[#76] Add UNIQUE keyword

Signed-off-by: Alejandro Lopez <a.lopez@yadro.com>

netmap/netmap.go

@@ -181,6 +181,10 @@ func (m NetMap) ContainerNodes(p PlacementPolicy, pivot []byte) ([][]NodeInfo, e
 
 	result := make([][]NodeInfo, len(p.replicas))
 
+	// Note that the cached selectors are not used when the policy contains the UNIQUE flag.
+	// This is necessary because each selection vector affects potentially the subsequent vectors
+	// and thus we call getSelection in such case, in order to take into account nodes previously
+	// marked as used by earlier replicas.
 	for i := range p.replicas {
 		sName := p.replicas[i].GetSelector()
 		if sName == "" {
@@ -198,18 +202,39 @@ func (m NetMap) ContainerNodes(p PlacementPolicy, pivot []byte) ([][]NodeInfo, e
 			}
 
 			for i := range p.selectors {
-				result[i] = append(result[i], flattenNodes(c.selections[p.selectors[i].GetName()])...)
+				if p.unique {
+					nodes, err := c.getSelection(p.selectors[i])
+					if err != nil {
+						return nil, err
+					}
+					result[i] = append(result[i], flattenNodes(nodes)...)
+				} else {
+					result[i] = append(result[i], flattenNodes(c.selections[p.selectors[i].GetName()])...)
+				}
+			}
+
+			if p.unique {
+				c.addUsedNodes(result[i]...)
 			}
 
 			continue
 		}
 
-		nodes, ok := c.selections[sName]
-		if !ok {
-			return nil, fmt.Errorf("selector not found: REPLICA '%s'", sName)
+		if p.unique {
+			nodes, err := c.getSelection(*c.processedSelectors[sName])
+			if err != nil {
+				return nil, err
+			}
+			result[i] = append(result[i], flattenNodes(nodes)...)
+			c.addUsedNodes(result[i]...)
+		} else {
+			nodes, ok := c.selections[sName]
+			if !ok {
+				return nil, fmt.Errorf("selector not found: REPLICA '%s'", sName)
+			}
+			result[i] = append(result[i], flattenNodes(nodes)...)
 		}
 
-		result[i] = append(result[i], flattenNodes(nodes)...)
 	}
 
 	return result, nil