Process events with processEventsWhileBlocked() when blocked.

When we are blocked and a few events a processed from time to time, it
is smarter to call the event handler a few times in order to handle the
accept, read, write, close cycle of a client in a single pass, otherwise
there is too much latency added for clients to receive a reply while the
server is busy in some way (for example during the DB loading).

src/networking.c

@@ -1621,3 +1621,26 @@ int clientsArePaused(void) {
     }
     return server.clients_paused;
 }
+
+/* This function is called by Redis in order to process a few events from
+ * time to time while blocked into some not interruptible operation.
+ * This allows to reply to clients with the -LOADING error while loading the
+ * data set at startup or after a full resynchronization with the master
+ * and so forth.
+ *
+ * It calls the event loop in order to process a few events. Specifically we
+ * try to call the event loop for times as long as we receive acknowledge that
+ * some event was processed, in order to go forward with the accept, read,
+ * write, close sequence needed to serve a client.
+ *
+ * The function returns the total number of events processed. */
+int processEventsWhileBlocked(void) {
+    int iterations = 4; /* See the function top-comment. */
+    int count = 0;
+    while (iterations--) {
+        int events = aeProcessEvents(server.el, AE_FILE_EVENTS|AE_DONT_WAIT);
+        if (!events) break;
+        count += events;
+    }
+    return count;
+}