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/shared/ask-password-api.c

@@ -730,8 +730,7 @@ int ask_password_agent(
         if (keyname)
                 (void) add_to_keyring_and_log(keyname, flags, l);
 
-        *ret = l;
-        l = NULL;
+        *ret = TAKE_PTR(l);
         r = 0;
 
 finish:

src/shared/bus-util.c

@@ -1094,8 +1094,7 @@ static int map_basic(sd_bus *bus, const char *member, sd_bus_message *m, sd_bus_
                         return r;
 
                 strv_free(*p);
-                *p = l;
-                l = NULL;
+                *p = TAKE_PTR(l);
                 return 0;
         }
 

src/shared/cgroup-show.c

@@ -189,8 +189,7 @@ int show_cgroup_by_path(
                         free(last);
                 }
 
-                last = k;
-                k = NULL;
+                last = TAKE_PTR(k);
         }
 
         if (r < 0)

src/shared/dns-domain.c

@@ -295,8 +295,7 @@ int dns_label_escape_new(const char *p, size_t l, char **ret) {
         if (r < 0)
                 return r;
 
-        *ret = s;
-        s = NULL;
+        *ret = TAKE_PTR(s);
 
         return r;
 }
@@ -601,8 +600,7 @@ int dns_name_endswith(const char *name, const char *suffix) {
 
                         /* Not the same, let's jump back, and try with the next label again */
                         s = suffix;
-                        n = saved_n;
-                        saved_n = NULL;
+                        n = TAKE_PTR(saved_n);
                 }
         }
 }

src/shared/efivars.c

@@ -249,8 +249,7 @@ int efi_get_variable(
         ((char*) buf)[st.st_size - 4] = 0;
         ((char*) buf)[st.st_size - 4 + 1] = 0;
 
-        *value = buf;
-        buf = NULL;
+        *value = TAKE_PTR(buf);
         *size = (size_t) st.st_size - 4;
 
         if (attribute)
@@ -563,8 +562,7 @@ int efi_get_boot_order(uint16_t **order) {
             l / sizeof(uint16_t) > INT_MAX)
                 return -EINVAL;
 
-        *order = buf;
-        buf = NULL;
+        *order = TAKE_PTR(buf);
         return (int) (l / sizeof(uint16_t));
 }
 

src/shared/fdset.c

@@ -25,6 +25,7 @@
 
 #include "sd-daemon.h"
 
+#include "alloc-util.h"
 #include "dirent-util.h"
 #include "fd-util.h"
 #include "fdset.h"
@@ -168,8 +169,7 @@ int fdset_new_fill(FDSet **_s) {
         }
 
         r = 0;
-        *_s = s;
-        s = NULL;
+        *_s = TAKE_PTR(s);
 
 finish:
         /* We won't close the fds here! */

src/shared/fstab-util.c

@@ -169,10 +169,8 @@ answer:
 
                 *filtered = f;
         }
-        if (value) {
-                *value = v;
-                v = NULL;
-        }
+        if (value)
+                *value = TAKE_PTR(v);
 
         return !!n;
 }
@@ -201,8 +199,7 @@ int fstab_extract_values(const char *opts, const char *name, char ***values) {
                         return r;
         }
 
-        *values = res;
-        res = NULL;
+        *values = TAKE_PTR(res);
 
         return !!*values;
 }

src/shared/import-util.c

@@ -153,8 +153,7 @@ int raw_strip_suffixes(const char *p, char **ret) {
                         break;
         }
 
-        *ret = q;
-        q = NULL;
+        *ret = TAKE_PTR(q);
 
         return 0;
 }

src/shared/install-printf.c

@@ -50,10 +50,8 @@ static int specifier_prefix_and_instance(char specifier, void *data, void *userd
                 if (!ans)
                         return -ENOMEM;
                 *ret = ans;
-        } else {
-                *ret = prefix;
-                prefix = NULL;
-        }
+        } else
+                *ret = TAKE_PTR(prefix);
 
         return 0;
 }

src/shared/install.c

@@ -1422,8 +1422,7 @@ static int unit_file_search(
                 r = unit_file_load_or_readlink(c, info, path, paths->root_dir, flags);
 
                 if (r >= 0) {
-                        info->path = path;
-                        path = NULL;
+                        info->path = TAKE_PTR(path);
                         result = r;
                         found_unit = true;
                         break;
@@ -1446,8 +1445,7 @@ static int unit_file_search(
 
