PSYNC2: stop sending newlines to sub-slaves when master is down.

This actually includes two changes:

1) No newlines to take the master-slave link up when the upstream master
is down. Doing this is dangerous because the sub-slave often is received
replication protocol for an half-command, so can't receive newlines
without desyncing the replication link, even with the code in order to
cancel out the bytes that PSYNC2 was using. Moreover this is probably
also not needed/sane, because anyway the slave can keep serving
requests, and because if it's configured to don't serve stale data, it's
a good idea, actually, to break the link.

2) When a +CONTINUE with a different ID is received, we now break
connection with the sub-slaves: they need to be notified as well. This
was part of the original specification but for some reason it was not
implemented in the code, and was alter found as a PSYNC2 bug in the
integration testing.

src/networking.c

@@ -1031,7 +1031,7 @@ int processInlineBuffer(client *c) {
     char *newline;
     int argc, j;
     sds *argv, aux;
-    size_t querylen, protolen;
+    size_t querylen;
 
     /* Search for end of line */
     newline = strchr(c->querybuf,'\n');
@@ -1044,7 +1044,6 @@ int processInlineBuffer(client *c) {
         }
         return C_ERR;
     }
-    protolen = (newline - c->querybuf)+1; /* Total protocol bytes of command. */
 
     /* Handle the \r\n case. */
     if (newline && newline != c->querybuf && *(newline-1) == '\r')
@@ -1067,15 +1066,6 @@ int processInlineBuffer(client *c) {
     if (querylen == 0 && c->flags & CLIENT_SLAVE)
         c->repl_ack_time = server.unixtime;
 
-    /* Newline from masters can be used to prevent timeouts, but should
-     * not affect the replication offset since they are always sent
-     * "out of band" directly writing to the socket and without passing
-     * from the output buffers. */
-    if (querylen == 0 && c->flags & CLIENT_MASTER) {
-        c->reploff -= protolen;
-        while (protolen--) chopReplicationBacklog();
-    }
-
     /* Leave data after the first line of the query in the buffer */
     sdsrange(c->querybuf,querylen+2,-1);
 

src/replication.c

@@ -148,22 +148,6 @@ void feedReplicationBacklog(void *ptr, size_t len) {
                               server.repl_backlog_histlen + 1;
 }
 
-/* Remove the last byte from the replication backlog. This
- * is useful when we receive an out of band "\n" to keep the connection
- * alive but don't want to count it as replication stream.
- *
- * As a side effect this function adjusts the master replication offset
- * of this instance to account for the missing byte. */
-void chopReplicationBacklog(void) {
-    if (!server.repl_backlog || !server.repl_backlog_histlen) return;
-    if (server.repl_backlog_idx == 0)
-        server.repl_backlog_idx = server.repl_backlog_size-1;
-    else
-        server.repl_backlog_idx--;
-    server.master_repl_offset--;
-    server.repl_backlog_histlen--;
-}
-
 /* Wrapper for feedReplicationBacklog() that takes Redis string objects
  * as input. */
 void feedReplicationBacklogWithObject(robj *o) {
@@ -1530,6 +1514,9 @@ int slaveTryPartialResynchronization(int fd, int read_reply) {
                  * new one. */
                 memcpy(server.replid,new,sizeof(server.replid));
                 memcpy(server.cached_master->replid,new,sizeof(server.replid));
+
+                /* Disconnect all the sub-slaves: they need to be notified. */
+                disconnectSlaves();
             }
         }
 
@@ -2553,10 +2540,8 @@ void replicationCron(void) {
             (slave->replstate == SLAVE_STATE_WAIT_BGSAVE_START ||
             (slave->replstate == SLAVE_STATE_WAIT_BGSAVE_END &&
              server.rdb_child_type != RDB_CHILD_TYPE_SOCKET));
-        int is_subslave = server.masterhost && server.master == NULL &&
-                          slave->replstate == SLAVE_STATE_ONLINE;
 
-        if (is_presync || is_subslave) {
+        if (is_presync) {
             if (write(slave->fd, "\n", 1) == -1) {
                 /* Don't worry about socket errors, it's just a ping. */
             }

tests/integration/psync2.tcl

@@ -87,6 +87,7 @@ start_server {} {
                     set slave_id [randomInt 5]
                     if {$disconnect} {
                         $R($slave_id) client kill type master
+                        puts "+++ Breaking link for slave #$slave_id"
                     }
                 }
             }