Undo slave-master handshake when SLAVEOF sets a new slave.

Issue #828 shows how Redis was not correctly undoing a non-blocking
connection attempt with the previous master when the master was set to a
new address using the SLAVEOF command.

This was also a result of lack of refactoring, so now there is a
function to cancel the non blocking handshake with the master.
The new function is now used when SLAVEOF NO ONE is called or when
SLAVEOF is used to set the master to a different address.

src/replication.c

@@ -695,6 +695,27 @@ void undoConnectWithMaster(void) {
     server.repl_state = REDIS_REPL_CONNECT;
 }
 
+/* This function aborts a non blocking replication attempt if there is one
+ * in progress, by canceling the non-blocking connect attempt or
+ * the initial bulk transfer.
+ *
+ * If there was a replication handshake in progress 1 is returned and
+ * the replication state (server.repl_state) set to REDIS_REPL_CONNECT.
+ *
+ * Otherwise zero is returned and no operation is perforemd at all. */
+int cancelReplicationHandshake(void) {
+    if (server.repl_state == REDIS_REPL_TRANSFER) {
+        replicationAbortSyncTransfer();
+    } else if (server.repl_state == REDIS_REPL_CONNECTING ||
+             server.repl_state == REDIS_REPL_RECEIVE_PONG)
+    {
+        undoConnectWithMaster();
+    } else {
+        return 0;
+    }
+    return 1;
+}
+
 void slaveofCommand(redisClient *c) {
     if (!strcasecmp(c->argv[1]->ptr,"no") &&
         !strcasecmp(c->argv[2]->ptr,"one")) {
@@ -702,11 +723,7 @@ void slaveofCommand(redisClient *c) {
             sdsfree(server.masterhost);
             server.masterhost = NULL;
             if (server.master) freeClient(server.master);
-            if (server.repl_state == REDIS_REPL_TRANSFER)
-                replicationAbortSyncTransfer();
-            else if (server.repl_state == REDIS_REPL_CONNECTING ||
-                     server.repl_state == REDIS_REPL_RECEIVE_PONG)
-                undoConnectWithMaster();
+            cancelReplicationHandshake();
             server.repl_state = REDIS_REPL_NONE;
             redisLog(REDIS_NOTICE,"MASTER MODE enabled (user request)");
         }
@@ -730,8 +747,7 @@ void slaveofCommand(redisClient *c) {
         server.masterport = port;
         if (server.master) freeClient(server.master);
         disconnectSlaves(); /* Force our slaves to resync with us as well. */
-        if (server.repl_state == REDIS_REPL_TRANSFER)
-            replicationAbortSyncTransfer();
+        cancelReplicationHandshake();
         server.repl_state = REDIS_REPL_CONNECT;
         redisLog(REDIS_NOTICE,"SLAVE OF %s:%d enabled (user request)",
             server.masterhost, server.masterport);