From 6cab6ffe490f8d71e845d4613ccb3307e2b78f48 Mon Sep 17 00:00:00 2001
From: Oleg Jukovec <oleg.jukovec@tarantool.org>
Date: Wed, 26 Feb 2025 14:56:55 +0300
Subject: [PATCH] core: failed to start if box.info.replication with gaps

The iterator does not handle the case where there are no records in
the box.info.replication table in case of deleting old records,
for examle:

```
- 1:
    id: 1
    uuid: 77f30d9a-d344-4f7a-a18c-578fb5ec2540
    lsn: 4
    name: null
  3:
    id: 3
    uuid: 0ce528ca-ea35-42be-96b5-35ba610f9a3e
    lsn: 0
    downstream:
      status: follow
      idle: 0.14953243732452
      vclock: {1: 4}
      lag: 0.0015106201171875
    name: null
```

It leads to the error and unable to start queue:

```
queue_state.lua:50: attempt to index a nil value
```
---
 CHANGELOG.md                   | 1 +
 queue/abstract/queue_state.lua | 5 +++--
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index bb89619..42a041a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -14,6 +14,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 ### Fixed
 
 - Grant method was added for `*_ready_buffer` spaces (#237).
+- Attempt to index a nil value if box.info.replication array has gaps.
 
 ## [1.4.2] - 2024-08-10
 
diff --git a/queue/abstract/queue_state.lua b/queue/abstract/queue_state.lua
index b61b531..289a7d3 100644
--- a/queue/abstract/queue_state.lua
+++ b/queue/abstract/queue_state.lua
@@ -47,8 +47,9 @@ local function max_lag()
     local n_replica = table.maxn(box.info.replication)
 
     for i = 1, n_replica do
-        if box.info.replication[i].upstream then
-            local lag = box.info.replication[i].upstream.lag
+        local replica = box.info.replication[i]
+        if replica and replica.upstream then
+            local lag = replica.upstream.lag
             if lag > max_lag then
                 max_lag = lag
             end