Add REPLPING command for use during replication (#329)

* added replping command for initiating replication

* backwards compatibility for replping (retry if not recognized)

* don't allow ping during loading (load balancer fix)

* changed replping warning to notice

Former-commit-id: d7f6bc16145206e96ffeb9941398d564c3dba6a9

src/replication.cpp

@@ -2759,18 +2759,19 @@ void syncWithMaster(connection *conn) {
         goto error;
     }
 
+retry_connect:
     /* Send a PING to check the master is able to reply without errors. */
-    if (mi->repl_state == REPL_STATE_CONNECTING) {
+    if (mi->repl_state == REPL_STATE_CONNECTING || mi->repl_state == REPL_STATE_RETRY_NOREPLPING) {
         serverLog(LL_NOTICE,"Non blocking connect for SYNC fired the event.");
         /* Delete the writable event so that the readable event remains
          * registered and we can wait for the PONG reply. */
         connSetReadHandler(conn, syncWithMaster);
         connSetWriteHandler(conn, NULL);
-        mi->repl_state = REPL_STATE_RECEIVE_PING_REPLY;
         /* Send the PING, don't check for errors at all, we have the timeout
          * that will take care about this. */
-        err = sendCommand(conn,"PING",NULL);
+        err = sendCommand(conn,mi->repl_state == REPL_STATE_RETRY_NOREPLPING ? "PING" : "REPLPING",NULL);
         if (err) goto write_error;
+        mi->repl_state = REPL_STATE_RECEIVE_PING_REPLY;
         return;
     }
 
@@ -2783,7 +2784,13 @@ void syncWithMaster(connection *conn) {
          * Note that older versions of Redis replied with "operation not
          * permitted" instead of using a proper error code, so we test
          * both. */
-        if (err[0] != '+' &&
+        if (strncmp(err,"-ERR unknown command",20) == 0) {
+            serverLog(LL_NOTICE,"Master does not support REPLPING, sending PING instead...");
+            mi->repl_state = REPL_STATE_RETRY_NOREPLPING;
+            sdsfree(err);
+            err = NULL;
+            goto retry_connect;
+        } else if (err[0] != '+' &&
             strncmp(err,"-NOAUTH",7) != 0 &&
             strncmp(err,"-NOPERM",7) != 0 &&
             strncmp(err,"-ERR operation not permitted",28) != 0)

src/server.cpp

@@ -753,6 +753,10 @@ struct redisCommand redisCommandTable[] = {
      "ok-stale ok-loading fast @connection @replication",
      0,NULL,0,0,0,0,0,0},
 
+     {"replping",pingCommand,-1,
+     "ok-stale fast @connection @replication",
+     0,NULL,0,0,0,0,0,0},
+
     {"echo",echoCommand,2,
      "fast @connection",
      0,NULL,0,0,0,0,0,0},
@@ -2768,6 +2772,7 @@ void createSharedObjects(void) {
     shared.lastid = makeObjectShared("LASTID",6);
     shared.default_username = makeObjectShared("default",7);
     shared.ping = makeObjectShared("ping",4);
+    shared.replping = makeObjectShared("replping", 8);
     shared.setid = makeObjectShared("SETID",5);
     shared.keepttl = makeObjectShared("KEEPTTL",7);
     shared.load = makeObjectShared("LOAD",4);

src/server.h

@@ -505,6 +505,7 @@ typedef enum {
     REPL_STATE_NONE = 0,            /* No active replication */
     REPL_STATE_CONNECT,             /* Must connect to master */
     REPL_STATE_CONNECTING,          /* Connecting to master */
+    REPL_STATE_RETRY_NOREPLPING,    /* Master does not support REPLPING, retry with PING */
     /* --- Handshake states, must be ordered --- */
     REPL_STATE_RECEIVE_PING_REPLY,  /* Wait for PING reply */
     REPL_STATE_SEND_HANDSHAKE,      /* Send handshake sequance to master */
@@ -1283,7 +1284,7 @@ struct sharedObjectsStruct {
     *emptyscan, *multi, *exec, *left, *right, *hset, *srem, *xgroup, *xclaim,  
     *script, *replconf, *eval, *persist, *set, *pexpireat, *pexpire, 
     *time, *pxat, *px, *retrycount, *force, *justid, 
-    *lastid, *ping, *setid, *keepttl, *load, *createconsumer,
+    *lastid, *ping, *replping, *setid, *keepttl, *load, *createconsumer,
     *getack, *special_asterick, *special_equals, *default_username,
     *hdel, *zrem, *mvccrestore, *pexpirememberat,
     *select[PROTO_SHARED_SELECT_CMDS],