From 1a5f7ce6ad4bf96548e65e597d8c01456cd91c25 Mon Sep 17 00:00:00 2001
From: Aleksandr Sarantsev <99037063+ephemeralsad@users.noreply.github.com>
Date: Tue, 17 Jun 2025 14:09:55 +0400
Subject: [PATCH] storcon: Exclude another secondaries while optimizing
 secondary (#12251)

## Problem

If the node intent includes more than one secondary, we can generate a
replace optimization using a candidate node that is already a secondary
location.

## Summary of changes

- Exclude all other secondary nodes from the scoring process to ensure
optimal candidate selection.
---
 storage_controller/src/tenant_shard.rs | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/storage_controller/src/tenant_shard.rs b/storage_controller/src/tenant_shard.rs
index c7b2628ec4..d6046e062e 100644
--- a/storage_controller/src/tenant_shard.rs
+++ b/storage_controller/src/tenant_shard.rs
@@ -1184,11 +1184,19 @@ impl TenantShard {
         for secondary in self.intent.get_secondary() {
             // Make sure we don't try to migrate a secondary to our attached location: this case happens
             // easily in environments without multiple AZs.
-            let exclude = match self.intent.attached {
+            let mut exclude = match self.intent.attached {
                 Some(attached) => vec![attached],
                 None => vec![],
             };
 
+            // Exclude all other secondaries from the scheduling process to avoid replacing
+            // one existing secondary with another existing secondary.
+            for another_secondary in self.intent.secondary.iter() {
+                if another_secondary != secondary {
+                    exclude.push(*another_secondary);
+                }
+            }
+
             let replacement = match &self.policy {
                 PlacementPolicy::Attached(_) => {
                     // Secondaries for an attached shard should be scheduled using `SecondaryShardTag`