Bug with :P modifier when current directory is the root

[Bart Schaefer <schaefer@brasslantern.com>]

Debugging in another thread, I added this line to
_canonical_paths_add_paths --

    : curpref is $curpref and :P is $curpref:P

and _complete_debug (with "$PWD" == "/") says

    +_canonical_paths_add_paths:11> : curpref is m and :P is //m

I can't think of any case [*] where the :P modifier should add a double
leading slash.

[*] Except on weird OSs where you refer to auto-mount points with //host/
but that's definitely not the case here.

Does this need an extra test that (here[strlen(here)-1] != '/') or some
such, or are we safe enough this way?  Can zgetcwd() ever return "//" or
"///" etc.?


diff --git a/Src/subst.c b/Src/subst.c
index 2214b3d..d68f16b 100644
--- a/Src/subst.c
+++ b/Src/subst.c
@@ -4336,7 +4336,11 @@ modify(char **str, char **ptr)
 			break;
 		    case 'P':
 			if (*copy != '/') {
-			    copy = zhtricat(metafy(zgetcwd(), -1, META_HEAPDUP), "/", copy);
+			    char *here = zgetcwd();
+			    if (here[1])
+				copy = zhtricat(metafy(here, -1, META_HEAPDUP), "/", copy);
+			    else
+				copy = dyncat(here, copy);
 			}
 			copy = xsymlink(copy, 1);
 			break;
@@ -4418,7 +4422,11 @@ modify(char **str, char **ptr)
 		    break;
 		case 'P':
 		    if (**str != '/') {
-			*str = zhtricat(metafy(zgetcwd(), -1, META_HEAPDUP), "/", *str);
+			char *here = zgetcwd();
+			if (here[1])
+			    *str = zhtricat(metafy(here, -1, META_HEAPDUP), "/", *str);
+			else
+			    *str = dyncat(here, *str);
 		    }
 		    *str = xsymlink(*str, 1);
 		    break;