macro: introduce TAKE_PTR() macro

This macro will read a pointer of any type, return it, and set the
pointer to NULL. This is useful as an explicit concept of passing
ownership of a memory area between pointers.

This takes inspiration from Rust:

https://doc.rust-lang.org/std/option/enum.Option.html#method.take

and was suggested by Alan Jenkins (@sourcejedi).

It drops ~160 lines of code from our codebase, which makes me like it.
Also, I think it clarifies passing of ownership, and thus helps
readability a bit (at least for the initiated who know the new macro)

1 files changed, 3 insertions, 6 deletions

diff --git a/src/basic/mount-util.c b/src/basic/mount-util.c
index db90194ff..b74d51a26 100644
--- a/src/basic/mount-util.c
+++ b/src/basic/mount-util.c
@@ -81,10 +81,8 @@ int name_to_handle_at_loop(
 
                 if (name_to_handle_at(fd, path, h, &mnt_id, flags) >= 0) {
 
-                        if (ret_handle) {
-                                *ret_handle = h;
-                                h = NULL;
-                        }
+                        if (ret_handle)
+                                *ret_handle = TAKE_PTR(h);
 
                         if (ret_mnt_id)
                                 *ret_mnt_id = mnt_id;
@@ -956,8 +954,7 @@ int mount_option_mangle(
         }
 
         *ret_mount_flags = mount_flags;
-        *ret_remaining_options = ret;
-        ret = NULL;
+        *ret_remaining_options = TAKE_PTR(ret);
 
         return 0;
 }