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/libsystemd-network/dhcp-option.c

@@ -253,10 +253,8 @@ int dhcp_option_parse(DHCPMessage *message, size_t len, dhcp_option_callback_t c
         if (message_type == 0)
                 return -ENOMSG;
 
-        if (_error_message && IN_SET(message_type, DHCP_NAK, DHCP_DECLINE)) {
-                *_error_message = error_message;
-                error_message = NULL;
-        }
+        if (_error_message && IN_SET(message_type, DHCP_NAK, DHCP_DECLINE))
+                *_error_message = TAKE_PTR(error_message);
 
         return message_type;
 }

src/libsystemd-network/dhcp6-option.c

@@ -617,8 +617,7 @@ int dhcp6_option_parse_domainname(const uint8_t *optval, uint16_t optlen, char *
                 idx++;
         }
 
-        *str_arr = names;
-        names = NULL;
+        *str_arr = TAKE_PTR(names);
 
         return idx;
 

src/libsystemd-network/network-internal.c

@@ -276,10 +276,9 @@ int config_parse_ifalias(const char *unit,
         }
 
         free(*s);
-        if (*n) {
-                *s = n;
-                n = NULL;
-        } else
+        if (*n)
+                *s = TAKE_PTR(n);
+        else
                 *s = NULL;
 
         return 0;
@@ -437,8 +436,7 @@ int deserialize_in_addrs(struct in_addr **ret, const char *string) {
                 size++;
         }
 
-        *ret = addresses;
-        addresses = NULL;
+        *ret = TAKE_PTR(addresses);
 
         return size;
 }
@@ -491,8 +489,7 @@ int deserialize_in6_addrs(struct in6_addr **ret, const char *string) {
                 size++;
         }
 
-        *ret = addresses;
-        addresses = NULL;
+        *ret = TAKE_PTR(addresses);
 
         return size;
 }
@@ -585,8 +582,7 @@ int deserialize_dhcp_routes(struct sd_dhcp_route **ret, size_t *ret_size, size_t
 
         *ret_size = size;
         *ret_allocated = allocated;
-        *ret = routes;
-        routes = NULL;
+        *ret = TAKE_PTR(routes);
 
         return 0;
 }

src/libsystemd-network/sd-dhcp-lease.c

@@ -807,8 +807,7 @@ int dhcp_lease_parse_search_domains(const uint8_t *option, size_t len, char ***d
                       pos = next_chunk;
         }
 
-        *domains = names;
-        names = NULL;
+        *domains = TAKE_PTR(names);
 
         return cnt;
 }