Allow blocked XREAD on a cluster replica (#7881)

I suppose that it was overlooked, since till recently none of the blocked commands were readonly.

other changes:
- add test for the above.
- add better support for additional (and deferring) clients for
  cluster tests
- improve a test which left the client in MULTI state.

src/cluster.c

@@ -5855,6 +5855,15 @@ int clusterRedirectBlockedClientIfNeeded(client *c) {
             int slot = keyHashSlot((char*)key->ptr, sdslen(key->ptr));
             clusterNode *node = server.cluster->slots[slot];
 
+            /* if the client is read-only and attempting to access key that our
+             * replica can handle, allow it. */
+            if ((c->flags & CLIENT_READONLY) &&
+                (c->lastcmd->flags & CMD_READONLY) &&
+                nodeIsSlave(myself) && myself->slaveof == node)
+            {
+                node = myself;
+            }
+
             /* We send an error and unblock the client if:
              * 1) The slot is unassigned, emitting a cluster down error.
              * 2) The slot is not handled by this node, nor being imported. */

tests/cluster/tests/16-transactions-on-replica.tcl

@@ -45,4 +45,25 @@ test "MULTI-EXEC with write operations is MOVED" {
     $replica MULTI
     catch {$replica HSET h b 4} err
     assert {[string range $err 0 4] eq {MOVED}}
+    catch {$replica exec} err
+    assert {[string range $err 0 8] eq {EXECABORT}}
+}
+
+test "read-only blocking operations from replica" {
+    set rd [redis_deferring_client redis 1]
+    $rd readonly
+    $rd read
+    $rd XREAD BLOCK 0 STREAMS k 0
+
+    wait_for_condition 1000 50 {
+        [RI 1 blocked_clients] eq {1}
+    } else {
+        fail "client wasn't blocked"
+    }
+
+    $primary XADD k * foo bar
+    set res [$rd read]
+    set res [lindex [lindex [lindex [lindex $res 0] 1] 0] 1]
+    assert {$res eq {foo bar}}
+    $rd close
 }

tests/instances.tcl

@@ -606,3 +606,16 @@ proc restart_instance {type id} {
     }
 }
 
+proc redis_deferring_client {type id} {
+    set port [get_instance_attrib $type $id port]
+    set host [get_instance_attrib $type $id host]
+    set client [redis $host $port 1 $::tls]
+    return $client
+}
+
+proc redis_client {type id} {
+    set port [get_instance_attrib $type $id port]
+    set host [get_instance_attrib $type $id host]
+    set client [redis $host $port 0 $::tls]
+    return $client
+}