Address review comments

graph/iterator/and_iterator.go

@@ -29,8 +29,8 @@ type And struct {
 	primaryIt         graph.Iterator
 	checkList         []graph.Iterator
 	result            graph.Value
-	err               error
 	runstats          graph.IteratorStats
+	err               error
 }
 
 // Creates a new And iterator.
@@ -154,9 +154,7 @@ func (it *And) Next() bool {
 			return graph.NextLogOut(it, curr, true)
 		}
 	}
-	if err := it.primaryIt.Err(); err != nil {
-		it.err = err
-	}
+	it.err = it.primaryIt.Err()
 	return graph.NextLogOut(it, nil, false)
 }
 
@@ -190,14 +188,26 @@ func (it *And) checkContainsList(val graph.Value, lastResult graph.Value) bool {
 	for i, c := range it.checkList {
 		ok = c.Contains(val)
 		if !ok {
-			if err := c.Err(); err != nil {
-				it.err = err
+			it.err = c.Err()
+			if it.err != nil {
 				return false
 			}
+
 			if lastResult != nil {
 				for j := 0; j < i; j++ {
-					// TODO(andrew-d): Should this result actually be used?
+					// One of the iterators has determined that this value doesn't
+					// match. However, the iterators that came before in the list
+					// may have returned "ok" to Contains().  We need to set all
+					// the tags back to what the previous result was -- effectively
+					// seeking back exactly one -- so we check all the prior iterators
+					// with the (already verified) result and throw away the result,
+					// which will be 'true'
 					it.checkList[j].Contains(lastResult)
+
+					it.err = it.checkList[j].Err()
+					if it.err != nil {
+						return false
+					}
 				}
 			}
 			break
@@ -254,16 +264,17 @@ func (it *And) NextPath() bool {
 	if it.primaryIt.NextPath() {
 		return true
 	}
-	if err := it.primaryIt.Err(); err != nil {
-		it.err = err
+	it.err = it.primaryIt.Err()
+	if it.err != nil {
 		return false
 	}
 	for _, sub := range it.internalIterators {
 		if sub.NextPath() {
 			return true
 		}
-		if err := sub.Err(); err != nil {
-			it.err = err
+
+		it.err = sub.Err()
+		if it.err != nil {
 			return false
 		}
 	}
@@ -275,23 +286,20 @@ func (it *And) cleanUp() {}
 
 // Close this iterator, and, by extension, close the subiterators.
 // Close should be idempotent, and it follows that if it's subiterators
-// follow this contract, the And follows the contract.
-//
-// Note: as this potentially involves closing multiple subiterators, only
-// the first error encountered while closing will be reported (if any).
+// follow this contract, the And follows the contract.  It closes all
+// subiterators it can, but returns the first error it encounters.
 func (it *And) Close() error {
 	it.cleanUp()
 
-	var ret error
-
-	ret = it.primaryIt.Close()
+	err := it.primaryIt.Close()
 	for _, sub := range it.internalIterators {
-		if err := sub.Close(); err != nil && ret != nil {
-			ret = err
+		serr := sub.Close()
+		if serr != nil && err == nil {
+			err = serr
 		}
 	}
 
-	return ret
+	return err
 }
 
 // Register this as an "and" iterator.