Improvements to PR #2425

1. Remove useless "cs" initialization.
2. Add a "select" var to capture a condition checked multiple times.
3. Avoid duplication of the same if (!copy) conditional.
4. Don't increment dirty if copy is given (no deletion is performed),
   otherwise we propagate MIGRATE when not needed.

src/cluster.c

@@ -4484,7 +4484,6 @@ void migrateCommand(redisClient *c) {
 
 try_again:
     /* Initialization */
-    cs = NULL;
     copy = 0;
     replace = 0;
     ttl = 0;
@@ -4522,13 +4521,15 @@ try_again:
 
     rioInitWithBuffer(&cmd,sdsempty());
 
-    /* Create RESTORE payload and generate the protocol to call the command. */
-    if (cs->last_dbid != dbid) {
+    /* Send the SELECT command if the current DB is not already selected. */
+    int select = cs->last_dbid != dbid; /* Should we emit SELECT? */
+    if (select) {
         redisAssertWithInfo(c,NULL,rioWriteBulkCount(&cmd,'*',2));
         redisAssertWithInfo(c,NULL,rioWriteBulkString(&cmd,"SELECT",6));
         redisAssertWithInfo(c,NULL,rioWriteBulkLongLong(&cmd,dbid));
     }
 
+    /* Create RESTORE payload and generate the protocol to call the command. */
     expireat = getExpire(c->db,c->argv[3]);
     if (expireat != -1) {
         ttl = expireat-mstime();
@@ -4578,12 +4579,12 @@ try_again:
         char buf2[1024];
 
         /* Read the two replies */
-        if (cs->last_dbid != dbid && syncReadLine(cs->fd, buf1, sizeof(buf1), timeout) <= 0)
+        if (select && syncReadLine(cs->fd, buf1, sizeof(buf1), timeout) <= 0)
             goto socket_rd_err;
         if (syncReadLine(cs->fd, buf2, sizeof(buf2), timeout) <= 0)
             goto socket_rd_err;
-        if ((cs->last_dbid != dbid && buf1[0] == '-') || buf2[0] == '-') {
-            /* If we got an error at all, assume that the last_dbid is no longer valid */
+        if ((select && buf1[0] == '-') || buf2[0] == '-') {
+            /* On error assume that last_dbid is no longer valid. */
             cs->last_dbid = -1;
             addReplyErrorFormat(c,"Target instance replied with error: %s",
                 (cs->last_dbid != dbid && buf1[0] == '-') ? buf1+1 : buf2+1);
@@ -4592,15 +4593,14 @@ try_again:
             cs->last_dbid = dbid;
             robj *aux;
 
+            addReply(c,shared.ok);
+
             if (!copy) {
                 /* No COPY option: remove the local key, signal the change. */
                 dbDelete(c->db,c->argv[3]);
                 signalModifiedKey(c->db,c->argv[3]);
-            }
-            addReply(c,shared.ok);
-            server.dirty++;
+                server.dirty++;
 
-            if (!copy) {
                 /* Translate MIGRATE as DEL for replication/AOF. */
                 aux = createStringObject("DEL",3);
                 rewriteClientCommandVector(c,2,aux,c->argv[3]);