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/mount/mount-tool.c b/src/mount/mount-tool.c
index dd139740c2..046def33ae 100644
--- a/src/mount/mount-tool.c
+++ b/src/mount/mount-tool.c
@@ -766,16 +766,14 @@ static int find_mount_points(const char *what, char ***list) {
                 if (!GREEDY_REALLOC(l, bufsize, n + 2))
                         return log_oom();
 
-                l[n++] = where;
-                where = NULL;
+                l[n++] = TAKE_PTR(where);
         }
 
         if (!GREEDY_REALLOC(l, bufsize, n + 1))
                 return log_oom();
 
         l[n] = NULL;
-        *list = l;
-        l = NULL; /* avoid freeing */
+        *list = TAKE_PTR(l);
 
         return n;
 }
@@ -827,8 +825,7 @@ static int find_loop_device(const char *backing_file, char **loop_dev) {
         if (!l)
                 return -ENXIO;
 
-        *loop_dev = l;
-        l = NULL; /* avoid freeing */
+        *loop_dev = TAKE_PTR(l);
 
         return 0;
 }