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)

src/network/netdev/netdev.c

@@ -488,8 +488,7 @@ int netdev_get_mac(const char *ifname, struct ether_addr **ret) {
         mac->ether_addr_octet[0] &= 0xfe;        /* clear multicast bit */
         mac->ether_addr_octet[0] |= 0x02;        /* set local assignment bit (IEEE802) */
 
-        *ret = mac;
-        mac = NULL;
+        *ret = TAKE_PTR(mac);
 
         return 0;
 }

src/network/netdev/wireguard.c

@@ -228,8 +228,7 @@ static int on_resolve_retry(sd_event_source *s, usec_t usec, void *userdata) {
 
         w->resolve_retry_event_source = sd_event_source_unref(w->resolve_retry_event_source);
 
-        w->unresolved_endpoints = w->failed_endpoints;
-        w->failed_endpoints = NULL;
+        w->unresolved_endpoints = TAKE_PTR(w->failed_endpoints);
 
         resolve_endpoints(netdev);
 

src/network/networkd-link-bus.c

@@ -82,8 +82,7 @@ int link_node_enumerator(sd_bus *bus, const char *path, void *userdata, char ***
         }
 
         l[c] = NULL;
-        *nodes = l;
-        l = NULL;
+        *nodes = TAKE_PTR(l);
 
         return 1;
 }

src/network/networkd-network-bus.c

@@ -122,8 +122,7 @@ int network_node_enumerator(sd_bus *bus, const char *path, void *userdata, char
                         return r;
         }
 
-        *nodes = l;
-        l = NULL;
+        *nodes = TAKE_PTR(l);
 
         return 1;
 }

src/network/networkd-network.c

@@ -72,8 +72,7 @@ int network_config_section_new(const char *filename, unsigned line, NetworkConfi
         strcpy(cs->filename, filename);
         cs->line = line;
 
-        *s = cs;
-        cs = NULL;
+        *s = TAKE_PTR(cs);
 
         return 0;
 }

src/network/networkd-radv.c

@@ -126,8 +126,7 @@ int prefix_new(Prefix **ret) {
         if (sd_radv_prefix_new(&prefix->radv_prefix) < 0)
                 return -ENOMEM;
 
-        *ret = prefix;
-        prefix = NULL;
+        *ret = TAKE_PTR(prefix);
 
         return 0;
 }
@@ -344,8 +343,7 @@ static int radv_get_ip6dns(Network *network, struct in6_addr **dns,
         }
 
         if (addresses) {
-                *dns = addresses;
-                addresses = NULL;
+                *dns = TAKE_PTR(addresses);
 
                 *n_dns = n_addresses;
         }

src/network/networkd-routing-policy-rule.c

@@ -857,8 +857,7 @@ static int routing_policy_rule_read_full_file(const char *state_file, char **ret
         if (size <= 0)
                 return -ENODATA;
 
-        *ret = s;
-        s = NULL;
+        *ret = TAKE_PTR(s);
 
         return size;
 }