logging: make nm-logging thread-safe

NetworkManager is single-threaded and uses a mainloop.

However, sometimes we may need multiple threads. For example, we will
need to write sysctl values asynchronously, using the glib thread-pool.
For that to work, we also need to switch the network-namespace of the
thread-pool thread. We want to use NMPNetns for that. Hence it's better
to have NMPNetns thread-safe, instead of coming up with a duplicate
implementation. But NMPNetns may want to log, so we also need nm-logging
thread-safe.

In general, code under "shared/nm-utils" and nm-logging should be usable
from multiple threads. It's simpler to make this code thread-safe than
re-implementing it. Also, it's a bad limitation to be unable to log
from other threads. If there is an error, the best we can often do is to
log about it.

Make nm-logging thread-safe. Actually, we only need to be able to log
from multiple threads. We don't need to setup or configure logging from
multiple threads. This restriction allows us to access logging from the
main-thread without any thread-synchronization (because all changes in
the logging setup are also done from the main-thread).

So, while logging from other threads requires a mutex, logging from the
main-thread is lock-free.

3 files changed, 11 insertions, 7 deletions

diff --git a/shared/nm-utils/nm-logging-fwd.h b/shared/nm-utils/nm-logging-fwd.h
index 303d59514c..900dfff812 100644
--- a/shared/nm-utils/nm-logging-fwd.h
+++ b/shared/nm-utils/nm-logging-fwd.h
@@ -94,18 +94,20 @@ typedef enum  { /*< skip >*/
 	_LOGL_N, /* the number of logging levels including "OFF" */
 } NMLogLevel;
 
-gboolean _nm_log_enabled (NMLogLevel level,
-                          NMLogDomain domain);
+gboolean _nm_log_enabled_impl (gboolean mt_require_locking,
+                               NMLogLevel level,
+                               NMLogDomain domain);
 
 void _nm_log_impl (const char *file,
                    guint line,
                    const char *func,
+                   gboolean mt_require_locking,
                    NMLogLevel level,
                    NMLogDomain domain,
                    int error,
                    const char *ifname,
                    const char *con_uuid,
                    const char *fmt,
-                   ...) _nm_printf (9, 10);
+                   ...) _nm_printf (10, 11);
 
 #endif /* __NM_LOGGING_DEFINES_H__ */
diff --git a/shared/systemd/nm-logging-stub.c b/shared/systemd/nm-logging-stub.c
index 80d4360783..5be69b4bf3 100644
--- a/shared/systemd/nm-logging-stub.c
+++ b/shared/systemd/nm-logging-stub.c
@@ -24,8 +24,9 @@
 /*****************************************************************************/
 
 gboolean
-_nm_log_enabled (NMLogLevel level,
-                 NMLogDomain domain)
+_nm_log_enabled_impl (gboolean mt_require_locking,
+                      NMLogLevel level,
+                      NMLogDomain domain)
 {
 	return FALSE;
 }
@@ -34,6 +35,7 @@ void
 _nm_log_impl (const char *file,
               guint line,
               const char *func,
+              gboolean mt_require_locking,
               NMLogLevel level,
               NMLogDomain domain,
               int error,
diff --git a/shared/systemd/sd-adapt-shared/nm-sd-adapt-shared.h b/shared/systemd/sd-adapt-shared/nm-sd-adapt-shared.h
index 905e45c0e1..f68144d896 100644
--- a/shared/systemd/sd-adapt-shared/nm-sd-adapt-shared.h
+++ b/shared/systemd/sd-adapt-shared/nm-sd-adapt-shared.h
@@ -54,10 +54,10 @@ _nm_log_get_max_level_realm (void)
 	const int _nm_e = (error); \
 	const NMLogLevel _nm_l = _slog_level_to_nm ((level)); \
 	\
-	if (_nm_log_enabled (_nm_l, LOGD_SYSTEMD)) { \
+	if (_nm_log_enabled_impl (!(NM_THREAD_SAFE_ON_MAIN_THREAD), _nm_l, LOGD_SYSTEMD)) { \
 		const char *_nm_location = strrchr ((""file), '/'); \
 		\
-		_nm_log_impl (_nm_location ? _nm_location + 1 : (""file), (line), (func), _nm_l, LOGD_SYSTEMD, _nm_e, NULL, NULL, ("%s"format), "libsystemd: ", ## __VA_ARGS__); \
+		_nm_log_impl (_nm_location ? _nm_location + 1 : (""file), (line), (func), !(NM_THREAD_SAFE_ON_MAIN_THREAD), _nm_l, LOGD_SYSTEMD, _nm_e, NULL, NULL, ("%s"format), "libsystemd: ", ## __VA_ARGS__); \
 	} \
 	(_nm_e > 0 ? -_nm_e : _nm_e); \
 })