cue: prepare API implementation for structure sharing

The CUE API relies on the Parent field to reflect the
path down which the user descended to obtain the
value. With structure sharing, this means this parent
field may vary by context.

This now adds another field to Value, which keeps
track of a differing parent Value chain. The field
only needs to be set in the case of actual structure
sharing.

Change-Id: Ia03de7d249ce72e72104740ce649827e7b07f2ad
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/9572
Reviewed-by: CUE cueckoo <cueckoo@gmail.com>
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
Reviewed-by: Paul Jolly <paul@myitcv.org.uk>

cue/context.go

@@ -72,7 +72,7 @@ func Scope(scope Value) BuildOption {
 		if o.Scope != nil {
 			panic("more than one scope is given")
 		}
-		o.Scope = scope.v
+		o.Scope = valueScope(scope)
 	}
 }
 
@@ -181,8 +181,8 @@ func (c *Context) BuildExpr(x ast.Expr, options ...BuildOption) Value {
 func errFn(pos token.Pos, msg string, args ...interface{}) {}
 
 // resolveExpr binds unresolved expressions to values in the expression or v.
-func resolveExpr(ctx *adt.OpContext, v *adt.Vertex, x ast.Expr) adt.Value {
-	cfg := &compile.Config{Scope: v}
+func resolveExpr(ctx *adt.OpContext, v Value, x ast.Expr) adt.Value {
+	cfg := &compile.Config{Scope: valueScope(v)}
 
 	astutil.ResolveExpr(x, errFn)
 

cue/instance.go

@@ -263,7 +263,7 @@ func (inst *hiddenInstance) Build(p *build.Instance) *Instance {
 
 	rErr := r.ResolveFiles(p)
 
-	cfg := &compile.Config{Scope: inst.root}
+	cfg := &compile.Config{Scope: valueScope(Value{idx: r, v: inst.root})}
 	v, err := compile.Files(cfg, r, p.ID(), p.Files...)
 
 	v.AddConjunct(adt.MakeRootConjunct(nil, inst.root))

cue/query.go

@@ -23,15 +23,15 @@ import (
 // getScopePrefix finds the Vertex that exists in v for the longest prefix of p.
 //
 // It is used to make the parent scopes visible when resolving expressions.
-func getScopePrefix(v Value, p Path) *adt.Vertex {
+func getScopePrefix(v Value, p Path) Value {
 	for _, sel := range p.Selectors() {
 		w := v.LookupPath(MakePath(sel))
 		if !w.Exists() {
 			break
 		}
 		v = w
 	}
-	return v.v
+	return v
 }
 
 // LookupPath reports the value for path p relative to v.
@@ -40,25 +40,28 @@ func (v Value) LookupPath(p Path) Value {
 		return Value{}
 	}
 	n := v.v
+	parent := v.parent_
 	ctx := v.ctx()
 
 outer:
 	for _, sel := range p.path {
 		f := sel.sel.feature(v.idx)
 		for _, a := range n.Arcs {
 			if a.Label == f {
+				parent = linkParent(parent, n, a)
 				n = a
 				continue outer
 			}
 		}
 		if sel.sel.optional() {
 			x := &adt.Vertex{
-				Parent: v.v,
+				Parent: n,
 				Label:  sel.sel.feature(ctx),
 			}
 			n.MatchAndInsert(ctx, x)
 			if len(x.Conjuncts) > 0 {
 				x.Finalize(ctx)
+				parent = linkParent(parent, n, x)
 				n = x
 				continue
 			}
@@ -71,8 +74,8 @@ outer:
 			// TODO: better message.
 			x = mkErr(v.idx, n, adt.NotExistError, "field %q not found", sel.sel)
 		}
-		v := makeValue(v.idx, n)
+		v := makeValue(v.idx, n, parent)
 		return newErrValue(v, x)
 	}
-	return makeValue(v.idx, n)
+	return makeValue(v.idx, n, parent)
 }