io: fix Pipe regression with differing error types

Usage of atomic.Value has a subtle requirement that the
value be of the same concrete type. In prior usage, the intention
was to consistently store a value of the error type.
Since error is an interface, the underlying concrete can differ.

Fix this by creating a type-safe abstraction over atomic.Value
that wraps errors in a struct{error} type to ensure consistent types.

Change-Id: Ica74f2daba15e4cff48d2b4f830d2cb51c608fb6
Reviewed-on: https://go-review.googlesource.com/75594
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/io/pipe.go

@@ -13,6 +13,18 @@ import (
 	"sync/atomic"
 )
 
+// atomicError is a type-safe atomic value for errors.
+// We use a struct{ error } to ensure consistent use of a concrete type.
+type atomicError struct{ v atomic.Value }
+
+func (a *atomicError) Store(err error) {
+	a.v.Store(struct{ error }{err})
+}
+func (a *atomicError) Load() error {
+	err, _ := a.v.Load().(struct{ error })
+	return err.error
+}
+
 // ErrClosedPipe is the error used for read or write operations on a closed pipe.
 var ErrClosedPipe = errors.New("io: read/write on closed pipe")
 
@@ -24,8 +36,8 @@ type pipe struct {
 
 	once sync.Once // Protects closing done
 	done chan struct{}
-	rerr atomic.Value
-	werr atomic.Value
+	rerr atomicError
+	werr atomicError
 }
 
 func (p *pipe) Read(b []byte) (n int, err error) {
@@ -46,8 +58,8 @@ func (p *pipe) Read(b []byte) (n int, err error) {
 }
 
 func (p *pipe) readCloseError() error {
-	_, rok := p.rerr.Load().(error)
-	if werr, wok := p.werr.Load().(error); !rok && wok {
+	rerr := p.rerr.Load()
+	if werr := p.werr.Load(); rerr == nil && werr != nil {
 		return werr
 	}
 	return ErrClosedPipe
@@ -85,8 +97,8 @@ func (p *pipe) Write(b []byte) (n int, err error) {
 }
 
 func (p *pipe) writeCloseError() error {
-	_, wok := p.werr.Load().(error)
-	if rerr, rok := p.rerr.Load().(error); !wok && rok {
+	werr := p.werr.Load()
+	if rerr := p.rerr.Load(); werr == nil && rerr != nil {
 		return rerr
 	}
 	return ErrClosedPipe

src/io/pipe_test.go

@@ -316,6 +316,31 @@ func TestWriteAfterWriterClose(t *testing.T) {
 	}
 }
 
+func TestPipeCloseError(t *testing.T) {
+	type testError1 struct{ error }
+	type testError2 struct{ error }
+
+	r, w := Pipe()
+	r.CloseWithError(testError1{})
+	if _, err := w.Write(nil); err != (testError1{}) {
+		t.Errorf("Write error: got %T, want testError1", err)
+	}
+	r.CloseWithError(testError2{})
+	if _, err := w.Write(nil); err != (testError2{}) {
+		t.Errorf("Write error: got %T, want testError2", err)
+	}
+
+	r, w = Pipe()
+	w.CloseWithError(testError1{})
+	if _, err := r.Read(nil); err != (testError1{}) {
+		t.Errorf("Read error: got %T, want testError1", err)
+	}
+	w.CloseWithError(testError2{})
+	if _, err := r.Read(nil); err != (testError2{}) {
+		t.Errorf("Read error: got %T, want testError2", err)
+	}
+}
+
 func TestPipeConcurrent(t *testing.T) {
 	const (
 		input    = "0123456789abcdef"