[6.x] Stop parallel Stache warming losing cached index usages

src/Stache/Stores/Store.php

@@ -121,9 +121,13 @@ public function cacheIndexUsage($index)
 
         $indexes->push($index);
 
-        $this->usedIndexes = $indexes;
+        // Another process may have recorded usages since this one memoized its
+        // list (e.g. parallel warming), so merge with the cache rather than replace it.
+        $cached = collect(Stache::cacheStore()->get($key = $this->indexUsageCacheKey(), []));
 
-        Stache::cacheStore()->put($this->indexUsageCacheKey(), $indexes->all());
+        $this->usedIndexes = $cached->merge($indexes)->unique()->sort()->values();
+
+        Stache::cacheStore()->put($key, $this->usedIndexes->all());
     }
 
     protected function indexUsageCacheKey()

tests/Stache/StoreTest.php

@@ -107,6 +107,24 @@ public function it_gets_the_paths_from_the_cache_again_after_memoized_state_is_r
         $this->assertEquals($expected, $store->paths());
         $this->assertEquals(2, $cacheHits);
     }
+
+    #[Test]
+    public function index_usage_cached_by_another_process_is_kept()
+    {
+        // Two instances of the same store stand in for two processes warming
+        // the Stache in parallel. Each memoizes its own usage list, so the
+        // second write must merge with the cache instead of replacing it.
+        $cacheKey = "stache::indexes::{$this->store->key()}::_indexes";
+        $other = new TestStore;
+
+        $this->store->cacheIndexUsage('path');
+        $other->cacheIndexUsage('path');
+
+        $this->store->cacheIndexUsage('site');
+        $other->cacheIndexUsage('uri');
+
+        $this->assertEquals(['path', 'site', 'uri'], Cache::get($cacheKey));
+    }
 }
 
 class TestStore extends Store