[3.3] Bugfix: Distinct collect block could create an empty result block (#5709)

arangod/Aql/CollectBlock.cpp

@@ -1016,8 +1016,15 @@ int DistinctCollectBlock::getOrSkipSome(size_t atLeast, size_t atMost,
             }
 
             TRI_ASSERT(cur != nullptr);
-            res->shrink(skipped);
-          } 
+            if (skipped > 0) {
+              res->shrink(skipped);
+            }
+            else {
+              AqlItemBlock* block = res.get();
+              returnBlock(block);
+              res.release();
+            }
+          }
           returnBlock(cur);
           _done = true;
           result = res.release();
@@ -1035,8 +1042,14 @@ int DistinctCollectBlock::getOrSkipSome(size_t atLeast, size_t atMost,
   }
 
   if (!skipping) {
-    TRI_ASSERT(skipped > 0);
-    res->shrink(skipped);
+    if (skipped > 0) {
+      res->shrink(skipped);
+    }
+    else {
+      AqlItemBlock* block = res.get();
+      returnBlock(block);
+      res.release();
+    }
   }
 
   result = res.release();

js/server/tests/aql/aql-distinct.js

@@ -300,7 +300,42 @@ function ahuacatlCollect () {
       var query = "FOR result IN UnitTestsCollection COLLECT key=result.a.key, name = result.a.name INTO group LIMIT 2,5 RETURN [key,name]";
       var result = AQL_EXECUTE(query).json;
       assertEqual([[0,2],[0,3],[0,4],[0,5],[0,6]], result);
-    }
+    },
+
+////////////////////////////////////////////////////////////////////////////////
+/// @brief distinct regression test:
+/// crash when a subsequent call to DistinctCollectBlock::getSome() added no new
+/// non-distinct results. The crash may occur ONLY IN MAINTAINER MODE, while
+/// in non-maintainer mode problems may occur only because either an empty
+/// result is returned (which would cause some parent blocks to crash), or
+/// an erroneous result is returned, but I didn't find a way to produce this
+/// reliably with an AQL query.
+////////////////////////////////////////////////////////////////////////////////
+
+    testDistinctRegressionGetSomeWithEmptyResult : function () {
+      const query = 'FOR i IN 1..2 FOR k IN 1..1000 RETURN DISTINCT k';
+      containsDistinct(query);
+      const options = {optimizer: {rules: [ "-interchange-adjacent-enumerations" ]}};
+
+      // check plan
+      const plan = helper.getCompactPlan(AQL_EXPLAIN(query, {}, options));
+      assertEqual(
+        [
+          "SingletonNode",
+          "CalculationNode",
+          "CalculationNode",
+          "EnumerateListNode",
+          "EnumerateListNode",
+          "CollectNode",
+          "ReturnNode",
+        ],
+        plan.map(node => node.type)
+      );
+
+      // execute query
+      const result = AQL_EXECUTE(query, {}, options).json;
+      assertEqual(1000, result.length);
+    },
 
   };
 }