Improve the DAG building and link detection

Only consider link B -> A once. If A depends from B in N different
ways, consider that a single link in the dag. Considering them
different won't help us detect cycles nor scheduling tasks.

In the cycle detection logic, use a string builder instead of '+'
to concatenate strings, since it more efficient.

pkg/apis/pipeline/v1beta1/pipeline_types.go

@@ -19,6 +19,7 @@ package v1beta1
 import (
 	"github.com/tektoncd/pipeline/pkg/reconciler/pipeline/dag"
 	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
+	"k8s.io/apimachinery/pkg/util/sets"
 )
 
 // +genclient
@@ -174,7 +175,16 @@ func (pt PipelineTask) Deps() []string {
 	deps = append(deps, pt.resourceDeps()...)
 	deps = append(deps, pt.orderingDeps()...)
 
-	return deps
+	uniqueDeps := sets.NewString()
+	for _, w := range deps {
+		if uniqueDeps.Has(w) {
+			continue
+		}
+		uniqueDeps.Insert(w)
+
+	}
+
+	return uniqueDeps.List()
 }
 
 func (pt PipelineTask) resourceDeps() []string {

pkg/reconciler/pipeline/dag/dag.go

@@ -137,12 +137,16 @@ func linkPipelineTasks(prev *Node, next *Node) error {
 }
 
 func visit(currentName string, nodes []*Node, path []string, visited map[string]bool) error {
+	var sb strings.Builder
 	for _, n := range nodes {
 		path = append(path, n.Task.HashKey())
 		if _, ok := visited[n.Task.HashKey()]; ok {
 			return errors.New(getVisitedPath(path))
 		}
-		visited[currentName+"."+n.Task.HashKey()] = true
+		sb.WriteString(currentName)
+		sb.WriteByte('.')
+		sb.WriteString(n.Task.HashKey())
+		visited[sb.String()] = true
 		if err := visit(n.Task.HashKey(), n.Prev, path, visited); err != nil {
 			return err
 		}

pkg/reconciler/pipelinerun/resources/pipelinerunstate_test.go

@@ -503,7 +503,7 @@ func buildPipelineStateWithLargeDepencyGraph(t *testing.T) PipelineRunState {
 			TaskSpec: &task.Spec,
 		},
 	}}
-	for i := 2; i < 60; i++ {
+	for i := 2; i < 400; i++ {
 		dependFrom := 1
 		if i > 10 {
 			if i%10 == 0 {