                         r = unit_file_load_or_readlink(c, info, path, paths->root_dir, flags);
                         if (r >= 0) {
-                                info->path = path;
-                                path = NULL;
+                                info->path = TAKE_PTR(path);
                                 result = r;
                                 found_unit = true;
                                 break;
@@ -1754,8 +1752,7 @@ static int install_info_symlink_wants(
                 if (r < 0)
                         return r;
 
-                path = instance.path;
-                instance.path = NULL;
+                path = TAKE_PTR(instance.path);
 
                 if (instance.type == UNIT_FILE_TYPE_MASKED) {
                         unit_file_changes_add(changes, n_changes, -ERFKILL, path, NULL);

src/shared/machine-image.c

@@ -80,7 +80,6 @@ Image *image_unref(Image *i) {
 
 static char **image_settings_path(Image *image) {
         _cleanup_strv_free_ char **l = NULL;
-        char **ret;
         const char *fn, *s;
         unsigned i = 0;
 
@@ -104,10 +103,7 @@ static char **image_settings_path(Image *image) {
         if (!l[i])
                 return NULL;
 
-        ret = l;
-        l = NULL;
-
-        return ret;
+        return TAKE_PTR(l);
 }
 
 static char *image_roothash_path(Image *image) {

src/shared/nsflags.c

@@ -114,8 +114,7 @@ int namespace_flag_to_string_many(unsigned long flags, char **ret) {
                         return -ENOMEM;
         }
 
-        *ret = s;
-        s = NULL;
+        *ret = TAKE_PTR(s);
 
         return 0;
 }

src/shared/path-lookup.c

@@ -181,7 +181,6 @@ static char** user_dirs(
         _cleanup_strv_free_ char **config_dirs = NULL, **data_dirs = NULL;
         _cleanup_free_ char *data_home = NULL;
         _cleanup_strv_free_ char **res = NULL;
-        char **tmp;
         int r;
 
         r = xdg_user_dirs(&config_dirs, &data_dirs);
@@ -242,10 +241,7 @@ static char** user_dirs(
         if (path_strv_make_absolute_cwd(res) < 0)
                 return NULL;
 
-        tmp = res;
-        res = NULL;
-
-        return tmp;
+        return TAKE_PTR(res);
 }
 
 bool path_is_user_data_dir(const char *path) {
@@ -374,8 +370,7 @@ static int acquire_config_dirs(UnitFileScope scope, char **persistent, char **ru
                         *runtime = NULL;
                 }
 
-                *persistent = a;
-                a = NULL;
+                *persistent = TAKE_PTR(a);
 
                 return 0;
 
@@ -413,8 +408,7 @@ static int acquire_control_dirs(UnitFileScope scope, char **persistent, char **r
                 if (!b)
                         return -ENOMEM;
 
-                *runtime = b;
-                b = NULL;
+                *runtime = TAKE_PTR(b);
 
                 break;
         }
@@ -443,8 +437,7 @@ static int acquire_control_dirs(UnitFileScope scope, char **persistent, char **r
                 assert_not_reached("Hmm, unexpected scope value.");
         }
 
-        *persistent = a;
-        a = NULL;
+        *persistent = TAKE_PTR(a);
 
         return 0;
 }

src/shared/specifier.c

@@ -117,8 +117,7 @@ int specifier_printf(const char *text, const Specifier table[], void *userdata,
                 *(t++) = '%';
 
         *t = 0;
-        *_ret = ret;
-        ret = NULL;
+        *_ret = TAKE_PTR(ret);
         return 0;
 }