daemon: Limit default log level to journald to VIR_LOG_INFO

Libvirt is really chatty when the DEBUG log level is enabled. When a
host uses journald we'd enable debug logging to journald when only
specifying the debug log level. As journald may employ rate throttling
this would lock up the daemon until it's able to flush all debug
messages.

This patch changes the default log level to VIR_LOG_INFO when using the
default (unconfigured) log output to journald.

To still allow debug logging to journald the user now has to explicitly
specify journald as a log output with priority 1 in the "log_outputs"
configuration option. This patch also changes the config file template
to be explicit about this change and notify the user about the possible
consequence of debug logging into journald.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1121955

daemon/libvirtd.c

@@ -678,7 +678,14 @@ daemonSetupLogging(struct daemonConfig *config,
         (godaemon || !isatty(STDIN_FILENO))) {
         char *tmp;
         if (access("/run/systemd/journal/socket", W_OK) >= 0) {
-            if (virAsprintf(&tmp, "%d:journald", virLogGetDefaultPriority()) < 0)
+            virLogPriority priority = virLogGetDefaultPriority();
+
+            /* By default we don't want to log too much stuff into journald as
+             * it may employ rate limiting and thus block libvirt execution. */
+            if (priority == VIR_LOG_DEBUG)
+                priority = VIR_LOG_INFO;
+
+            if (virAsprintf(&tmp, "%d:journald", priority) < 0)
                 goto error;
             virLogParseOutputs(tmp);
             VIR_FREE(tmp);

daemon/libvirtd.conf

@@ -309,6 +309,10 @@
 
 # Logging level: 4 errors, 3 warnings, 2 information, 1 debug
 # basically 1 will log everything possible
+# Note: Journald may employ rate limiting of the messages logged
+# and thus lock up the libvirt daemon. To use the debug level with
+# journald you have to specify it explicitly in 'log_outputs', otherwise
+# only information level messages will be logged.
 #log_level = 3
 
 # Logging filters: