Use a dependency graph to start services

The algorithm is like so:
* get all the leaves of the graph, these are all the service that don't have any dependency
* once a service is started we take the list of its parents (dependents)
* if all the dependencies of each of those dependents are started then we can start it as well
* if not then we continue to the next dependent

Signed-off-by: Djordje Lukic <djordje.lukic@docker.com>

local/compose.go

@@ -174,7 +174,7 @@ func (s *local) Down(ctx context.Context, projectName string) error {
 		return err
 	}
 
-	eg, ctx := errgroup.WithContext(ctx)
+	eg, _ := errgroup.WithContext(ctx)
 	w := progress.ContextWriter(ctx)
 	for _, c := range list {
 		container := c
@@ -625,7 +625,7 @@ func (s *local) ensureNetwork(ctx context.Context, n types.NetworkConfig) error
 				StatusText: "Create",
 				Done:       false,
 			})
-			if _, err := s.containerService.apiClient.NetworkCreate(context.Background(), n.Name, createOpts); err != nil {
+			if _, err := s.containerService.apiClient.NetworkCreate(ctx, n.Name, createOpts); err != nil {
 				return errors.Wrapf(err, "failed to create network %s", n.Name)
 			}
 			w.Event(progress.Event{

local/convergence.go

@@ -57,7 +57,7 @@ func (s *local) ensureService(ctx context.Context, project *types.Project, servi
 
 	scale := getScale(service)
 
-	eg, ctx := errgroup.WithContext(ctx)
+	eg, _ := errgroup.WithContext(ctx)
 	if len(actual) < scale {
 		next, err := nextContainerNumber(actual)
 		if err != nil {
@@ -115,7 +115,7 @@ func (s *local) ensureService(ctx context.Context, project *types.Project, servi
 }
 
 func (s *local) waitDependencies(ctx context.Context, project *types.Project, service types.ServiceConfig) error {
-	eg, ctx := errgroup.WithContext(ctx)
+	eg, _ := errgroup.WithContext(ctx)
 	for dep, config := range service.DependsOn {
 		switch config.Condition {
 		case "service_healthy":

local/dependencies.go

@@ -20,99 +20,175 @@ package local
 
 import (
 	"context"
+	"fmt"
+	"sync"
 
 	"github.com/compose-spec/compose-go/types"
 	"golang.org/x/sync/errgroup"
 )
 
-func inDependencyOrder(ctx context.Context, project *types.Project, fn func(context.Context, types.ServiceConfig) error) error {
+type ServiceStatus int
-	graph := buildDependencyGraph(project.Services)
+
+const (
+	ServiceStopped ServiceStatus = iota
+	ServiceStarted
+)
+
+func inDependencyOrder(ctx context.Context, project *types.Project, fn func(context.Context, types.ServiceConfig) error) error {
+	g := NewGraph(project.Services)
+	leaves := g.Leaves()
+
+	eg, _ := errgroup.WithContext(ctx)
+	eg.Go(func() error {
+		return run(ctx, g, eg, leaves, fn)
+	})
 
-	eg, ctx := errgroup.WithContext(ctx)
-	results := make(chan string)
-	errors := make(chan error)
-	scheduled := map[string]bool{}
-	for len(graph) > 0 {
-		for _, n := range graph.independents() {
-			service := n.service
-			if scheduled[service.Name] {
-				continue
-			}
-			eg.Go(func() error {
-				err := fn(ctx, service)
-				if err != nil {
-					errors <- err
-					return err
-				}
-				results <- service.Name
-				return nil
-			})
-			scheduled[service.Name] = true
-		}
-		select {
-		case result := <-results:
-			graph.resolved(result)
-		case err := <-errors:
-			return err
-		}
-	}
 	return eg.Wait()
 }
 
-type dependencyGraph map[string]node
+// Note: this could be `graph.walk` or whatever
-
+func run(ctx context.Context, graph *Graph, eg *errgroup.Group, nodes []*Vertex, fn func(context.Context, types.ServiceConfig) error) error {
-type node struct {
+	for _, node := range nodes {
-	service      types.ServiceConfig
+		n := node
-	dependencies []string
+		// Don't start this service yet if all of their children have
-	dependent    []string
+		// not been started yet.
-}
+		if len(graph.FilterChildren(n.Service.Name, ServiceStopped)) != 0 {
-
+			continue
-func (graph dependencyGraph) independents() []node {
-	var nodes []node
-	for _, node := range graph {
-		if len(node.dependencies) == 0 {
-			nodes = append(nodes, node)
 		}
+
+		eg.Go(func() error {
+			err := fn(ctx, n.Service)
+			if err != nil {
+				return err
+			}
+
+			graph.UpdateStatus(n.Service.Name, ServiceStarted)
+
+			return run(ctx, graph, eg, n.GetParents(), fn)
+		})
 	}
-	return nodes
+
+	return nil
 }
 
