core/logging: define nm_log() to check for nm_logging_enabled() first

Change the definition of nm_log() to first check whether logging is
enabled. This has the benefit, that the logging arguments don't have
to be evaluated if logging is disabled.

With this change, you must ensure, that calling any logging function
is side-effect-free. For example:
    nm_log_debug ("This is a bug %s, %d", has_side_effect (), ++i);
would be a bug, because the logging arguments get only evaluated
depending on the logging setup.

Signed-off-by: Thomas Haller <thaller@redhat.com>

src/nm-logging.h

@@ -97,8 +97,14 @@ GQuark nm_logging_error_quark    (void);
 #define nm_log_info(domain, ...)    nm_log (LOGL_INFO,  (domain), __VA_ARGS__)
 #define nm_log_dbg(domain, ...)     nm_log (LOGL_DEBUG, (domain), __VA_ARGS__)
 
+/* nm_log() only evaluates it's argument list after checking
+ * whether logging for the given level/domain is enabled.  */
 #define nm_log(level, domain, ...) \
-	_nm_log (G_STRLOC, G_STRFUNC, (level), (domain), __VA_ARGS__)
+    G_STMT_START { \
+        if (nm_logging_enabled ((level), (domain))) { \
+            _nm_log (G_STRLOC, G_STRFUNC, (level), (domain), __VA_ARGS__); \
+        } \
+    } G_STMT_END
 
 void _nm_log (const char *loc,
               const char *func,