Add depends_on field for core service ordering

* Adds depends_on fields to compose YAML
* Updates parsing code to copy across depends_on field to
openfaas service from compose service definition
* Adds algorithm and unit tests for finding order
* Applies order to up.go command
* Makes unit testing on MacOS possible through build directives

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <alexellis2@gmail.com>

pkg/graph.go

@@ -0,0 +1,69 @@
+package pkg
+
+import "log"
+
+// resolve finds the order of dependencies for a graph
+// of nodes.
+// Inspired by algorithm from
+// https://www.electricmonk.nl/log/2008/08/07/dependency-resolving-algorithm/
+func resolve(node *Node, resolved, unresolved *Graph) {
+	unresolved.nodes = append(unresolved.nodes, node)
+
+	for _, edge := range node.Edges {
+
+		if !resolved.Contains(edge) && unresolved.Contains(edge) {
+			log.Panicf("edge: %s may be a circular dependency", edge.Name)
+		}
+
+		resolve(edge, resolved, unresolved)
+	}
+
+	for _, r := range resolved.nodes {
+		if r.Name == node.Name {
+			return
+		}
+	}
+
+	resolved.nodes = append(resolved.nodes, node)
+
+	unresolved.Remove(node)
+}
+
+func newNode(name string, edges []*Node) *Node {
+	return &Node{
+		Name:  name,
+		Edges: edges,
+	}
+}
+
+type Node struct {
+	Name  string
+	Edges []*Node
+}
+
+type Graph struct {
+	nodes []*Node
+}
+
+func (g *Graph) Contains(target *Node) bool {
+	for _, g := range g.nodes {
+		if g.Name == target.Name {
+			return true
+		}
+	}
+
+	return false
+}
+func (g *Graph) Remove(target *Node) {
+	var found *int
+	for i, n := range g.nodes {
+		if n == target {
+			found = &i
+			break
+		}
+	}
+
+	if found != nil {
+		g.nodes = append(g.nodes[:*found], g.nodes[*found+1:]...)
+	}
+}