mjwall commented on a change in pull request #2293:
URL: https://github.com/apache/accumulo/pull/2293#discussion_r735164620



##########
File path: 
server/gc/src/main/java/org/apache/accumulo/gc/GarbageCollectionAlgorithm.java
##########
@@ -217,11 +221,43 @@ private void confirmDeletes(GarbageCollectionEnvironment 
gce,
         throw new RuntimeException(
             "Scanner over metadata table returned unexpected column : " + 
entry.getKey());
     }
+    Set<String> tableIdsAfter = gce.getTableIDs();
+    ensureAllTablesChecked(Collections.unmodifiableSet(tableIdsBefore),
+        Collections.unmodifiableSet(tableIdsSeen), 
Collections.unmodifiableSet(tableIdsAfter));
 
     confirmDeletesFromReplication(gce.getReplicationNeededIterator(),
         candidateMap.entrySet().iterator());
   }
 
+  @VisibleForTesting
+  /**
+   *
+   */
+  protected void ensureAllTablesChecked(Set<String> tableIdsBefore, 
Set<String> tableIdsSeen,
+      Set<String> tableIdsAfter) {
+
+    // if a table was added or deleted during this run, it is acceptable to not
+    // have seen those tables ids when scanning the metadata table. So get the 
intersection
+    Set<String> tableIdsMustHaveSeen = new HashSet<>(tableIdsBefore);
+    tableIdsMustHaveSeen.retainAll(tableIdsAfter);
+
+    if (tableIdsMustHaveSeen.isEmpty() && !tableIdsSeen.isEmpty()) {
+      // we saw no table ids in ZK but did in the metadata table. This is 
unexpected.
+      throw new RuntimeException(
+          "Saw no table ids in ZK but did see table ids in metadata table: " + 
tableIdsSeen);
+    }
+
+    // From that intersection, remove all the table ids that were seen.
+    tableIdsMustHaveSeen.removeAll(tableIdsSeen);
+
+    // If anything is left then we missed a table and may not have removed 
rfiles still in use

Review comment:
       should say "may not have removed rfile references from the list of 
candidates that are actually still in use", will update




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