minor rename to MetadataRowReadTracker and added a test from PR comment

Author: Ed Coleman

server/gc/src/main/java/org/apache/accumulo/gc/GarbageCollectionAlgorithm.java

@@ -176,7 +176,7 @@ private void confirmDeletes(GarbageCollectionEnvironment gce,
 
     // it is important that the tracker is closed before performing deletes so that last row is
     // checked
-    try (MetadataReadTracker readTracker = new MetadataReadTracker()) {
+    try (MetadataRowReadTracker readTracker = new MetadataRowReadTracker()) {
       Iterator<Entry<Key,Value>> iter = gce.getReferenceIterator();
       while (iter.hasNext()) {
         Entry<Key,Value> entry = iter.next();
@@ -364,14 +364,14 @@ public void collect(GarbageCollectionEnvironment gce)
    * Track metadata rows read to help validate that gc scan has complete information to make a
    * decision on deleting files
    */
-  private static class MetadataReadTracker implements AutoCloseable {
+  private static class MetadataRowReadTracker implements AutoCloseable {
     private boolean hasDir = false;
     private boolean hasPrevRow = false;
     private Text row;
 
     private boolean closed = false;
 
-    public MetadataReadTracker() {
+    public MetadataRowReadTracker() {
       this.row = null;
     }
 

server/gc/src/test/java/org/apache/accumulo/gc/GarbageCollectionTest.java

@@ -870,4 +870,14 @@ public void testPrevRowOnly() {
     assertEquals(3, gce.candidates.size());
   }
 
+  @Test
+  public void testNoPrevRowNoDir() throws Exception {
+
+    GarbageCollectionAlgorithm gca = new GarbageCollectionAlgorithm();
+
+    TestGCE gce = new TestGCE();
+    gce.candidates.add("/1636/default_tablet");
+    gce.addFileReference("b", "m", "hdfs://foo.com:6000/user/foo/tables/b/t-0/F00.rf");
+    assertThrows(IllegalStateException.class, () -> gca.collect(gce));
+  }
 }