store: new error type (noElementsLeft)

coordinator/store/etcdstore.go

@@ -243,10 +243,10 @@ type EtcdIterator struct {
 	keys []string
 }
 
-// GetNext gets the next element.
+// GetNext returns the next element of the iterator.
 func (i *EtcdIterator) GetNext() (string, error) {
 	if i.idx >= len(i.keys) {
-		return "", fmt.Errorf("index out of range [%d] with length %d", i.idx, len(i.keys))
+		return "", &NoElementsLeftError{idx: i.idx}
 	}
 	key := i.keys[i.idx]
 	i.idx++

coordinator/store/stdstore.go

@@ -1,7 +1,6 @@
 package store
 
 import (
-	"fmt"
 	"strings"
 	"sync"
 )
@@ -170,11 +169,11 @@ type StdIterator struct {
 // GetNext returns the next element of the iterator.
 func (i *StdIterator) GetNext() (string, error) {
 	if i.idx >= len(i.keys) {
-		return "", fmt.Errorf("index out of range [%d] with length %d", i.idx, len(i.keys))
+		return "", &NoElementsLeftError{idx: i.idx}
 	}
-	val := i.keys[i.idx]
+	key := i.keys[i.idx]
 	i.idx++
-	return val, nil
+	return key, nil
 }
 
 // HasNext returns true if there are elements left to get with GetNext().

coordinator/store/store.go

@@ -53,3 +53,14 @@ type ValueUnsetError struct {
 func (s *ValueUnsetError) Error() string {
 	return fmt.Sprintf("store: requested value not set: %s", s.requestedValue)
 }
+
+// NoElementsLeftError occurs when trying to get an element from an interator that
+// doesn't have elements left.
+type NoElementsLeftError struct {
+	idx int
+}
+
+// Error implements the Error interface.
+func (n *NoElementsLeftError) Error() string {
+	return fmt.Sprintf("index out of range [%d]", n.idx)
+}