Fixed SGX main loop not handling signals properly

Main loop blocks while waiting for client data. For this reason, we can't
just check for the stop condition synchronously. In any case, we still need
to ensure that the finalise logic is only called once.

This commit introduces a global counter to ensure finalise_with is only called
once.

firmware/src/sgx/src/untrusted/main.c

@@ -47,6 +47,9 @@ static struct argp_option options[] = {
     {"port", 'p', "PORT", 0, "Port to listen on", 0},
     {0}};
 
+// Global counter to avoid multiple calls to finalise_with
+static sig_atomic_t G_signal_counter = 0;
+
 // Argument definitions for argp
 struct arguments {
     char *bind;
@@ -103,17 +106,22 @@ static void finalise_with(int exit_code) {
     exit(exit_code);
 }
 
-static void finalise(int signum) {
+static void signal_handler(int signum) {
     (void)signum; // Suppress unused parameter warning
 
+    if (G_signal_counter++ > 0) {
+        // Signal has already been handled
+        return;
+    }
+
     finalise_with(0);
 }
 
 static void set_signal_handlers() {
-    signal(SIGINT, finalise);
-    signal(SIGTERM, finalise);
-    signal(SIGHUP, finalise);
-    signal(SIGABRT, finalise);
+    signal(SIGINT, signal_handler);
+    signal(SIGTERM, signal_handler);
+    signal(SIGHUP, signal_handler);
+    signal(SIGABRT, signal_handler);
 }
 
 int main(int argc, char **argv) {