logging: warn about invalid logging backends and drop "debug" backend

"debug" was documentation in `man NetworkManager.conf` as a valid
logging backend. However, it was completely ignored by
nm_logging_syslog_openlog().
In fact, it makes not sense. Passing debug = TRUE to
nm_logging_syslog_openlog(), means that all messages will be
printed to stderr in addition to syslog/journal. However, when
NetworkManager is daemonizing, stderr is closed.
Whether NetworkManager is daemonizing depends entirely on command
line options --no-daemon and --debug. Hence, the logging backend "debug"
from the configuration file either conflicts or is redundant.

Also, adjust logging backend description in `man NetworkManager.conf`.

Also, log a warning about invalid/unsupported logging backend.

(cherry picked from commit 2ccf6168dc)

configure.ac

@@ -391,10 +391,8 @@ AC_ARG_WITH(config-logging-backend-default,
             nm_config_logging_backend_default="$withval",
             nm_config_logging_backend_default="")
 
-if test "$nm_config_logging_backend_default" != 'debug' \
-     -a "$nm_config_logging_backend_default" != 'syslog' \
-     -a "$nm_config_logging_backend_default" != 'journal' \
-     -a "$nm_config_logging_backend_default" != 'journal-syslog-style'; then
+if test "$nm_config_logging_backend_default" != 'syslog' \
+     -a "$nm_config_logging_backend_default" != 'journal'; then
 	# unknown backend. Reset to default. Silently accept the invalid value to
 	# be future proof.
 	nm_config_logging_backend_default=''

man/NetworkManager.conf.xml

@@ -603,12 +603,10 @@ unmanaged-devices=mac:00:22:68:1c:59:b1;mac:00:1E:65:30:D1:C4;interface-name:eth
         <varlistentry>
           <term><varname>backend</varname></term>
           <listitem><para>The logging backend. Supported values
-          are "<literal>debug</literal>", "<literal>syslog</literal>",
-          "<literal>journal</literal>".
-          "<literal>debug</literal>" uses syslog and logs to standard error.
-          If NetworkManager is started in debug mode (<literal>--debug</literal>)
-          this option is ignored and "<literal>debug</literal>" is always used.
-          Otherwise, the default is "<literal>&NM_CONFIG_DEFAULT_LOGGING_BACKEND_TEXT;</literal>".
+          are "<literal>syslog</literal>" and "<literal>journal</literal>".
+          When NetworkManager is started with "<literal>--debug</literal>"
+          in addition all messages will be printed to stderr.
+          If unspecified, the default is "<literal>&NM_CONFIG_DEFAULT_LOGGING_BACKEND_TEXT;</literal>".
           </para></listitem>
         </varlistentry>
         <varlistentry>

meson_options.txt

@@ -21,7 +21,7 @@ option('modify_system', type: 'boolean', value: false, description: 'Allow users
 option('polkit_agent', type: 'boolean', value: false, description: 'enable polkit agent for clients')
 option('selinux', type: 'boolean', value: true, description: 'Build with SELinux')
 option('systemd_journal', type: 'boolean', value: true, description: 'Use systemd journal for logging')
-option('config_logging_backend_default', type: 'combo', choices: ['default', 'debug', 'syslog', 'journal', 'journal-syslog-style'], value: 'default', description: 'Default value for logging.backend')
+option('config_logging_backend_default', type: 'combo', choices: ['default', 'syslog', 'journal'], value: 'default', description: 'Default value for logging.backend')
 option('hostname_persist', type: 'combo', choices: ['default', 'suse', 'gentoo', 'slackware'], value: 'default', description: 'Hostname persist method')
 option('libaudit', type: 'combo', choices: ['yes', 'yes-disabled-by-default', 'no'], value: 'yes', description: 'Build with audit daemon support. yes-disabled-by-default enables support, but disables it unless explicitly configured via NetworkManager.conf')
 

src/devices/nm-device.c

@@ -13756,12 +13756,10 @@ nm_device_spawn_iface_helper (NMDevice *self)
 		g_ptr_array_add (argv, g_strdup_printf ("%d %s", (int) stable_type, stable_id));
 	}
 
