fix: copy the catalog's stored name set instead of mutating it in place

nitrite/src/main/java/org/dizitart/no2/store/MapMetaData.java

@@ -51,9 +51,11 @@ public Document getInfo() {
 
     @SuppressWarnings("unchecked")
     private void populateInfo(Document metadata) {
-        mapNames = (Set<String>) metadata.get(TAG_MAP_METADATA, Set.class);
-        if (mapNames == null) {
-            mapNames = new HashSet<>();
-        }
+        // copy, never adopt: the set handed back here is the instance stored in the catalog
+        // page, and MVStore serializes pages on a background thread that any write may start.
+        // Adding to it in place raced that thread (ConcurrentModificationException inside
+        // HashSet.writeObject, then a store panic) whenever collections were created in a burst.
+        Set<String> stored = (Set<String>) metadata.get(TAG_MAP_METADATA, Set.class);
+        mapNames = stored == null ? new HashSet<>() : new HashSet<>(stored);
     }
 }

nitrite/src/test/java/org/dizitart/no2/store/StoreCatalogCopyOnWriteTest.java

@@ -0,0 +1,57 @@
+/*
+ * Copyright (c) 2017-2020. Nitrite author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.dizitart.no2.store;
+
+import org.dizitart.no2.collection.Document;
+import org.dizitart.no2.store.memory.InMemoryStore;
+import org.junit.Test;
+
+import java.util.Set;
+
+import static org.dizitart.no2.common.Constants.COLLECTION_CATALOG;
+import static org.dizitart.no2.common.Constants.TAG_COLLECTIONS;
+import static org.dizitart.no2.common.Constants.TAG_MAP_METADATA;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertNotSame;
+import static org.junit.Assert.assertTrue;
+
+/**
+ * The catalog must replace the stored name set on every write, never mutate it: MVStore hands
+ * out the stored instance and may be serializing it on another thread at that moment.
+ */
+public class StoreCatalogCopyOnWriteTest {
+    @Test
+    @SuppressWarnings("unchecked")
+    public void testWritingAnEntryLeavesTheStoredSetUntouched() {
+        InMemoryStore store = new InMemoryStore();
+        StoreCatalog catalog = new StoreCatalog(store);
+        catalog.writeCollectionEntry("first");
+
+        NitriteMap<String, Document> catalogMap = store.openMap(COLLECTION_CATALOG, String.class, Document.class);
+        Set<String> storedBefore = (Set<String>) catalogMap.get(TAG_COLLECTIONS).get(TAG_MAP_METADATA, Set.class);
+        assertEquals(Set.of("first"), storedBefore);
+
+        catalog.writeCollectionEntry("second");
+
+        Set<String> storedAfter = (Set<String>) catalogMap.get(TAG_COLLECTIONS).get(TAG_MAP_METADATA, Set.class);
+        assertNotSame("the write must store a new set, not the one a serializer may be reading", storedBefore, storedAfter);
+        assertFalse("the previously stored set must not have been mutated", storedBefore.contains("second"));
+        assertEquals(Set.of("first", "second"), storedAfter);
+        assertTrue(catalog.getCollectionNames().containsAll(Set.of("first", "second")));
+    }
+}