journal: normalize priority of logging sources

The stream event source has a priority of SD_EVENT_PRIORITY_NORMAL+5,
and stdout source +10, but the native and syslog event sources are left
at the default of 0.

As a result, any heavy native or syslog logger can cause starvation of
the other loggers.  This is trivially demonstrated by running:

 dd if=/dev/urandom bs=8k | od | systemd-cat & # native spammer
 systemd-run echo hello & # stream logger
 journalctl --follow --output=verbose --no-pager --identifier=echo &

... and wait, and wait, the "hello" never comes.

Now kill %1, "hello" arrives finally.

1 files changed, 4 insertions, 0 deletions

diff --git a/src/journal/journald-syslog.c b/src/journal/journald-syslog.c
index cfc50d889b..2f7a805f14 100644
--- a/src/journal/journald-syslog.c
+++ b/src/journal/journald-syslog.c
@@ -430,6 +430,10 @@ int server_open_syslog_socket(Server *s) {
         if (r < 0)
                 return log_error_errno(r, "Failed to add syslog server fd to event loop: %m");
 
+        r = sd_event_source_set_priority(s->syslog_event_source, SD_EVENT_PRIORITY_NORMAL+5);
+        if (r < 0)
+                return log_error_errno(r, "Failed to adjust syslog event source priority: %m");
+
         return 0;
 }