-	logging_backend = nm_config_get_is_debug (nm_config_get ())
-	                  ? g_strdup ("debug")
-	                  : nm_config_data_get_value (NM_CONFIG_GET_DATA_ORIG,
-	                                              NM_CONFIG_KEYFILE_GROUP_LOGGING,
-	                                              NM_CONFIG_KEYFILE_KEY_LOGGING_BACKEND,
-	                                              NM_CONFIG_GET_VALUE_STRIP | NM_CONFIG_GET_VALUE_NO_EMPTY);
+	logging_backend = nm_config_data_get_value (NM_CONFIG_GET_DATA_ORIG,
+	                                            NM_CONFIG_KEYFILE_GROUP_LOGGING,
+	                                            NM_CONFIG_KEYFILE_KEY_LOGGING_BACKEND,
+	                                            NM_CONFIG_GET_VALUE_STRIP | NM_CONFIG_GET_VALUE_NO_EMPTY);
 	if (logging_backend) {
 		g_ptr_array_add (argv, g_strdup ("--logging-backend"));
 		g_ptr_array_add (argv, logging_backend);

src/nm-logging.c

@@ -839,6 +839,11 @@ void
 nm_logging_syslog_openlog (const char *logging_backend, gboolean debug)
 {
 	gboolean fetch_monotonic_timestamp = FALSE;
+	gboolean obsolete_debug_backend = FALSE;
+
+	nm_assert (NM_IN_STRSET (""NM_CONFIG_DEFAULT_LOGGING_BACKEND,
+	                         NM_LOG_CONFIG_BACKEND_JOURNAL,
+	                         NM_LOG_CONFIG_BACKEND_SYSLOG));
 
 	if (global.log_backend != LOG_BACKEND_GLIB)
 		g_return_if_reached ();
@@ -846,8 +851,19 @@ nm_logging_syslog_openlog (const char *logging_backend, gboolean debug)
 	if (!logging_backend)
 		logging_backend = ""NM_CONFIG_DEFAULT_LOGGING_BACKEND;
 
+	if (nm_streq (logging_backend, NM_LOG_CONFIG_BACKEND_DEBUG)) {
+		/* "debug" was wrongly documented as a valid logging backend. It makes no sense however,
+		 * because printing to stderr only makes sense when not demonizing. Whether to daemonize
+		 * is only controlled via command line arguments (--no-daemon, --debug) and not via the
+		 * logging backend from configuration.
+		 *
+		 * Fall back to the default. */
+		logging_backend = ""NM_CONFIG_DEFAULT_LOGGING_BACKEND;
+		obsolete_debug_backend = TRUE;
+	}
+
 #if SYSTEMD_JOURNAL
-	if (strcmp (logging_backend, "syslog") != 0) {
+	if (!nm_streq (logging_backend, NM_LOG_CONFIG_BACKEND_SYSLOG)) {
 		global.log_backend = LOG_BACKEND_JOURNAL;
 		global.uses_syslog = TRUE;
 		global.debug_stderr = debug;
@@ -871,4 +887,24 @@ nm_logging_syslog_openlog (const char *logging_backend, gboolean debug)
 		 * time causes a logging message. We don't want to do that during _nm_log_impl. */
 		nm_utils_get_monotonic_timestamp_ns ();
 	}
+
+	if (obsolete_debug_backend)
+		nm_log_dbg (LOGD_CORE, "config: ignore deprecated logging backend 'debug', fallback to '%s'", logging_backend);
+
+	if (nm_streq (logging_backend, NM_LOG_CONFIG_BACKEND_SYSLOG)) {
+		/* good */
+	} else if (nm_streq (logging_backend, NM_LOG_CONFIG_BACKEND_JOURNAL)) {
+#if !SYSTEMD_JOURNAL
+		nm_log_warn (LOGD_CORE, "config: logging backend 'journal' is not available, fallback to 'syslog'");
+#endif
+	} else {
+		nm_log_warn (LOGD_CORE, "config: invalid logging backend '%s', fallback to '%s'",
+		             logging_backend,
+#if SYSTEMD_JOURNAL
+		             NM_LOG_CONFIG_BACKEND_JOURNAL
+#else
+		             NM_LOG_CONFIG_BACKEND_SYSLOG
+#endif
+		             );
+	}
 }

src/nm-logging.h

@@ -28,6 +28,10 @@
 #error nm-test-utils.h must be included as last header
 #endif
 
+#define NM_LOG_CONFIG_BACKEND_DEBUG   "debug"
+#define NM_LOG_CONFIG_BACKEND_SYSLOG  "syslog"
+#define NM_LOG_CONFIG_BACKEND_JOURNAL "journal"
+
 /* Log domains */
 typedef enum  { /*< skip >*/
 	LOGD_NONE       = 0LL,