Mask SIGALRM everything but in the main thread.

This is required to ensure that the signal will be delivered to the main
thread when the watchdog timer expires.

src/bio.c

@@ -108,9 +108,18 @@ void bioCreateBackgroundJob(int type, void *arg1, void *arg2, void *arg3) {
 void *bioProcessBackgroundJobs(void *arg) {
     struct bio_job *job;
     unsigned long type = (unsigned long) arg;
+    sigset_t sigset;
 
     pthread_detach(pthread_self());
     pthread_mutex_lock(&bio_mutex[type]);
+    /* Block SIGALRM so we are sure that only the main thread will
+     * receive the watchdog signal. */
+    sigemptyset(&sigset);
+    sigaddset(&sigset, SIGALRM);
+    if (pthread_sigmask(SIG_BLOCK, &sigset, NULL))
+        redisLog(REDIS_WARNING,
+            "Warning: can't mask SIGALRM in bio.c thread: %s", strerror(errno));
+
     while(1) {
         listNode *ln;