cmd/vet,sync: check lock values more precisely

Fixes #26165

Change-Id: I1f3bd193af9b6f8461c736330952b6e50d3e00d9
Reviewed-on: https://go-review.googlesource.com/121876Reviewed-by: Alan Donovan <adonovan@google.com>
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/vet/copylock.go

@@ -234,14 +234,12 @@ func lockPath(tpkg *types.Package, typ types.Type) typePath {
 		return nil
 	}
 
-	// We're looking for cases in which a reference to this type
-	// can be locked, but a value cannot. This differentiates
+	// We're looking for cases in which a pointer to this type
+	// is a sync.Locker, but a value is not. This differentiates
 	// embedded interfaces from embedded values.
-	if plock := types.NewMethodSet(types.NewPointer(typ)).Lookup(tpkg, "Lock"); plock != nil {
-		if lock := types.NewMethodSet(typ).Lookup(tpkg, "Lock"); lock == nil {
+	if types.Implements(types.NewPointer(typ), lockerType) && !types.Implements(typ, lockerType) {
 		return []types.Type{typ}
 	}
-	}
 
 	nfields := styp.NumFields()
 	for i := 0; i < nfields; i++ {
@@ -254,3 +252,15 @@ func lockPath(tpkg *types.Package, typ types.Type) typePath {
 
 	return nil
 }
+
+var lockerType *types.Interface
+
+// Construct a sync.Locker interface type.
+func init() {
+	nullary := types.NewSignature(nil, nil, nil, false) // func()
+	methods := []*types.Func{
+		types.NewFunc(token.NoPos, nil, "Lock", nullary),
+		types.NewFunc(token.NoPos, nil, "Unlock", nullary),
+	}
+	lockerType = types.NewInterface(methods, nil).Complete()
+}

src/sync/cond.go

@@ -95,3 +95,4 @@ type noCopy struct{}
 
 // Lock is a no-op used by -copylocks checker from `go vet`.
 func (*noCopy) Lock()   {}
+func (*noCopy) Unlock() {}