go/types: don't write during sanitizeInfo if nothing has changed

In its final phase, the typechecker walks the types it produces to
ensure that no unexpanded type instances leak through the API. However,
this also walks shared types (such as those in the universe scope),
resulting in a potential data race during concurrent typechecking
passes.

Fix this by being careful not to write if nothing needs to be changed.
Since any shared types should already be sanitized, this should
eliminate data races.

For #44434

Change-Id: Iadb2e78863efe0e974e69a00e255f26cfaf9386a
Reviewed-on: https://go-review.googlesource.com/c/go/+/294411
Trust: Robert Findley <rfindley@google.com>
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>

src/go/types/sanitize.go

@@ -6,6 +6,11 @@ package types
 
 // sanitizeInfo walks the types contained in info to ensure that all instances
 // are expanded.
+//
+// This includes some objects that may be shared across concurrent
+// type-checking passes (such as those in the universe scope), so we are
+// careful here not to write types that are already sanitized. This avoids a
+// data race as any shared types should already be sanitized.
 func sanitizeInfo(info *Info) {
 	var s sanitizer = make(map[Type]Type)
 
@@ -13,27 +18,42 @@ func sanitizeInfo(info *Info) {
 	// If modified, they must be assigned back.
 
 	for e, tv := range info.Types {
-		tv.Type = s.typ(tv.Type)
-		info.Types[e] = tv
+		if typ := s.typ(tv.Type); typ != tv.Type {
+			tv.Type = typ
+			info.Types[e] = tv
+		}
 	}
 
 	for e, inf := range info.Inferred {
+		changed := false
 		for i, targ := range inf.Targs {
-			inf.Targs[i] = s.typ(targ)
+			if typ := s.typ(targ); typ != targ {
+				inf.Targs[i] = typ
+				changed = true
+			}
+		}
+		if typ := s.typ(inf.Sig); typ != inf.Sig {
+			inf.Sig = typ.(*Signature)
+			changed = true
+		}
+		if changed {
+			info.Inferred[e] = inf
 		}
-		inf.Sig = s.typ(inf.Sig).(*Signature)
-		info.Inferred[e] = inf
 	}
 
 	for _, obj := range info.Defs {
 		if obj != nil {
-			obj.setType(s.typ(obj.Type()))
+			if typ := s.typ(obj.Type()); typ != obj.Type() {
+				obj.setType(typ)
+			}
 		}
 	}
 
 	for _, obj := range info.Uses {
 		if obj != nil {
-			obj.setType(s.typ(obj.Type()))
+			if typ := s.typ(obj.Type()); typ != obj.Type() {
+				obj.setType(typ)
+			}
 		}
 	}
 
@@ -57,16 +77,22 @@ func (s sanitizer) typ(typ Type) Type {
 		// nothing to do
 
 	case *Array:
-		t.elem = s.typ(t.elem)
+		if elem := s.typ(t.elem); elem != t.elem {
+			t.elem = elem
+		}
 
 	case *Slice:
-		t.elem = s.typ(t.elem)
+		if elem := s.typ(t.elem); elem != t.elem {
+			t.elem = elem
+		}
 
 	case *Struct:
 		s.varList(t.fields)
 
 	case *Pointer:
-		t.base = s.typ(t.base)
+		if base := s.typ(t.base); base != t.base {
+			t.base = base
+		}
 
 	case *Tuple:
 		s.tuple(t)
@@ -87,20 +113,32 @@ func (s sanitizer) typ(typ Type) Type {
 		s.typ(t.allTypes)
 
 	case *Map:
-		t.key = s.typ(t.key)
-		t.elem = s.typ(t.elem)
+		if key := s.typ(t.key); key != t.key {
+			t.key = key
+		}
+		if elem := s.typ(t.elem); elem != t.elem {
+			t.elem = elem
+		}
 
 	case *Chan:
-		t.elem = s.typ(t.elem)
+		if elem := s.typ(t.elem); elem != t.elem {
+			t.elem = elem
+		}
 
 	case *Named:
-		t.orig = s.typ(t.orig)
-		t.underlying = s.typ(t.underlying)
+		if orig := s.typ(t.orig); orig != t.orig {
+			t.orig = orig
+		}
+		if under := s.typ(t.underlying); under != t.underlying {
+			t.underlying = under
+		}
 		s.typeList(t.targs)
 		s.funcList(t.methods)
 
 	case *TypeParam:
-		t.bound = s.typ(t.bound)
+		if bound := s.typ(t.bound); bound != t.bound {
+			t.bound = bound
+		}
 
 	case *instance:
 		typ = t.expand()
@@ -115,7 +153,9 @@ func (s sanitizer) typ(typ Type) Type {
 
 func (s sanitizer) var_(v *Var) {
 	if v != nil {
-		v.typ = s.typ(v.typ)
+		if typ := s.typ(v.typ); typ != v.typ {
+			v.typ = typ
+		}
 	}
 }
 
@@ -133,7 +173,9 @@ func (s sanitizer) tuple(t *Tuple) {
 
 func (s sanitizer) func_(f *Func) {
 	if f != nil {
-		f.typ = s.typ(f.typ)
+		if typ := s.typ(f.typ); typ != f.typ {
+			f.typ = typ
+		}
 	}
 }
 
@@ -145,6 +187,8 @@ func (s sanitizer) funcList(list []*Func) {
 
 func (s sanitizer) typeList(list []Type) {
 	for i, t := range list {
-		list[i] = s.typ(t)
+		if typ := s.typ(t); typ != t {
+			list[i] = typ
+		}
 	}
 }