cue/load: fix Instances race

The shared `*cue.Instance` used by the filetypes logic was causing
a race when loading. Using `cue.Value` instead avoids the race because
`cue.Value` is already finalized.

Updates #1746, #2480.

Signed-off-by: Roger Peppe <rogpeppe@gmail.com>
Change-Id: I48819188f1f0bbda62909f3aea8675953e2d8fc2
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/556519
Reviewed-by: Marcel van Lohuizen <mpvl@gmail.com>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
TryBot-Result: CUEcueckoo <cueckoo+gerrithub@cuelang.org>
Reviewed-by: Paul Jolly <paul@myitcv.io>

cue/load/loader_test.go

@@ -20,6 +20,7 @@ import (
 	"path/filepath"
 	"strconv"
 	"strings"
+	"sync"
 	"testing"
 	"text/template"
 	"unicode"
@@ -439,3 +440,23 @@ func TestOverlays(t *testing.T) {
 		}
 	}
 }
+
+func TestLoadInstancesConcurrent(t *testing.T) {
+	// This test is designed to fail when run with the race detector
+	// if there's an underlying race condition.
+	// See https:/cuelang.org/issue/1746
+	race(func() {
+		Instances([]string{"."}, nil)
+	})
+}
+
+func race(f func()) {
+	var wg sync.WaitGroup
+	for i := 0; i < 2; i++ {
+		wg.Add(1)
+		go func() {
+			f()
+			wg.Done()
+		}()
+	}
+}

encoding/gocode/templates.go

@@ -78,7 +78,7 @@ func {{if .func}}{{.complete}}{{.cueName}}{{$sig}}
 var loadCode = template.Must(template.New("load").Funcs(template.FuncMap{
 	"normalizeHex": normalizeHex,
 }).Parse(`
-var {{.prefix}}Codec, {{.prefix}}Instance = func() (*gocodec.Codec, *cue.Instance) {
+var {{.prefix}}Codec, {{.prefix}}Instance_, {{.prefix}}Value = func() (*gocodec.Codec, *cue.Instance, cue.Value) {
 	var r *cue.Runtime
 	r = {{if .runtime}}{{.runtime}}{{else}}&cue.Runtime{}{{end}}
 	instances, err := r.Unmarshal({{.prefix}}InstanceData)
@@ -88,13 +88,16 @@ var {{.prefix}}Codec, {{.prefix}}Instance = func() (*gocodec.Codec, *cue.Instanc
 	if len(instances) != 1 {
 		panic("expected encoding of exactly one instance")
 	}
-	return gocodec.New(r, nil), instances[0]
+	return gocodec.New(r, nil), instances[0], instances[0].Value()
 }()
 
+// Deprecated: cue.Instance is deprecated. Use {{.prefix}}Value instead.
+var {{.prefix}}Instance = {{.prefix}}Instance_
+
 // {{.prefix}}Make is called in the init phase to initialize CUE values for
 // validation functions.
 func {{.prefix}}Make(name string, x interface{}) cue.Value {
-	f, err := {{.prefix}}Instance.Value().FieldByName(name, true)
+	f, err := {{.prefix}}Value.FieldByName(name, true)
 	if err != nil {
 		panic(fmt.Errorf("could not find type %q in instance", name))
 	}

internal/filetypes/filetypes.go

@@ -96,8 +96,7 @@ func FromFile(b *build.File, mode Mode) (*FileInfo, error) {
 		}, nil
 	}
 
-	i := cuegenInstance.Value()
-	i, errs := update(nil, i, i, "modes", mode.String())
+	i, errs := update(nil, cuegenValue, cuegenValue, "modes", mode.String())
 	v := i.LookupDef("FileInfo")
 	v = v.Fill(b)
 
@@ -283,7 +282,7 @@ func toFile(i, v cue.Value, filename string) (*build.File, error) {
 }
 
 func parseType(s string, mode Mode) (inst, val cue.Value, err error) {
-	i := cuegenInstance.Value()
+	i := cuegenValue
 	i = i.Unify(i.Lookup("modes", mode.String()))
 	v := i.LookupDef("File")