-func (graph dependencyGraph) resolved(result string) {
+type Graph struct {
-	for _, parent := range graph[result].dependent {
+	Vertices map[string]*Vertex
-		node := graph[parent]
+	lock     sync.RWMutex
-		node.dependencies = remove(node.dependencies, result)
-		graph[parent] = node
-	}
-	delete(graph, result)
 }
 
-func buildDependencyGraph(services types.Services) dependencyGraph {
+type Vertex struct {
-	graph := dependencyGraph{}
+	Key      string
-	for _, s := range services {
+	Service  types.ServiceConfig
-		graph[s.Name] = node{
+	Status   ServiceStatus
-			service: s,
+	Children map[string]*Vertex
-		}
+	Parents  map[string]*Vertex
+}
+
+func (v *Vertex) GetParents() []*Vertex {
+	var res []*Vertex
+	for _, p := range v.Parents {
+		res = append(res, p)
+	}
+	return res
+}
+
+func NewGraph(services types.Services) *Graph {
+	graph := &Graph{
+		lock:     sync.RWMutex{},
+		Vertices: map[string]*Vertex{},
+	}
+
+	for _, s := range services {
+		graph.AddVertex(s.Name, s)
 	}
 
 	for _, s := range services {
-		node := graph[s.Name]
 		for _, name := range s.GetDependencies() {
-			dependency := graph[name]
+			graph.AddEdge(s.Name, name)
-			node.dependencies = append(node.dependencies, name)
-			dependency.dependent = append(dependency.dependent, s.Name)
-			graph[name] = dependency
 		}
-		graph[s.Name] = node
 	}
+
 	return graph
 }
 
-func remove(slice []string, item string) []string {
+// We then create a constructor function for the Vertex
-	var s []string
+func NewVertex(key string, service types.ServiceConfig) *Vertex {
-	for _, i := range slice {
+	return &Vertex{
-		if i != item {
+		Key:      key,
-			s = append(s, i)
+		Service:  service,
+		Status:   ServiceStopped,
+		Parents:  map[string]*Vertex{},
+		Children: map[string]*Vertex{},
+	}
+}
+
+func (g *Graph) AddVertex(key string, service types.ServiceConfig) {
+	g.lock.Lock()
+	defer g.lock.Unlock()
+
+	v := NewVertex(key, service)
+	g.Vertices[key] = v
+}
+
+func (g *Graph) AddEdge(source string, destination string) error {
+	g.lock.Lock()
+	defer g.lock.Unlock()
+
+	sourceVertex := g.Vertices[source]
+	destinationVertex := g.Vertices[destination]
+
+	if sourceVertex == nil {
+		return fmt.Errorf("could not find %s", source)
+	}
+	if destinationVertex == nil {
+		return fmt.Errorf("could not find %s", destination)
+	}
+
+	// If they are already connected
+	if _, ok := sourceVertex.Children[destination]; ok {
+		return nil
+	}
+
+	sourceVertex.Children[destination] = destinationVertex
+	destinationVertex.Parents[source] = sourceVertex
+
+	g.Vertices[source] = sourceVertex
+	g.Vertices[destination] = destinationVertex
+	return nil
+}
+
+func (g *Graph) Leaves() []*Vertex {
+	g.lock.Lock()
+	defer g.lock.Unlock()
+
+	var res []*Vertex
+	for _, v := range g.Vertices {
+		if len(v.Children) == 0 {
+			res = append(res, v)
 		}
 	}
-	return s
+
+	return res
+}
+
+func (g *Graph) UpdateStatus(key string, status ServiceStatus) {
+	g.lock.Lock()
+	defer g.lock.Unlock()
+	g.Vertices[key].Status = status
+}
+
+func (g *Graph) FilterChildren(key string, status ServiceStatus) []*Vertex {
+	g.lock.Lock()
+	defer g.lock.Unlock()
+
+	var res []*Vertex
+	vertex := g.Vertices[key]
+
+	for _, child := range vertex.Children {
+		if child.Status == status {
+			res = append(res, child)
+		}
+	}
+
+	return res
 }