encoding/jsonschema: allow mapping URLs to values within a package

A JSON Schema value might not be situated at the root of a CUE package.
This change allows `MapURL` to return not only the import path of
package in which a JSON Schema URI can be found in CUE, but also a
reference within that package.

This is technically a backwardly incompatible change to the `MapURL`
signature, but as this is a relatively new part of the API (added in
v0.10.0) and is unlikely to have many importers use it, I think that's
probably OK.

Signed-off-by: Roger Peppe <rogpeppe@gmail.com>
Change-Id: I3e44b2623fe3c5778afd1e6d2ad50948395a0145
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1201910
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>

encoding/jsonschema/decode_test.go

@@ -216,9 +216,9 @@ properties: x: $ref: "https://something.test/foo#/definitions/blah"
 `)
 	var calls []string
 	expr, err := jsonschema.Extract(v, &jsonschema.Config{
-		MapURL: func(u *url.URL) (string, error) {
+		MapURL: func(u *url.URL) (string, cue.Path, error) {
 			calls = append(calls, u.String())
-			return "other.test/something:blah", nil
+			return "other.test/something:blah", cue.ParsePath("#Foo.bar"), nil
 		},
 	})
 	qt.Assert(t, qt.IsNil(err))
@@ -230,7 +230,7 @@ properties: x: $ref: "https://something.test/foo#/definitions/blah"
 	qt.Assert(t, qt.Equals(string(b), `
 import "other.test/something:blah"
 
-x?: blah.#blah
+x?: blah.#Foo.bar.#blah
 ...
 `[1:]))
 }
@@ -244,8 +244,8 @@ properties: {
 }
 `, cue.Filename("foo.cue"))
 	_, err := jsonschema.Extract(v, &jsonschema.Config{
-		MapURL: func(u *url.URL) (string, error) {
-			return "", fmt.Errorf("some error")
+		MapURL: func(u *url.URL) (string, cue.Path, error) {
+			return "", cue.Path{}, fmt.Errorf("some error")
 		},
 	})
 	qt.Assert(t, qt.Equals(errors.Details(err, nil), `

encoding/jsonschema/jsonschema.go

@@ -97,9 +97,9 @@ type Config struct {
 	Map func(pos token.Pos, path []string) ([]ast.Label, error)
 
 	// MapURL maps a URL reference as found in $ref to
-	// an import path for a package.
+	// an import path for a package and a path within that package.
 	// If this is nil, [DefaultMapURL] will be used.
-	MapURL func(u *url.URL) (importPath string, err error)
+	MapURL func(u *url.URL) (importPath string, path cue.Path, err error)
 
 	// TODO: configurability to make it compatible with OpenAPI, such as
 	// - locations of definitions: #/components/schemas, for instance.

encoding/jsonschema/ref.go

@@ -126,7 +126,7 @@ func (s *state) makeCUERef(n cue.Value, u *url.URL, fragmentParts []string) (_e
 		return sel
 	}
 
-	var ident *ast.Ident
+	var refExpr ast.Expr
 
 	for ; ; s = s.up {
 		if s.up == nil {
@@ -138,7 +138,7 @@ func (s *state) makeCUERef(n cue.Value, u *url.URL, fragmentParts []string) (_e
 					return s.rootRef()
 				}
 
-				ident, fragmentParts = s.getNextIdent(n.Pos(), fragmentParts)
+				refExpr, fragmentParts = s.getNextIdent(n.Pos(), fragmentParts)
 
 			case u.Host != "":
 				// Reference not found within scope. Create an import reference.
@@ -147,7 +147,7 @@ func (s *state) makeCUERef(n cue.Value, u *url.URL, fragmentParts []string) (_e
 				// referenced. We could consider doing an extra pass to record
 				// all '$id's in a file to be able to link to them even if they
 				// are not in scope.
-				importPath, err := s.cfg.MapURL(u)
+				importPath, refPath, err := s.cfg.MapURL(u)
 				if err != nil {
 					ustr := u.String()
 					// Avoid producing many errors for the same URL.
@@ -162,8 +162,13 @@ func (s *state) makeCUERef(n cue.Value, u *url.URL, fragmentParts []string) (_e
 					s.errf(n, "cannot determine package name from import path %q", importPath)
 					return nil
 				}
-				ident = ast.NewIdent(ip.Qualifier)
+				ident := ast.NewIdent(ip.Qualifier)
 				ident.Node = &ast.ImportSpec{Path: ast.NewString(importPath)}
+				refExpr, err = pathRefSyntax(refPath, ident)
+				if err != nil {
+					s.errf(n, "invalid CUE path for URL %q: %v", u, err)
+					return nil
+				}
 
 			default:
 				// Just a path, not sure what that means.
@@ -206,13 +211,14 @@ func (s *state) makeCUERef(n cue.Value, u *url.URL, fragmentParts []string) (_e
 				}
 				return newSel(e, s.idRef[1])
 			}
-			ident, fragmentParts = s.getNextIdent(n.Pos(), fragmentParts)
+			ident, fragmentParts0 := s.getNextIdent(n.Pos(), fragmentParts)
 			ident.Node = s.obj
+			refExpr, fragmentParts = ident, fragmentParts0
 			break
 		}
 	}
 
-	return s.newSel(n.Pos(), ident, fragmentParts)
+	return s.newSel(n.Pos(), refExpr, fragmentParts)
 }
 
 // getNextSelector translates a JSON Reference path into a CUE path by consuming
@@ -424,7 +430,7 @@ func jsonSchemaRef(p token.Pos, a []string) ([]ast.Label, error) {
 // path mapping. It trims off any ".json" suffix and uses the
 // package name "schema" if the final component of the path
 // isn't a valid CUE identifier.
-func DefaultMapURL(u *url.URL) (importPath string, err error) {
+func DefaultMapURL(u *url.URL) (string, cue.Path, error) {
 	p := u.Path
 	base := path.Base(p)
 	if !ast.IsValidIdent(base) {
@@ -436,5 +442,32 @@ func DefaultMapURL(u *url.URL) (importPath string, err error) {
 		}
 		p += ":" + base
 	}
-	return u.Host + p, nil
+	return u.Host + p, cue.Path{}, nil
+}
+
+// pathRefSyntax returns the syntax for an expression which
+// looks up the path inside the given root expression's value.
+// It returns an error if the path contains any elements with
+// type [cue.OptionalConstraint], [cue.RequiredConstraint], or [cue.PatternConstraint],
+// none of which are expressible as a CUE index expression.
+//
+// TODO implement this properly and move to a method on [cue.Path].
+func pathRefSyntax(cuePath cue.Path, root ast.Expr) (ast.Expr, error) {
+	expr := root
+	for _, sel := range cuePath.Selectors() {
+		switch sel.LabelType() {
+		case cue.StringLabel, cue.DefinitionLabel:
+			ident := sel.String()
+			if !ast.IsValidIdent(ident) {
+				return nil, fmt.Errorf("cannot form expression for path %q", cuePath)
+			}
+			expr = &ast.SelectorExpr{
+				X:   expr,
+				Sel: ast.NewIdent(sel.String()),
+			}
+		default:
+			return nil, fmt.Errorf("cannot form expression for path %q", cuePath)
+		}
+	}
+	return expr, nil
 }