[6.x] Keep which indexes are used when the Stache is cleared

src/Stache/Stores/Store.php

@@ -406,8 +406,8 @@ public function clear()
             app('stache.indexes')->forget("{$this->key()}.{$index->name()}");
         });
 
-        $this->usedIndexes = collect();
-        Stache::cacheStore()->forget($this->indexUsageCacheKey());
+        // Which indexes queries use is kept. It's what tells the next warm to build them,
+        // rather than leaving the first requests to build each one from every item.
 
         $this->clearCachedPaths();
 

tests/Stache/StoreTest.php

@@ -125,6 +125,39 @@ public function index_usage_cached_by_another_process_is_kept()
 
         $this->assertEquals(['path', 'site', 'uri'], Cache::get($cacheKey));
     }
+
+    #[Test]
+    public function clearing_keeps_which_indexes_are_used_so_the_next_warm_builds_them()
+    {
+        $cacheKey = "stache::indexes::{$this->store->key()}::_indexes";
+
+        $this->store->cacheIndexUsage('title');
+
+        $this->store->clear();
+
+        $this->assertEquals(['title'], Cache::get($cacheKey));
+        $this->assertTrue($this->store->indexes()->has('title'));
+
+        // A process that starts after the clear, as the one that warms usually is.
+        $this->assertTrue((new TestStore)->indexes()->has('title'));
+    }
+
+    #[Test]
+    public function an_index_used_again_after_clearing_is_still_recorded()
+    {
+        // Asset containers are cleared on every request when the watcher is on, and use
+        // their path index straight after. See https://github.com/statamic/cms/issues/3476
+        $cacheKey = "stache::indexes::{$this->store->key()}::_indexes";
+
+        $this->store->cacheIndexUsage('path');
+
+        $this->store->clear();
+
+        $this->store->cacheIndexUsage('path');
+
+        $this->assertEquals(['path'], Cache::get($cacheKey));
+        $this->assertTrue((new TestStore)->indexes()->has('path'));
+    }
 }
 
 class TestStore extends Store