statb.st_dev;
-	if(*name=='/' && onstdpath(name))
+	if(*name=='/' && onstdpath(shp, name))
 		flag = PATH_STD_DIR;
 	first = (pp->flags&PATH_CDPATH)?(Pathcomp_t*)shp->cdpathlist:path_get(shp,"");
 	for(oldpp=first; oldpp && oldpp!=pp; oldpp=oldpp->next)
@@ -450,6 +453,8 @@ Pathcomp_t *path_nextcomp(Shell_t *shp,register Pathcomp_t *pp, const char *name
 
 static Pathcomp_t* defpath_init(Shell_t *shp)
 {
+	if(!std_path && !(std_path=astconf("PATH",NIL(char*),NIL(char*))))
+		std_path = e_defpath;
 	Pathcomp_t *pp = (void*)path_addpath(shp,(Pathcomp_t*)0,(std_path),PATH_PATH);
 	return(pp);
 }
@@ -458,8 +463,6 @@ static void path_init(Shell_t *shp)
 {
 	const char *val;
 	Pathcomp_t *pp;
-	if(!std_path && !(std_path=astconf("PATH",NIL(char*),NIL(char*))))
-		std_path = e_defpath;
 	if(val=sh_scoped(shp,(PATHNOD))->nvalue.cp)
 	{
 		shp->pathlist = pp = (void*)path_addpath(shp,(Pathcomp_t*)shp->pathlist,val,PATH_PATH);

src/cmd/ksh93/tests/path.sh

@@ -397,5 +397,11 @@ ${.sh.version}
 END
 ) || err_exit '${.sh.xxx} variables causes cat not be found'
 
+# ======
+# Check that 'command -p' searches the default OS utility PATH.
+v=$(PATH=/dev/null "$SHELL" -c 'command -p ls /dev/null')
+[[ $v == /dev/null ]] || err_exit 'command -p fails to find standard utility'
+
+# ======
 exit $((Errors<125?Errors:125))