generate systemd: support pods and geneartig files

Support generating systemd unit files for a pod.  Podman generates one
unit file for the pod including the PID file for the infra container's
conmon process and one unit file for each container (excluding the infra
container).

Note that this change implies refactorings in the `pkg/systemdgen` API.

Signed-off-by: Valentin Rothberg <rothberg@redhat.com>

libpod/container_graph.go

@@ -16,14 +16,30 @@ type containerNode struct {
 	dependedOn []*containerNode
 }
 
-type containerGraph struct {
+// ContainerGraph is a dependency graph based on a set of containers.
+type ContainerGraph struct {
 	nodes              map[string]*containerNode
 	noDepNodes         []*containerNode
 	notDependedOnNodes map[string]*containerNode
 }
 
-func buildContainerGraph(ctrs []*Container) (*containerGraph, error) {
-	graph := new(containerGraph)
+// DependencyMap returns the dependency graph as map with the key being a
+// container and the value being the containers the key depends on.
+func (cg *ContainerGraph) DependencyMap() (dependencies map[*Container][]*Container) {
+	dependencies = make(map[*Container][]*Container)
+	for _, node := range cg.nodes {
+		dependsOn := make([]*Container, len(node.dependsOn))
+		for i, d := range node.dependsOn {
+			dependsOn[i] = d.container
+		}
+		dependencies[node.container] = dependsOn
+	}
+	return dependencies
+}
+
+// BuildContainerGraph builds a dependency graph based on the container slice.
+func BuildContainerGraph(ctrs []*Container) (*ContainerGraph, error) {
+	graph := new(ContainerGraph)
 	graph.nodes = make(map[string]*containerNode)
 	graph.notDependedOnNodes = make(map[string]*containerNode)
 
@@ -78,7 +94,7 @@ func buildContainerGraph(ctrs []*Container) (*containerGraph, error) {
 // Detect cycles in a container graph using Tarjan's strongly connected
 // components algorithm
 // Return true if a cycle is found, false otherwise
-func detectCycles(graph *containerGraph) (bool, error) {
+func detectCycles(graph *ContainerGraph) (bool, error) {
 	type nodeInfo struct {
 		index   int
 		lowLink int