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)

10 files changed, 36 insertions, 74 deletions

diff --git a/src/core/cgroup.c b/src/core/cgroup.c
index 65ed86580f..62a3d86ef1 100644
--- a/src/core/cgroup.c
+++ b/src/core/cgroup.c
@@ -1381,8 +1381,7 @@ int unit_set_cgroup_path(Unit *u, const char *path) {
 
         unit_release_cgroup(u);
 
-        u->cgroup_path = p;
-        p = NULL;
+        u->cgroup_path = TAKE_PTR(p);
 
         return 1;
 }
diff --git a/src/core/dbus-execute.c b/src/core/dbus-execute.c
index 635213a866..7344623ebf 100644
--- a/src/core/dbus-execute.c
+++ b/src/core/dbus-execute.c
@@ -424,10 +424,8 @@ static int property_get_syscall_filter(
                                 if (r < 0)
                                         return -ENOMEM;
                         }
-                } else {
-                        s = name;
-                        name = NULL;
-                }
+                } else
+                        s = TAKE_PTR(name);
 
                 r = strv_consume(&l, s);
                 if (r < 0)
@@ -1125,8 +1123,7 @@ int bus_set_transient_exec_command(
                                 return -ENOMEM;
                         }
 
-                        c->argv = argv;
-                        argv = NULL;
+                        c->argv = TAKE_PTR(argv);
 
                         c->flags = b ? EXEC_COMMAND_IGNORE_FAILURE : 0;
 
diff --git a/src/core/dbus-job.c b/src/core/dbus-job.c
index 0802fc9773..15d0e5dd8b 100644
--- a/src/core/dbus-job.c
+++ b/src/core/dbus-job.c
@@ -274,8 +274,7 @@ int bus_job_coldplug_bus_track(Job *j) {
 
         assert(j);
 
-        deserialized_clients = j->deserialized_clients;
-        j->deserialized_clients = NULL;
+        deserialized_clients = TAKE_PTR(j->deserialized_clients);
 
         if (strv_isempty(deserialized_clients))
                 return 0;
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;
 }
diff --git a/src/core/dynamic-user.c b/src/core/dynamic-user.c
index de6aadd597..f87a5a20d4 100644
--- a/src/core/dynamic-user.c
+++ b/src/core/dynamic-user.c
@@ -770,8 +770,7 @@ int dynamic_user_lookup_uid(Manager *m, uid_t uid, char **ret) {
         if (check_uid != uid) /* lock file doesn't match our own idea */
                 return -ESRCH;
 
-        *ret = user;
-        user = NULL;
+        *ret = TAKE_PTR(user);
 
         return 0;
 }
diff --git a/src/core/execute.c b/src/core/execute.c
index 7292b815db..bfd3dfdafc 100644
--- a/src/core/execute.c
+++ b/src/core/execute.c
@@ -1733,8 +1733,7 @@ static int build_environment(
         our_env[n_env++] = NULL;
         assert(n_env <= 12);
 
-        *ret = our_env;
-        our_env = NULL;
+        *ret = TAKE_PTR(our_env);
 
         return 0;
 }
@@ -1763,8 +1762,7 @@ static int build_pass_environment(const ExecContext *c, char ***ret) {
                 x = NULL;
         }
 
-        *ret = pass_env;
-        pass_env = NULL;
+        *ret = TAKE_PTR(pass_env);
 
         return 0;
 }
@@ -2290,9 +2288,7 @@ static int compile_bind_mounts(
 
         *ret_bind_mounts = bind_mounts;
         *ret_n_bind_mounts = n;
-        *ret_empty_directories = empty_directories;
-
-        empty_directories = NULL;
+        *ret_empty_directories = TAKE_PTR(empty_directories);
 
         return (int) n;
 
@@ -2696,8 +2692,7 @@ static int compile_suggested_paths(const ExecContext *c, const ExecParameters *p
                 }
         }
 
-        *ret = list;
-        list = NULL;
+        *ret = TAKE_PTR(list);
 
         return 0;
 }
diff --git a/src/core/load-fragment.c b/src/core/load-fragment.c
index 94605ab0d0..887eb1cf49 100644
--- a/src/core/load-fragment.c
+++ b/src/core/load-fragment.c
@@ -1248,8 +1248,7 @@ int config_parse_exec_cpu_affinity(const char *unit,
         }
 
         if (!c->cpuset) {
-                c->cpuset = cpuset;
-                cpuset = NULL;
+                c->cpuset = TAKE_PTR(cpuset);
                 c->cpuset_ncpus = (unsigned) ncpus;
                 return 0;
         }
@@ -1257,8 +1256,7 @@ int config_parse_exec_cpu_affinity(const char *unit,
         if (c->cpuset_ncpus < (unsigned) ncpus) {
                 CPU_OR_S(CPU_ALLOC_SIZE(c->cpuset_ncpus), cpuset, c->cpuset, cpuset);
                 CPU_FREE(c->cpuset);
-                c->cpuset = cpuset;
-                cpuset = NULL;
+                c->cpuset = TAKE_PTR(cpuset);
                 c->cpuset_ncpus = (unsigned) ncpus;
                 return 0;
         }
@@ -2089,8 +2087,7 @@ int config_parse_user_group(
                         return -ENOEXEC;
                 }
 
