prevent stealing of values from Aql const registers

CHANGELOG

@@ -1,6 +1,11 @@
 devel
 -----
 
+* Prevent stealing of values from AQL const value registers. This fixes an
+  issue for queries that produce constant results (known at query compile time)
+  when the queries are executed directly on a DB server in a cluster (which is
+  not supported, but may happen for troubleshooting).
+
 * Fixed BTS-562: reduce-extraction-to-projection optimization returns null for 
   one attribute if nested attributes are named the same.
 

arangod/Aql/InputAqlItemRow.cpp

@@ -157,6 +157,11 @@ AqlValue InputAqlItemRow::stealValue(RegisterId registerId) {
   TRI_ASSERT(registerId.isConstRegister() || registerId < getNumRegisters());
   AqlValue const& a = block().getValueReference(_baseIndex, registerId);
   if (!a.isEmpty() && a.requiresDestruction()) {
+    if (registerId.isConstRegister()) {
+      // we cannot steal the value of a const register!
+      return a.clone();
+    }
+
     // Now no one is responsible for AqlValue a
     block().steal(a);
   }

tests/js/client/shell/shell-query-dbserver-cluster.js

@@ -44,7 +44,7 @@ function queriesTestSuite () {
   'use strict';
 
   return {
-    setUp: function() {
+    setUpAll: function() {
       arango.reconnect(originalEndpoint, "_system", arango.connectedUser(), "");
       db._drop(cn);
 
@@ -56,10 +56,18 @@ function queriesTestSuite () {
       c.insert(docs);
     },
 
-    tearDown: function() {
+    tearDownAll: function() {
       arango.reconnect(originalEndpoint, "_system", arango.connectedUser(), "");
       db._drop(cn);
     },
+
+    setUp: function() {
+      arango.reconnect(originalEndpoint, "_system", arango.connectedUser(), "");
+    },
+
+    tearDown: function() {
+      arango.reconnect(originalEndpoint, "_system", arango.connectedUser(), "");
+    },
 
     // test executing operations on the coordinator
     testCoordinator: function() {
@@ -97,6 +105,39 @@ function queriesTestSuite () {
       assertEqual(100, totalToArray);
       assertEqual(100, totalQuery);
     },
+
+    // test executing operations on the coordinator, without collection
+    testCoordinatorNoCollection: function() {
+      let result = db._query("RETURN [DECODE_REV('_dpq8a-----'), DECODE_REV('_bpq8a-----')]").toArray();
+
+      assertEqual([
+        [ 
+          { "date" : "2022-02-02T16:22:18.368Z", "count" : 0 }, 
+          { "date" : "2021-01-01T00:00:00.000Z", "count" : 0 } 
+        ]
+      ], result);
+    },
+
+    // test executing operations on the DB-Server, without collection
+    testDBServerNoCollection: function() {
+      const dbservers = getServers("dbserver");
+      assertTrue(dbservers.length > 0, "no dbservers found");
+
+      dbservers.forEach(function(dbserver, i) {
+        let id = dbserver.id;
+        require("console").warn("connecting to dbserver", dbserver.endpoint, id);
+        arango.reconnect(dbserver.endpoint, "_system", arango.connectedUser(), "");
+
+        let result = db._query("RETURN [DECODE_REV('_dpq8a-----'), DECODE_REV('_bpq8a-----')]").toArray();
+
+        assertEqual([
+          [ 
+            { "date" : "2022-02-02T16:22:18.368Z", "count" : 0 }, 
+            { "date" : "2021-01-01T00:00:00.000Z", "count" : 0 } 
+          ]
+        ], result);
+      });
+    },
 
   };
 }