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, 2 insertions, 4 deletions

diff --git a/src/core/dbus.c b/src/core/dbus.c
index 56b43adcda..eb9ec44f5b 100644
--- a/src/core/dbus.c
+++ b/src/core/dbus.c
@@ -501,8 +501,7 @@ static int bus_job_enumerate(sd_bus *bus, const char *path, void *userdata, char
 
         assert(hashmap_size(m->jobs) == k);
 
-        *nodes = l;
-        l = NULL;
+        *nodes = TAKE_PTR(l);
 
         return k;
 }
@@ -526,8 +525,7 @@ static int bus_unit_enumerate(sd_bus *bus, const char *path, void *userdata, cha
                 k++;
         }
 
-        *nodes = l;
-        l = NULL;
+        *nodes = TAKE_PTR(l);
 
         return k;
 }