Test Sync() call when calling AddSync

If call to `Sync()` fails, wrap the WriteSyncer with a fake `Sync()`.
Fixes uber-go#328

zapcore/write_syncer.go

@@ -40,12 +40,26 @@ type WriteSyncer interface {
 func AddSync(w io.Writer) WriteSyncer {
 	switch w := w.(type) {
 	case WriteSyncer:
+		if !checkSync(w.(WriteSyncer)) {
+			return writerWrapper{w}
+		}
 		return w
 	default:
 		return writerWrapper{w}
 	}
 }
 
+// checkSync executes a Sync() on the WriteSyncer to check whether it operates
+// cleanly.  In the particular case of the WriteSyncer being a special file,
+// calling Sync will return an `os.ErrInvalid` error.  If, for whatever reason,
+// the call of `Sync()` returns an error, this will return a `false`.
+func checkSync(w WriteSyncer) bool {
+	if err := w.Sync(); err != nil {
+		return false
+	}
+	return true
+}
+
 type lockedWriteSyncer struct {
 	sync.Mutex
 	ws WriteSyncer

zapcore/write_syncer_test.go

@@ -23,6 +23,8 @@ package zapcore
 import (
 	"bytes"
 	"errors"
+	"io/ioutil"
+	"os"
 	"testing"
 
 	"io"
@@ -65,6 +67,20 @@ func TestAddSyncWriter(t *testing.T) {
 	assert.NoError(t, ws.Sync(), "Unexpected error calling a no-op Sync method.")
 }
 
+func TestCheckSync(t *testing.T) {
+	assert.False(t, checkSync(os.Stdout), "Expected os.Stdout to fail sync check")
+	assert.False(t, checkSync(os.Stderr), "Expected os.Stderr to fail sync check")
+
+	// Test a real file, which should pass
+	tmpFile, err := ioutil.TempFile("/tmp", "zapcore-test")
+	if err != nil {
+		t.SkipNow()
+		return
+	}
+	defer tmpFile.Close()
+	assert.True(t, checkSync(tmpFile), "Expected os.Stderr to fail sync check")
+}
+
 func TestMultiWriteSyncerWritesBoth(t *testing.T) {
 	first := &bytes.Buffer{}
 	second := &bytes.Buffer{}