-                n = k;
-                k = NULL;
+                n = TAKE_PTR(k);
         }
 
         free(*user);
@@ -2320,10 +2317,8 @@ int config_parse_environ(
                                            "Failed to resolve specifiers, ignoring: %s", word);
                                 continue;
                         }
-                } else {
-                        k = word;
-                        word = NULL;
-                }
+                } else
+                        k = TAKE_PTR(word);
 
                 if (!env_assignment_is_valid(k)) {
                         log_syntax(unit, LOG_ERR, filename, line, 0,
@@ -2390,10 +2385,8 @@ int config_parse_pass_environ(
                                            "Failed to resolve specifiers, ignoring: %s", word);
                                 continue;
                         }
-                } else {
-                        k = word;
-                        word = NULL;
-                }
+                } else
+                        k = TAKE_PTR(word);
 
                 if (!env_name_is_valid(k)) {
                         log_syntax(unit, LOG_ERR, filename, line, 0,
@@ -2469,10 +2462,8 @@ int config_parse_unset_environ(
                                            "Failed to resolve specifiers, ignoring: %s", word);
                                 continue;
                         }
-                } else {
-                        k = word;
-                        word = NULL;
-                }
+                } else
+                        k = TAKE_PTR(word);
 
                 if (!env_assignment_is_valid(k) && !env_name_is_valid(k)) {
                         log_syntax(unit, LOG_ERR, filename, line, 0,
@@ -3515,8 +3506,7 @@ int config_parse_device_allow(
         if (!a)
                 return log_oom();
 
-        a->path = path;
-        path = NULL;
+        a->path = TAKE_PTR(path);
         a->r = !!strchr(m, 'r');
         a->w = !!strchr(m, 'w');
         a->m = !!strchr(m, 'm');
@@ -3615,8 +3605,7 @@ int config_parse_io_device_weight(
         if (!w)
                 return log_oom();
 
-        w->path = path;
-        path = NULL;
+        w->path = TAKE_PTR(path);
 
         w->weight = u;
 
@@ -3701,8 +3690,7 @@ int config_parse_io_limit(
                 if (!l)
                         return log_oom();
 
-                l->path = path;
-                path = NULL;
+                l->path = TAKE_PTR(path);
                 for (ttype = 0; ttype < _CGROUP_IO_LIMIT_TYPE_MAX; ttype++)
                         l->limits[ttype] = cgroup_io_limit_defaults[ttype];
 
@@ -3804,8 +3792,7 @@ int config_parse_blockio_device_weight(
         if (!w)
                 return log_oom();
 
-        w->path = path;
-        path = NULL;
+        w->path = TAKE_PTR(path);
 
         w->weight = u;
 
@@ -3885,8 +3872,7 @@ int config_parse_blockio_bandwidth(
                 if (!b)
                         return log_oom();
 
-                b->path = path;
-                path = NULL;
+                b->path = TAKE_PTR(path);
                 b->rbps = CGROUP_LIMIT_MAX;
                 b->wbps = CGROUP_LIMIT_MAX;
 
diff --git a/src/core/mount-setup.c b/src/core/mount-setup.c
index 9c27972aff..b7d654619c 100644
--- a/src/core/mount-setup.c
+++ b/src/core/mount-setup.c
@@ -315,10 +315,8 @@ int mount_cgroup_controllers(char ***join_controllers) {
                         options = strv_join(*k, ",");
                         if (!options)
                                 return log_oom();
-                } else {
-                        options = controller;
-                        controller = NULL;
-                }
+                } else
+                        options = TAKE_PTR(controller);
 
                 where = strappend("/sys/fs/cgroup/", options);
                 if (!where)
diff --git a/src/core/service.c b/src/core/service.c
index 23a5bcd1c4..588f08fef3 100644
--- a/src/core/service.c
+++ b/src/core/service.c
@@ -2554,8 +2554,7 @@ static int service_deserialize_exec_command(Unit *u, const char *key, const char
                         state = STATE_EXEC_COMMAND_PATH;
                         break;
                 case STATE_EXEC_COMMAND_PATH:
-                        path = arg;
-                        arg = NULL;
+                        path = TAKE_PTR(arg);
                         state = STATE_EXEC_COMMAND_ARGS;
 
                         if (!path_is_absolute(path))
diff --git a/src/core/unit.c b/src/core/unit.c
index 52851b6ffc..90ec73231b 100644
--- a/src/core/unit.c
+++ b/src/core/unit.c
@@ -709,10 +709,8 @@ static int set_complete_move(Set **s, Set **other) {
 
         if (*s)
                 return set_move(*s, *other);
-        else {
-                *s = *other;
-                *other = NULL;
-        }
+        else
+                *s = TAKE_PTR(*other);
 
         return 0;
 }
@@ -726,10 +724,8 @@ static int hashmap_complete_move(Hashmap **s, Hashmap **other) {
 
         if (*s)
                 return hashmap_move(*s, *other);
-        else {
-                *s = *other;
-                *other = NULL;
-        }
+        else
+                *s = TAKE_PTR(*other);
 
         return 0;
 }
@@ -4017,8 +4013,7 @@ static int user_from_unit_name(Unit *u, char **ret) {
                 return r;
 
         if (valid_user_group_name(n)) {
-                *ret = n;
-                n = NULL;
+                *ret = TAKE_PTR(n);
                 return 0;
         }
 
@@ -4220,7 +4215,7 @@ char* unit_escape_setting(const char *s, UnitWriteFlags flags, char **buf) {
 char* unit_concat_strv(char **l, UnitWriteFlags flags) {
         _cleanup_free_ char *result = NULL;
         size_t n = 0, allocated = 0;
-        char **i, *ret;
+        char **i;
 
         /* Takes a list of strings, escapes them, and concatenates them. This may be used to format command lines in a
          * way suitable for ExecStart= stanzas */
@@ -4255,10 +4250,7 @@ char* unit_concat_strv(char **l, UnitWriteFlags flags) {
 
         result[n] = 0;
 
-        ret = result;
-        result = NULL;
-
-        return ret;
+        return TAKE_PTR(result);
 }
 
 int unit_write_setting(Unit *u, UnitWriteFlags flags, const char *name, const char *data) {