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)

17 files changed, 45 insertions, 66 deletions

diff --git a/src/basic/alloc-util.h b/src/basic/alloc-util.h
index ec7808c1f7..b1e0edbb7f 100644
--- a/src/basic/alloc-util.h
+++ b/src/basic/alloc-util.h
@@ -130,3 +130,12 @@ void* greedy_realloc0(void **p, size_t *allocated, size_t need, size_t size);
                 _new_ = alloca_align(_size_, (align));                  \
                 (void*)memset(_new_, 0, _size_);                        \
         })
+
+/* Takes inspiration from Rusts's Option::take() method: reads and returns a pointer, but at the same time resets it to
+ * NULL. See: https://doc.rust-lang.org/std/option/enum.Option.html#method.take */
+#define TAKE_PTR(ptr)                           \
+        ({                                      \
+                typeof(ptr) _ptr_ = (ptr);      \
+                (ptr) = NULL;                   \
+                _ptr_;                          \
+        })
diff --git a/src/basic/cap-list.c b/src/basic/cap-list.c
index c4557666ef..9416391a53 100644
--- a/src/basic/cap-list.c
+++ b/src/basic/cap-list.c
@@ -103,8 +103,7 @@ int capability_set_to_string_alloc(uint64_t set, char **s) {
 
         str[n > 0 ? n - 1 : 0] = '\0'; /* truncate the last space, if it's there */
 
-        *s = str;
-        str = NULL;
+        *s = TAKE_PTR(str);
 
         return 0;
 }
diff --git a/src/basic/cgroup-util.c b/src/basic/cgroup-util.c
index 1a2d7fc087..5934ee6e8a 100644
--- a/src/basic/cgroup-util.c
+++ b/src/basic/cgroup-util.c
@@ -1424,10 +1424,9 @@ int cg_pid_get_path_shifted(pid_t pid, const char *root, char **cgroup) {
         if (r < 0)
                 return r;
 
-        if (c == raw) {
-                *cgroup = raw;
-                raw = NULL;
-        } else {
+        if (c == raw)
+                *cgroup = TAKE_PTR(raw);
+        else {
                 char *n;
 
                 n = strdup(c);
@@ -2010,8 +2009,7 @@ int cg_slice_to_path(const char *unit, char **ret) {
         if (!strextend(&s, e, NULL))
                 return -ENOMEM;
 
-        *ret = s;
-        s = NULL;
+        *ret = TAKE_PTR(s);
 
         return 0;
 }
@@ -2301,8 +2299,7 @@ int cg_mask_to_string(CGroupMask mask, char **ret) {
         assert(s);
 
         s[n] = 0;
-        *ret = s;
-        s = NULL;
+        *ret = TAKE_PTR(s);
 
         return 0;
 }
diff --git a/src/basic/extract-word.c b/src/basic/extract-word.c
index 5e42560487..7a6b56f071 100644
--- a/src/basic/extract-word.c
+++ b/src/basic/extract-word.c
@@ -194,8 +194,7 @@ finish:
 
 finish_force_next:
         s[sz] = 0;
-        *ret = s;
-        s = NULL;
+        *ret = TAKE_PTR(s);
 
         return 1;
 }
diff --git a/src/basic/fileio.c b/src/basic/fileio.c
index fb26274afa..f807842c31 100644
--- a/src/basic/fileio.c
+++ b/src/basic/fileio.c
@@ -330,8 +330,7 @@ int read_full_stream(FILE *f, char **contents, size_t *size) {
         }
 
         buf[l] = 0;
-        *contents = buf;
-        buf = NULL; /* do not free */
+        *contents = TAKE_PTR(buf);
 
         if (size)
                 *size = l;
@@ -1432,8 +1431,7 @@ int open_tmpfile_linkable(const char *target, int flags, char **ret_path) {
         if (fd < 0)
                 return -errno;
 
-        *ret_path = tmp;
-        tmp = NULL;
+        *ret_path = TAKE_PTR(tmp);
 
         return fd;
 }
@@ -1519,8 +1517,7 @@ int read_nul_string(FILE *f, char **ret) {
                         return -ENOMEM;
         }
 
