[KS-136] Disallow non-trigger steps with no dependents

core/services/workflows/engine.go

@@ -564,6 +564,7 @@ func NewEngine(cfg Config) (engine *Engine, err error) {
 	// - that there are no step `ref` called `trigger` as this is reserved for any triggers
 	// - that there are no duplicate `ref`s
 	// - that the `ref` for any triggers is empty -- and filled in with `trigger`
+	// - that the resulting graph is strongly connected (i.e. no disjointed subgraphs exist)
 	// - etc.
 
 	workflow, err := Parse(cfg.Spec)

core/services/workflows/models.go

@@ -1,6 +1,7 @@
 package workflows
 
 import (
+	"errors"
 	"fmt"
 
 	"github.com/dominikbraun/graph"
@@ -182,6 +183,10 @@ func Parse(yamlWorkflow string) (*workflow, error) {
 		}
 		step.dependencies = refs
 
+		if stepRef != keywordTrigger && len(refs) == 0 {
+			return nil, errors.New("all non-trigger steps must have a dependent ref")
+		}
+
 		for _, r := range refs {
 			innerErr = g.AddEdge(r, step.Ref)
 			if innerErr != nil {

core/services/workflows/models_test.go

@@ -186,6 +186,31 @@ targets:
 				"a-target": {},
 			},
 		},
+		{
+			name: "non-trigger step with no dependent refs",
+			yaml: `
+triggers:
+  - type: "a-trigger"
+  - type: "a-second-trigger"
+actions:
+  - type: "an-action"
+    ref: "an-action"
+    inputs:
+      hello: "world"
+consensus:
+  - type: "a-consensus"
+    ref: "a-consensus"
+    inputs:
+      trigger_output: $(trigger.outputs)
+      action_output: $(an-action.outputs)
+targets:
+  - type: "a-target"
+    ref: "a-target"
+    inputs:
+      consensus_output: $(a-consensus.outputs)
+`,
+			errMsg: "all non-trigger steps must have a dependent ref",
+		},
 	}
 
 	for _, tc := range testCases {