manager: process exec_fd (i.e. Type=exec) events before SIGCHLD events

We want to make sure we don't confuse the case "process started
successfully but then failed quickly" from the case "process failed to
start". Hence we need to make sure we take notice of Type=exec before we
bother with SIGCHLD.

Hence move EVENT_PRIORITY_EXEC_FD to the front. In fact, let's move it
even further up than SIGCHLD, i.e. before sd_notify() handling, so that
we don't end up processing service state change notifications before we
even considered that the service is properly started.

This also gives the cgroup OOM handling and the exec_fd handling
different priorities, to improve robustness of the system, we should act
quickly on OOM, and it doesn't matter if a service started succcessfully
if we have to act on OOM anyway.

This is based on Andrew Onyshchuk <andryk.rv@gmail.com> work here:

See: #30799
Fixes: #28304

diff --git a/src/core/manager.h b/src/core/manager.h
index cfcf1929f544504de1732b18f2f2870c33ea9e43..4193c10e4b5fedb4abf2774933016967f5608aac 100644 (file)
--- a/src/core/manager.h
+++ b/src/core/manager.h
@@ -653,11 +653,11 @@ enum {
         EVENT_PRIORITY_CGROUP_AGENT     = SD_EVENT_PRIORITY_NORMAL-9,
         EVENT_PRIORITY_CGROUP_INOTIFY   = SD_EVENT_PRIORITY_NORMAL-9,
         EVENT_PRIORITY_CGROUP_OOM       = SD_EVENT_PRIORITY_NORMAL-8,
-        EVENT_PRIORITY_NOTIFY           = SD_EVENT_PRIORITY_NORMAL-8,
-        EVENT_PRIORITY_SIGCHLD          = SD_EVENT_PRIORITY_NORMAL-7,
-        EVENT_PRIORITY_SIGNALS          = SD_EVENT_PRIORITY_NORMAL-6,
-        EVENT_PRIORITY_CGROUP_EMPTY     = SD_EVENT_PRIORITY_NORMAL-5,
-        EVENT_PRIORITY_EXEC_FD          = SD_EVENT_PRIORITY_NORMAL-3,
+        EVENT_PRIORITY_EXEC_FD          = SD_EVENT_PRIORITY_NORMAL-6,
+        EVENT_PRIORITY_NOTIFY           = SD_EVENT_PRIORITY_NORMAL-5,
+        EVENT_PRIORITY_SIGCHLD          = SD_EVENT_PRIORITY_NORMAL-4,
+        EVENT_PRIORITY_SIGNALS          = SD_EVENT_PRIORITY_NORMAL-3,
+        EVENT_PRIORITY_CGROUP_EMPTY     = SD_EVENT_PRIORITY_NORMAL-2,
         EVENT_PRIORITY_TIME_CHANGE      = SD_EVENT_PRIORITY_NORMAL-1,
         EVENT_PRIORITY_TIME_ZONE        = SD_EVENT_PRIORITY_NORMAL-1,
         EVENT_PRIORITY_IPC              = SD_EVENT_PRIORITY_NORMAL,

diff --git a/src/core/service.c b/src/core/service.c
index 79859a2a1ea26b601be3679d8594284b3896ed6e..aacfe0d57e855a8c2268f9e9122b79d9c79adea5 100644 (file)
--- a/src/core/service.c
+++ b/src/core/service.c
@@ -1512,7 +1512,8 @@ static int service_allocate_exec_fd_event_source(
         if (r < 0)
                 return log_unit_error_errno(UNIT(s), r, "Failed to allocate exec_fd event source: %m");
 
-        /* This is a bit lower priority than SIGCHLD, as that carries a lot more interesting failure information */
+        /* This is a bit higher priority than SIGCHLD, to make sure we don't confuse the case "failed to
+         * start" from the case "succeeded to start, but failed immediately after". */
 
         r = sd_event_source_set_priority(source, EVENT_PRIORITY_EXEC_FD);
         if (r < 0)