swrap: Suppress intentional fall through warning

-Wimplicit-fallthrough compiler flag introduced with gcc v7
results in the following warning during compilation:

[  7%] Building C object src/CMakeFiles/socket_wrapper.dir/socket_wrapper.c.o
src/socket_wrapper.c: In function ‘sockaddr_convert_to_un’:
src/socket_wrapper.c:1846:18: warning: this statement may fall through [-Wimplicit-fallthrough=]
  case AF_UNSPEC: {
                  ^
/src/socket_wrapper.c:1866:2: note: here
  case AF_INET:
  ^~~~

Default level for -Wimplicit-fallthrough(which is 3) allows
us to get rid of the above warning using specific comments
made within switch cases matching the regular expressions
outlined in gcc docs[1].

But for us the presence of preprocessor directives in the
vicinity of such comments nullifies its effect[2]. So our
best bet would be to make use of the reliable fallthrough
attribute and supress such warnings in future.

[1] https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-Wimplicit-fallthrough
[2] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77817

Signed-off-by: Anoop C S <anoopcs@redhat.com>
Reviewed-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Michael Adam <obnox@samba.org>

1 files changed, 27 insertions, 0 deletions

diff --git a/ConfigureChecks.cmake b/ConfigureChecks.cmake
index 0691c8a..cb2ea92 100644
--- a/ConfigureChecks.cmake
+++ b/ConfigureChecks.cmake
@@ -198,6 +198,33 @@ int main(void) {
 }" HAVE_DESTRUCTOR_ATTRIBUTE)
 
 check_c_source_compiles("
+#define FALL_THROUGH __attribute__((fallthrough))
+
+enum direction_e {
+    UP = 0,
+    DOWN,
+};
+
+int main(void) {
+    enum direction_e key = UP;
+    int i = 10;
+    int j = 0;
+
+    switch (key) {
+    case UP:
+        i = 5;
+        FALL_THROUGH;
+    case DOWN:
+        j = i * 2;
+        break;
+    default:
+        break;
+    }
+
+    return 0;
+}" HAVE_FALLTHROUGH_ATTRIBUTE)
+
+check_c_source_compiles("
 __thread int tls;
 
 int main(void) {