-        *ret = x;
-        x = NULL;
+        *ret = TAKE_PTR(x);
 
         return 0;
 }
diff --git a/src/basic/fs-util.c b/src/basic/fs-util.c
index c65ba4bfe5..aec8b00744 100644
--- a/src/basic/fs-util.c
+++ b/src/basic/fs-util.c
@@ -458,10 +458,8 @@ int get_files_in_directory(const char *path, char ***list) {
                         n++;
         }
 
-        if (list) {
-                *list = l;
-                l = NULL; /* avoid freeing */
-        }
+        if (list)
+                *list = TAKE_PTR(l);
 
         return n;
 }
@@ -838,10 +836,9 @@ int chase_symlinks(const char *path, const char *original_root, unsigned flags,
                 }
 
                 /* If this is not a symlink, then let's just add the name we read to what we already verified. */
-                if (!done) {
-                        done = first;
-                        first = NULL;
-                } else {
+                if (!done)
+                        done = TAKE_PTR(first);
+                else {
                         /* If done is "/", as first also contains slash at the head, then remove this redundant slash. */
                         if (streq(done, "/"))
                                 *done = '\0';
@@ -863,10 +860,8 @@ int chase_symlinks(const char *path, const char *original_root, unsigned flags,
                         return -ENOMEM;
         }
 
-        if (ret) {
-                *ret = done;
-                done = NULL;
-        }
+        if (ret)
+                *ret = TAKE_PTR(done);
 
         if (flags & CHASE_OPEN) {
                 int q;
diff --git a/src/basic/locale-util.c b/src/basic/locale-util.c
index 266cb29936..de3d7c8c89 100644
--- a/src/basic/locale-util.c
+++ b/src/basic/locale-util.c
@@ -341,8 +341,7 @@ int get_keymaps(char ***ret) {
 
         strv_sort(l);
 
-        *ret = l;
-        l = NULL;
+        *ret = TAKE_PTR(l);
 
         return 0;
 }
diff --git a/src/basic/mount-util.c b/src/basic/mount-util.c
index 8151b3a4e4..b6e7c94017 100644
--- a/src/basic/mount-util.c
+++ b/src/basic/mount-util.c
@@ -81,10 +81,8 @@ int name_to_handle_at_loop(
 
                 if (name_to_handle_at(fd, path, h, &mnt_id, flags) >= 0) {
 
-                        if (ret_handle) {
-                                *ret_handle = h;
-                                h = NULL;
-                        }
+                        if (ret_handle)
+                                *ret_handle = TAKE_PTR(h);
 
                         if (ret_mnt_id)
                                 *ret_mnt_id = mnt_id;
@@ -951,8 +949,7 @@ int mount_option_mangle(
         }
 
         *ret_mount_flags = mount_flags;
-        *ret_remaining_options = ret;
-        ret = NULL;
+        *ret_remaining_options = TAKE_PTR(ret);
 
         return 0;
 }
diff --git a/src/basic/parse-util.c b/src/basic/parse-util.c
index fa5b4a353a..8d4912fdbf 100644
--- a/src/basic/parse-util.c
+++ b/src/basic/parse-util.c
@@ -324,8 +324,7 @@ int parse_syscall_and_errno(const char *in, char **name, int *error) {
                 return -EINVAL;
 
         *error = e;
-        *name = n;
-        n = NULL;
+        *name = TAKE_PTR(n);
 
         return 0;
 }
diff --git a/src/basic/path-util.c b/src/basic/path-util.c
index df94629385..d4c4a02cb3 100644
--- a/src/basic/path-util.c
+++ b/src/basic/path-util.c
@@ -290,8 +290,7 @@ char **path_strv_resolve(char **l, const char *root) {
                 r = chase_symlinks(t, root, 0, &u);
                 if (r == -ENOENT) {
                         if (root) {
-                                u = orig;
-                                orig = NULL;
+                                u = TAKE_PTR(orig);
                                 free(t);
                         } else
                                 u = t;
diff --git a/src/basic/proc-cmdline.c b/src/basic/proc-cmdline.c
index c5d1fb1d41..c51e3c0a3b 100644
--- a/src/basic/proc-cmdline.c
+++ b/src/basic/proc-cmdline.c
@@ -204,10 +204,8 @@ int proc_cmdline_get_key(const char *key, unsigned flags, char **value) {
                 }
         }
 
