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/sd-bus/bus-objects.c

@@ -1470,8 +1470,7 @@ static struct node *bus_node_allocate(sd_bus *bus, const char *path) {
                 return NULL;
 
         n->parent = parent;
-        n->path = s;
-        s = NULL; /* do not free */
+        n->path = TAKE_PTR(s);
 
         r = hashmap_put(bus->nodes, n->path, n);
         if (r < 0) {

src/libsystemd/sd-bus/sd-bus.c

@@ -1342,8 +1342,7 @@ int bus_set_address_user(sd_bus *b) {
         if (asprintf(&s, DEFAULT_USER_BUS_ADDRESS_FMT, ee) < 0)
                 return -ENOMEM;
 
-        b->address = s;
-        s = NULL;
+        b->address = TAKE_PTR(s);
 
         return 0;
 }

src/libsystemd/sd-daemon/sd-daemon.c

@@ -141,8 +141,7 @@ _public_ int sd_listen_fds_with_names(int unset_environment, char ***names) {
                         return r;
         }
 
-        *names = l;
-        l = NULL;
+        *names = TAKE_PTR(l);
 
         return n_fds;
 }

src/libsystemd/sd-device/sd-device.c

@@ -1297,8 +1297,7 @@ int device_get_id_filename(sd_device *device, const char **ret) {
                         }
                 }
 
-                device->id_filename = id;
-                id = NULL;
+                device->id_filename = TAKE_PTR(id);
         }
 
         *ret = device->id_filename;

src/libsystemd/sd-login/sd-login.c

@@ -325,8 +325,7 @@ _public_ int sd_uid_get_display(uid_t uid, char **session) {
         if (isempty(s))
                 return -ENODATA;
 
-        *session = s;
-        s = NULL;
+        *session = TAKE_PTR(s);
 
         return 0;
 }
@@ -355,8 +354,7 @@ static int file_of_seat(const char *seat, char **_p) {
         if (!p)
                 return -ENOMEM;
 
-        *_p = p;
-        p = NULL;
+        *_p = TAKE_PTR(p);
         return 0;
 }
 
@@ -529,8 +527,7 @@ _public_ int sd_session_get_state(const char *session, char **state) {
         if (isempty(s))
                 return -EIO;
 
-        *state = s;
-        s = NULL;
+        *state = TAKE_PTR(s);
 
         return 0;
 }
@@ -575,8 +572,7 @@ static int session_get_string(const char *session, const char *field, char **val
         if (isempty(s))
                 return -ENODATA;
 
-        *value = s;
-        s = NULL;
+        *value = TAKE_PTR(s);
         return 0;
 }
 
@@ -681,10 +677,8 @@ _public_ int sd_seat_get_active(const char *seat, char **session, uid_t *uid) {
                         return r;
         }
 
-        if (session && s) {
-                *session = s;
-                s = NULL;
-        }
+        if (session && s)
+                *session = TAKE_PTR(s);
 
         return 0;
 }
@@ -909,10 +903,9 @@ _public_ int sd_get_machine_names(char ***machines) {
                 *b = NULL;
         }
 
-        if (machines) {
-                *machines = l;
-                l = NULL;
-        }
+        if (machines)
+                *machines = TAKE_PTR(l);
+
         return r;
 }
 
@@ -933,8 +926,7 @@ _public_ int sd_machine_get_class(const char *machine, char **class) {
         if (!c)
                 return -EIO;
 
-        *class = c;
-        c = NULL;
+        *class = TAKE_PTR(c);
 
         return 0;
 }

src/libsystemd/sd-netlink/netlink-message.c

@@ -803,8 +803,7 @@ static int netlink_container_parse(sd_netlink_message *m,
                 attributes[type].net_byteorder = RTA_FLAGS(rta) & NLA_F_NET_BYTEORDER;
         }
 
-        container->attributes = attributes;
-        attributes = NULL;
+        container->attributes = TAKE_PTR(attributes);
         container->n_attributes = count;
 
         return 0;

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

@@ -51,8 +51,7 @@ _public_ int sd_network_get_operational_state(char **state) {
         if (isempty(s))
                 return -ENODATA;
 
-        *state = s;
-        s = NULL;
+        *state = TAKE_PTR(s);
 
         return 0;
 }
@@ -81,8 +80,7 @@ static int network_get_strv(const char *key, char ***ret) {
         strv_uniq(a);
         r = strv_length(a);
 
-        *ret = a;
-        a = NULL;
+        *ret = TAKE_PTR(a);
 
         return r;
 }
@@ -121,8 +119,7 @@ static int network_link_get_string(int ifindex, const char *field, char **ret) {
         if (isempty(s))
                 return -ENODATA;
 
-        *ret = s;
-        s = NULL;
+        *ret = TAKE_PTR(s);
 
         return 0;
 }
@@ -154,8 +151,7 @@ static int network_link_get_strv(int ifindex, const char *key, char ***ret) {
         strv_uniq(a);
         r = strv_length(a);
 
-        *ret = a;
-        a = NULL;
+        *ret = TAKE_PTR(a);
 
         return r;
 }
@@ -263,8 +259,7 @@ static int network_link_get_ifindexes(int ifindex, const char *key, int **ret) {
         if (ifis)
                 ifis[c] = 0; /* Let's add a 0 ifindex to the end, to be nice */
 
-        *ret = ifis;
-        ifis = NULL;
+        *ret = TAKE_PTR(ifis);
 
         return c;
 }