fs-util: add new CHASE_TRAIL_SLASH flag for chase_symlinks()

This rearranges chase_symlinks() a bit: if no special flags are
specified it will now revert to behaviour before
b12d25a8d6. However, if the new
CHASE_TRAIL_SLASH flag is specified it will follow the behaviour
introduced by that commit.

I wasn't sure which one to make the beaviour that requires specification
of a flag to enable. I opted to make the "append trailing slash"
behaviour the one to enable by a flag, following the thinking that the
function should primarily be used to generate a normalized path, and I
am pretty sure a path without trailing slash is the more "normalized"
one, as the trailing slash is not really a part of it, but merely a
"decorator" that tells various system calls to generate ENOTDIR if the
path doesn't refer to a path.

Or to say this differently: if the slash was part of normalization then
we really should add it in all cases when the final path is a directory,
not just when the user originally specified it.

Fixes: #8544
Replaces: #8545

src/basic/fs-util.c

@@ -685,8 +685,10 @@ int chase_symlinks(const char *path, const char *original_root, unsigned flags,
                 /* Just a single slash? Then we reached the end. */
                 if (path_equal(first, "/")) {
                         /* Preserve the trailing slash */
-                        if (!strextend(&done, "/", NULL))
-                                return -ENOMEM;
+
+                        if (flags & CHASE_TRAIL_SLASH)
+                                if (!strextend(&done, "/", NULL))
+                                        return -ENOMEM;
 
                         break;
                 }

src/basic/fs-util.h

@@ -86,6 +86,7 @@ enum {
         CHASE_NO_AUTOFS   = 1U << 2,   /* If set, return -EREMOTE if autofs mount point found */
         CHASE_SAFE        = 1U << 3,   /* If set, return EPERM if we ever traverse from unprivileged to privileged files or directories */
         CHASE_OPEN        = 1U << 4,   /* If set, return an O_PATH object to the final component */
+        CHASE_TRAIL_SLASH = 1U << 5,   /* If set, any trailing slash will be preserved */
 };
 
 int chase_symlinks(const char *path_with_prefix, const char *root, unsigned flags, char **ret);

src/basic/mount-util.c

@@ -296,7 +296,7 @@ int path_is_mount_point(const char *t, const char *root, int flags) {
          * /bin -> /usr/bin/ and /usr is a mount point, then the parent that we
          * look at needs to be /usr, not /. */
         if (flags & AT_SYMLINK_FOLLOW) {
-                r = chase_symlinks(t, root, 0, &canonical);
+                r = chase_symlinks(t, root, CHASE_TRAIL_SLASH, &canonical);
                 if (r < 0)
                         return r;