go/types: fix race-y initialization of Struct.offsets

Use sync.Once to ensure, that 'offsets' field is initialized
once only in a threadsafe way.

Fixes #12887

Change-Id: I90ef929c421ccd3094339c67a39b02d8f2e47211
Reviewed-on: https://go-review.googlesource.com/16013Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/types/sizes.go

@@ -132,13 +132,8 @@ func (s *StdSizes) Sizeof(T Type) int64 {
 		if n == 0 {
 			return 0
 		}
-		offsets := t.offsets
-		if t.offsets == nil {
-			// compute offsets on demand
-			offsets = s.Offsetsof(t.fields)
-			t.offsets = offsets
-		}
-		return offsets[n-1] + s.Sizeof(t.fields[n-1].typ)
+		setOffsets(t, s)
+		return t.offsets[n-1] + s.Sizeof(t.fields[n-1].typ)
 	case *Interface:
 		return s.WordSize * 2
 	}
@@ -159,11 +154,13 @@ func (conf *Config) alignof(T Type) int64 {
 }
 
 func (conf *Config) offsetsof(T *Struct) []int64 {
-	offsets := T.offsets
-	if offsets == nil && T.NumFields() > 0 {
+	var offsets []int64
+	if T.NumFields() > 0 {
 		// compute offsets on demand
 		if s := conf.Sizes; s != nil {
-			offsets = s.Offsetsof(T.fields)
+			calculated := setOffsets(T, s)
+			offsets = T.offsets
+			if calculated {
 				// sanity checks
 				if len(offsets) != T.NumFields() {
 					panic("Config.Sizes.Offsetsof returned the wrong number of offsets")
@@ -173,10 +170,11 @@ func (conf *Config) offsetsof(T *Struct) []int64 {
 						panic("Config.Sizes.Offsetsof returned an offset < 0")
 					}
 				}
+			}
 		} else {
-			offsets = stdSizes.Offsetsof(T.fields)
+			setOffsets(T, &stdSizes)
+			offsets = T.offsets
 		}
-		T.offsets = offsets
 	}
 	return offsets
 }
@@ -209,3 +207,15 @@ func align(x, a int64) int64 {
 	y := x + a - 1
 	return y - y%a
 }
+
+// setOffsets sets the offsets of s for the given sizes if necessary.
+// The result is true if the offsets were not set before; otherwise it
+// is false.
+func setOffsets(s *Struct, sizes Sizes) bool {
+	var calculated bool
+	s.offsetsOnce.Do(func() {
+		calculated = true
+		s.offsets = sizes.Offsetsof(s.fields)
+	})
+	return calculated
+}

src/go/types/type.go

@@ -4,9 +4,10 @@
 
 package types
 
-import "sort"
-
-// TODO(gri) Revisit factory functions - make sure they have all relevant parameters.
+import (
+	"sort"
+	"sync"
+)
 
 // A Type represents a type of Go.
 // All types implement the Type interface.
@@ -122,8 +123,8 @@ func (s *Slice) Elem() Type { return s.elem }
 type Struct struct {
 	fields      []*Var
 	tags        []string  // field tags; nil if there are no tags
-	// TODO(gri) access to offsets is not threadsafe - fix this
 	offsets     []int64   // field offsets in bytes, lazily initialized
+	offsetsOnce sync.Once // for threadsafe lazy initialization of offsets
 }
 
 // NewStruct returns a new struct with the given fields and corresponding field tags.