internal/core/dep: prevent crash

Dep may sometimes assign an "empty" Vertex which has a nil
BaseValue. The tasks processing assumed BaseValue to be non-nil.

Fixed in dep to not pass `empty` to users.

Fixes #1405

Signed-off-by: Marcel van Lohuizen <mpvl@golang.org>
Change-Id: Ie49e309a567d0cfd4e972bf8c118008ee25ffd39
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/533475
Reviewed-by: Paul Jolly <paul@myitcv.io>

Author: mpvl

cmd/cue/cmd/testdata/script/cmd_issue1405.txt

@@ -0,0 +1,41 @@
+exec cue cmd alias ./alias_tool.cue
+cmp stdout stdout-golden
+
+-- alias_tool.cue --
+import (
+	"tool/exec"
+	"text/template"
+)
+
+command: alias: {
+	rendered: template.Execute(t, instance & {
+		enabled: true
+	})
+
+    echo: exec.Run & {
+		cmd: ["echo", rendered]
+	}
+}
+
+t: """
+	# {{ .name }}
+	"""
+
+#base: {
+	enabled: bool | *false
+	name:    string
+	value?:  int
+}
+
+#combined: X=#base & {
+	name: "OK"
+	if X.enabled {
+		value: 123456
+	}
+}
+
+instance: #combined & {
+	enabled: true
+}
+-- stdout-golden --
+# OK

internal/core/adt/context.go

@@ -735,6 +735,8 @@ func (c *OpContext) unifyNode(v Expr, state VertexStatus) (result Value) {
 
 		if v.isUndefined() || state > v.status {
 			// Keep a minimum state of AllArcs.
+			// TODO: AllArcs may still not be achieved if a node is currently
+			// evaluating.
 			state := state
 			if state < AllArcs {
 				state = AllArcs

internal/core/dep/dep.go

@@ -95,6 +95,7 @@ func VisitFields(c *adt.OpContext, n *adt.Vertex, f VisitFunc) error {
 var empty *adt.Vertex
 
 func init() {
+	// TODO: Consider setting a non-nil BaseValue.
 	empty = &adt.Vertex{}
 	empty.UpdateStatus(adt.Finalized)
 }
@@ -222,7 +223,7 @@ func (c *visitor) markResolver(env *adt.Environment, r adt.Resolver) {
 	}
 
 	if ref, _ := c.ctxt.Resolve(env, r); ref != nil {
-		if ref != c.node {
+		if ref != c.node && ref != empty {
 			d := Dependency{
 				Node:      ref,
 				Reference: r,

internal/core/dep/testdata/selfref.txtar

@@ -0,0 +1,29 @@
+#Issue: 1405
+-- in.cue --
+a: {
+	command: alias: {
+		// TODO: fix inner referene.
+		val: (instance & { enabled: true }).value
+		echo: cmd: val
+	}
+
+	#base: {
+		enabled: bool | *false
+		value?:  string
+	}
+
+	#combined: X=#base & {
+		if X.enabled {
+			value: "123456"
+		}
+	}
+
+	instance: #combined & { enabled: true }
+}
+-- out/dependencies/field --
+-- out/dependencies/all --
+-- out/dependencies/dynamic --
+value
+a.command.alias.val
+a.#base
+a.#combined