-        if (value) {
-                *value = ret;
-                ret = NULL;
-        }
+        if (value)
+                *value = TAKE_PTR(ret);
 
         return found;
 }
diff --git a/src/basic/process-util.c b/src/basic/process-util.c
index 853e0e3449..2583b310a4 100644
--- a/src/basic/process-util.c
+++ b/src/basic/process-util.c
@@ -623,8 +623,7 @@ int get_process_environ(pid_t pid, char **env) {
         } else
                 outcome[sz] = '\0';
 
-        *env = outcome;
-        outcome = NULL;
+        *env = TAKE_PTR(outcome);
 
         return 0;
 }
diff --git a/src/basic/securebits-util.c b/src/basic/securebits-util.c
index 441d386f9e..75675bf4c0 100644
--- a/src/basic/securebits-util.c
+++ b/src/basic/securebits-util.c
@@ -48,8 +48,7 @@ int secure_bits_to_string_alloc(int i, char **s) {
         if (len != 0)
                 str[len - 1] = '\0';
 
-        *s = str;
-        str = NULL;
+        *s = TAKE_PTR(str);
 
         return 0;
 }
diff --git a/src/basic/socket-util.c b/src/basic/socket-util.c
index b91b093132..fd26ae7137 100644
--- a/src/basic/socket-util.c
+++ b/src/basic/socket-util.c
@@ -985,8 +985,7 @@ int getpeersec(int fd, char **ret) {
         if (isempty(s))
                 return -EOPNOTSUPP;
 
-        *ret = s;
-        s = NULL;
+        *ret = TAKE_PTR(s);
 
         return 0;
 }
diff --git a/src/basic/strv.c b/src/basic/strv.c
index e80ff4a62c..8010016451 100644
--- a/src/basic/strv.c
+++ b/src/basic/strv.c
@@ -341,8 +341,7 @@ int strv_split_extract(char ***t, const char *s, const char *separators, Extract
                 if (!GREEDY_REALLOC(l, allocated, n + 2))
                         return -ENOMEM;
 
-                l[n++] = word;
-                word = NULL;
+                l[n++] = TAKE_PTR(word);
 
                 l[n] = NULL;
         }
@@ -353,8 +352,7 @@ int strv_split_extract(char ***t, const char *s, const char *separators, Extract
                         return -ENOMEM;
         }
 
-        *t = l;
-        l = NULL;
+        *t = TAKE_PTR(l);
 
         return (int) n;
 }
diff --git a/src/basic/terminal-util.c b/src/basic/terminal-util.c
index eacfd14677..87691d1d38 100644
--- a/src/basic/terminal-util.c
+++ b/src/basic/terminal-util.c
@@ -707,10 +707,9 @@ int vtnr_from_tty(const char *tty) {
                 tty = active;
         }
 
-        if (tty == active) {
-                *ret = active;
-                active = NULL;
-        } else {
+        if (tty == active)
+                *ret = TAKE_PTR(active);
+        else {
                 char *tmp;
 
                 tmp = strdup(tty);
@@ -778,8 +777,7 @@ int get_kernel_consoles(char ***ret) {
                 goto fallback;
         }
 
-        *ret = l;
-        l = NULL;
+        *ret = TAKE_PTR(l);
 
         return 0;
 
@@ -788,8 +786,7 @@ fallback:
         if (r < 0)
                 return r;
 
-        *ret = l;
-        l = NULL;
+        *ret = TAKE_PTR(l);
 
         return 0;
 }
diff --git a/src/basic/unit-name.c b/src/basic/unit-name.c
index 3937dfc5ee..7d9367334b 100644
--- a/src/basic/unit-name.c
+++ b/src/basic/unit-name.c
@@ -363,8 +363,7 @@ int unit_name_unescape(const char *f, char **ret) {
 
         *t = 0;
 
-        *ret = r;
-        r = NULL;
+        *ret = TAKE_PTR(r);
 
         return 0;
 }