Fix wrong BitSet result in MemoryAccountingBitsetCollector

lucene/CHANGES.txt

@@ -251,6 +251,8 @@ Bug Fixes
 * GITHUB#15731: Fix tessellator failure when a hole is touching another hole in the middle of a segment. In this case
   we might fail to detect it and choose a wrong bridge to eliminate the hole. (Ignacio Vera)
 
+* GITHUB#15656: Fix wrong BitSet result in MemoryAccountingBitsetCollector (Binlong Gao)
+
 Other
 ---------------------
 * GITHUB#15586: Document that scoring and ranking may change across major Lucene versions, and that applications requiring stable ranking should explicitly configure Similarity. (Parveen Saini)

lucene/misc/src/java/org/apache/lucene/misc/search/MemoryAccountingBitsetCollector.java

@@ -39,6 +39,7 @@ public MemoryAccountingBitsetCollector(CollectorMemoryTracker tracker) {
 
   @Override
   protected void doSetNextReader(LeafReaderContext context) throws IOException {
+    docBase = context.docBase;
     length += context.reader().maxDoc();
     FixedBitSet newBitSet = FixedBitSet.ensureCapacity(bitSet, length);
     if (newBitSet != bitSet) {

lucene/misc/src/test/org/apache/lucene/misc/search/TestMemoryAccountingBitsetCollector.java

@@ -70,4 +70,18 @@ public void testMemoryAccountingBitsetCollectorMemoryLimit() {
           searcher.search(MatchAllDocsQuery.INSTANCE, bitSetCollector);
         });
   }
+
+  public void testCollectedResult() throws Exception {
+    CollectorMemoryTracker tracker =
+        new CollectorMemoryTracker("testMemoryTracker", Long.MAX_VALUE);
+    MemoryAccountingBitsetCollector collector = new MemoryAccountingBitsetCollector(tracker);
+
+    IndexSearcher searcher = new IndexSearcher(reader);
+    searcher.search(MatchAllDocsQuery.INSTANCE, collector);
+
+    assertEquals(1000, collector.bitSet.cardinality());
+    for (int i = 0; i < 1000; i++) {
+      assertTrue(collector.bitSet.get(i));
+    }
+  }
 }