run: propagate return code/status from the child

Fixes #13756. We were returning things that didn't make much sense:
we would always use the exit_code value as the exit code. But it sometimes
contains a exit code from the process, and sometimes the number of a signal
that was used to kill the process. We would also ignore SuccessExitStatus=
and in general whether systemd thinks the service exited successfully
(hence the issue in #13756, where systemd would return success/SIGTERM,
but we'd just look at the SIGTERM part.)

If we are doing --wait, let's always propagate the exit code/status from
the child.

While at it, make the documentation useful.

1 files changed, 8 insertions, 2 deletions

diff --git a/src/run/run.c b/src/run/run.c
index 3d63cf0254..de968caf3f 100644
--- a/src/run/run.c
+++ b/src/run/run.c
@@ -16,6 +16,7 @@
 #include "bus-wait-for-jobs.h"
 #include "calendarspec.h"
 #include "env-util.h"
+#include "exit-status.h"
 #include "fd-util.h"
 #include "format-util.h"
 #include "main-func.h"
@@ -1303,9 +1304,14 @@ static int start_transient_service(
                         }
                 }
 
-                /* Try to propagate the service's return value */
-                if (c.result && STR_IN_SET(c.result, "success", "exit-code") && c.exit_code == CLD_EXITED)
+                /* Try to propagate the service's return value. But if the service defines
+                 * e.g. SuccessExitStatus, honour this, and return 0 to mean "success". */
+                if (streq_ptr(c.result, "success"))
+                        *retval = 0;
+                else if (streq_ptr(c.result, "exit-code") && c.exit_status > 0)
                         *retval = c.exit_status;
+                else if (streq_ptr(c.result, "signal"))
+                        *retval = EXIT_EXCEPTION;
                 else
                         *retval = EXIT_FAILURE;
         }