core: reuse the same /tmp, /var/tmp and inaccessible dir

All Execs within the service, will get mounted the same
/tmp and /var/tmp directories, if service is configured with
PrivateTmp=yes. Temporary directories are cleaned up by service
itself in addition to systemd-tmpfiles. Directory which is mounted
as inaccessible is created at runtime in /run/systemd.

src/shared/util.c

@@ -5682,3 +5682,47 @@ int search_and_fopen_nulstr(const char *path, const char *mode, const char *sear
 
         return search_and_fopen_internal(path, mode, s, _f);
 }
+
+int create_tmp_dir(char template[], mode_t mask, bool need_sticky, char** dir_name) {
+        int r = 0;
+        char *d = NULL;
+        bool remove = false;
+        mode_t _cleanup_umask_ u;
+
+        assert(dir_name);
+
+        u = umask(mask);
+        d = mkdtemp(template);
+        if (!d) {
+                r = -errno;
+                log_debug("Can't create directory");
+                goto fail;
+        }
+
+        remove = true;
+
+        log_debug("Created temporary directory : %s", template);
+
+        d = strdup(template);
+        if (!d) {
+                r = log_oom();
+                goto fail;
+        }
+
+        if (need_sticky) {
+                r = chmod(template, 0777 | S_ISVTX);
+                if (r < 0) {
+                        r = -errno;
+                        goto fail;
+                }
+                log_debug("Setting sticky bit on : %s", template);
+        }
+
+        *dir_name = d;
+
+        return 0;
+fail:
+        if (remove)
+                rmdir(template);
+        return r;
+}

src/shared/util.h

@@ -574,6 +574,7 @@ int on_ac_power(void);
 
 int search_and_fopen(const char *path, const char *mode, const char **search, FILE **_f);
 int search_and_fopen_nulstr(const char *path, const char *mode, const char *search, FILE **_f);
+int create_tmp_dir(char template[], mode_t mask, bool need_sticky, char** dir_name);
 
 #define FOREACH_LINE(line, f, on_error)                         \
         for (;;)                                                \