core: fix group ownership when Group is set

When Group is set in the unit, the runtime directories are owned by
this group and not the default group of the user (same for cgroup paths
and standard outputs)

Fix #1231

src/core/execute.c

@@ -629,15 +629,6 @@ static int enforce_groups(const ExecContext *context, const char *username, gid_
          * we avoid NSS lookups for gid=0. */
 
         if (context->group || username) {
-
-                if (context->group) {
-                        const char *g = context->group;
-
-                        r = get_group_creds(&g, &gid);
-                        if (r < 0)
-                                return r;
-                }
-
                 /* First step, initialize groups from /etc/groups */
                 if (username && gid != 0) {
                         if (initgroups(username, gid) < 0)
@@ -1414,6 +1405,17 @@ static int exec_child(
                 }
         }
 
+        if (context->group) {
+                const char *g = context->group;
+
+                r = get_group_creds(&g, &gid);
+                if (r < 0) {
+                        *exit_status = EXIT_GROUP;
+                        return r;
+                }
+        }
+
+
         /* If a socket is connected to STDIN/STDOUT/STDERR, we
          * must sure to drop O_NONBLOCK */
         if (socket_fd >= 0)

src/test/test-execute.c

@@ -140,6 +140,7 @@ static void test_exec_umask(Manager *m) {
 static void test_exec_runtimedirectory(Manager *m) {
         test(m, "exec-runtimedirectory.service", 0, CLD_EXITED);
         test(m, "exec-runtimedirectory-mode.service", 0, CLD_EXITED);
+        test(m, "exec-runtimedirectory-owner.service", 0, CLD_EXITED);
 }
 
 int main(int argc, char *argv[]) {

test/exec-runtimedirectory-owner.service

@@ -0,0 +1,9 @@
+[Unit]
+Description=Test for RuntimeDirectory owner (must not be the default group of the user if Group is set)
+
+[Service]
+ExecStart=/bin/sh -c 'f=/tmp/test-exec_runtimedirectory-owner;g=$(stat -c %G $f); echo "$g"; exit $(test $g = "nobody")'
+Type=oneshot
+Group=nobody
+User=root
+RuntimeDirectory=test-exec_runtimedirectory-owner