cmd/cue,pkg/tool/exec: rely on cue schema for exec.Run defaults

Currently, exec.Run is unable to properly leverage its defined schema
to evaluate defaults, due to the fields not being defined in the legacy
schema definitions.

This updates the logic in the task runner to unify the old schemas with
the new, to allow for new fields to be referenced.

Signed-off-by: Nick Figgins <figginsn@gmail.com>
Change-Id: I7b1eda6a002bb12a5059ddbc7ced42c76102ecc2
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1176665
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: CUEcueckoo <cueckoo@gmail.com>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>

cmd/cue/cmd/custom.go

@@ -221,8 +221,10 @@ func newTaskFunc(cmd *Command) flow.TaskFunc {
 				return nil, errors.Promote(err1, "newTask")
 			}
 		}
+		var isLegacy bool
 		if k, ok := legacyKinds[kind]; ok {
 			kind = k
+			isLegacy = true
 		}
 		rf := itask.Lookup(kind)
 		if rf == nil {
@@ -242,12 +244,17 @@ func newTaskFunc(cmd *Command) flow.TaskFunc {
 		}
 
 		return flow.RunnerFunc(func(t *flow.Task) error {
+			obj := t.Value()
+
+			if isLegacy {
+				obj = obj.Unify(v)
+			}
 			c := &itask.Context{
 				Context: t.Context(),
 				Stdin:   cmd.InOrStdin(),
 				Stdout:  cmd.OutOrStdout(),
 				Stderr:  cmd.OutOrStderr(),
-				Obj:     t.Value(),
+				Obj:     obj,
 			}
 			value, err := runner.Run(c)
 			if err != nil {

pkg/tool/exec/exec.go

@@ -45,13 +45,8 @@ func (c *execCmd) Run(ctx *task.Context) (res interface{}, err error) {
 
 	// TODO: set environment variables, if defined.
 	stream := func(name string) (stream cue.Value, ok bool) {
-		c := ctx.Obj.Lookup(name)
-		// Although the schema defines a default versions, older implementations
-		// may not use it yet.
-		if !c.Exists() {
-			return
-		}
-		if err := c.Null(); ctx.Err != nil || err == nil {
+		c := ctx.Obj.LookupPath(cue.ParsePath(name))
+		if err := c.Null(); c.Err() != nil || err == nil {
 			return
 		}
 		return c, true
@@ -71,14 +66,10 @@ func (c *execCmd) Run(ctx *task.Context) (res interface{}, err error) {
 		cmd.Stderr = ctx.Stderr
 	}
 
-	// TODO(mvdan): exec.Run declares mustSucceed as a regular field with a default of true.
-	// We should be able to rely on that here, removing the need for Exists and repeating the default.
-	mustSucceed := true
-	if v := ctx.Obj.LookupPath(cue.ParsePath("mustSucceed")); v.Exists() {
-		mustSucceed, err = v.Bool()
-		if err != nil {
-			return nil, errors.Wrapf(err, v.Pos(), "invalid bool value")
-		}
+	v := ctx.Obj.LookupPath(cue.ParsePath("mustSucceed"))
+	mustSucceed, err := v.Bool()
+	if err != nil {
+		return nil, errors.Wrapf(err, v.Pos(), "invalid bool value")
 	}
 
 	update := map[string]interface{}{}
@@ -102,6 +93,7 @@ func (c *execCmd) Run(ctx *task.Context) (res interface{}, err error) {
 			update["stderr"] = err.Error()
 		}
 	}
+
 	if !mustSucceed {
 		return update, nil
 	}