logging: use static buffer in nm_logging_all_domains_to_string()

Don't create a heap allocated GString to hold the static
result of nm_logging_all_domains_to_string().

Instead, use a static buffer of the exactly required size.

The main reason to do this, is to get the exact size of
"_all_logging_domains_to_str" buffer. This is the upper
boundary for the size of a string buffer to hold all domain
names.
We will need that boundary in the next commit. The attractive
thing here is that we will have a unit-test failure if this
boundery no longer matches (--with-more-asserts). That means,
this boundary is guarded by unit tests and we don't accidentally
get it wrong when the domains change.

Also, take care to initialize the buffer in a thread-safe manner.
It's easy enough to get right, so there is no excuse for having
non-thread-safe code in logging.

2 files changed, 45 insertions, 9 deletions

diff --git a/src/nm-logging.c b/src/nm-logging.c
index 0e81749e40..b30c8815c7 100644
--- a/src/nm-logging.c
+++ b/src/nm-logging.c
@@ -549,27 +549,50 @@ _domains_to_string (gboolean include_level_override,
 	return g_string_free (str, FALSE);
 }
 
+static char _all_logging_domains_to_str[273];
+
 const char *
 nm_logging_all_domains_to_string (void)
 {
-	static GString *str;
+	static const char *volatile str = NULL;
+	const char *s;
 
-	if (G_UNLIKELY (!str)) {
+again:
+	s = g_atomic_pointer_get (&str);
+	if (G_UNLIKELY (!s)) {
+		static gsize once = 0;
 		const LogDesc *diter;
+		gsize buf_l;
+		char *buf_p;
+
+		if (!g_once_init_enter (&once))
+			goto again;
+
+		buf_p = _all_logging_domains_to_str;
+		buf_l = sizeof (_all_logging_domains_to_str);
 
-		str = g_string_new (LOGD_DEFAULT_STRING);
+		nm_utils_strbuf_append_str (&buf_p, &buf_l, LOGD_DEFAULT_STRING);
 		for (diter = &domain_desc[0]; diter->name; diter++) {
-			g_string_append_c (str, ',');
-			g_string_append (str, diter->name);
+			nm_utils_strbuf_append_c (&buf_p, &buf_l, ',');
+			nm_utils_strbuf_append_str (&buf_p, &buf_l, diter->name);
 			if (diter->num == LOGD_DHCP6)
-				g_string_append (str, "," LOGD_DHCP_STRING);
+				nm_utils_strbuf_append_str (&buf_p, &buf_l, ","LOGD_DHCP_STRING);
 			else if (diter->num == LOGD_IP6)
-				g_string_append (str, "," LOGD_IP_STRING);
+				nm_utils_strbuf_append_str (&buf_p, &buf_l, ","LOGD_IP_STRING);
 		}
-		g_string_append (str, "," LOGD_ALL_STRING);
+		nm_utils_strbuf_append_str (&buf_p, &buf_l, LOGD_ALL_STRING);
+
+		/* Did you modify the logging domains (or their names)? Adjust the size of
+		 * _all_logging_domains_to_str buffer above to have the exact size. */
+		nm_assert (strlen (_all_logging_domains_to_str) == sizeof (_all_logging_domains_to_str) - 1);
+		nm_assert (buf_l == 1);
+
+		s = _all_logging_domains_to_str;
+		g_atomic_pointer_set (&str, s);
+		g_once_init_leave (&once, 1);
 	}
 
-	return str->str;
+	return s;
 }
 
 /**
diff --git a/src/tests/test-general.c b/src/tests/test-general.c
index 0dee566eaa..cedf5b8a9a 100644
--- a/src/tests/test-general.c
+++ b/src/tests/test-general.c
@@ -109,6 +109,17 @@ test_nm_utils_ip6_address_clear_host_address (void)
 /*****************************************************************************/
 
 static void
+test_logging_domains (void)
+{
+	const char *s;
+
+	s = nm_logging_all_domains_to_string ();
+	g_assert (s && s[0]);
+}
+
+/*****************************************************************************/
+
+static void
 _test_same_prefix (const char *a1, const char *a2, guint8 plen)
 {
 	struct in6_addr a = *nmtst_inet6_from_string (a1);
@@ -2180,6 +2191,8 @@ main (int argc, char **argv)
 {
 	nmtst_init_with_logging (&argc, &argv, NULL, "ALL");
 
+	g_test_add_func ("/general/test_logging_domains", test_logging_domains);
+
 	g_test_add_func ("/general/nm_utils_strbuf_append", test_nm_utils_strbuf_append);
 
 	g_test_add_func ("/general/nm_utils_ip6_address_clear_host_address", test_nm_utils_ip6_address_clear_host_address);