Move EINVAL check into syncwriter

Closes #44

Author: nhooyr

Diff for: internal/syncwriter/syncwriter.go

@@ -2,9 +2,11 @@
 package syncwriter
 
 import (
+	"errors"
 	"io"
 	"os"
 	"sync"
+	"syscall"
 )
 
 // Writer implements a concurrency safe io.Writer wrapper.
@@ -39,19 +41,20 @@ func (w *Writer) Sync() error {
 	w.mu.Lock()
 	defer w.mu.Unlock()
 
-	if f, ok := w.w.(*os.File); ok {
-		// We do not want to sync if the writer is os.Stdout or os.Stderr
-		// as that is unsupported on both Linux and MacOS and will return
-		// an error about the fd being an invalid argument.
+	s, ok := w.w.(syncer)
+	if !ok {
+		return nil
+	}
+	err := s.Sync()
+	if _, ok := w.w.(*os.File); ok {
+		// Opened files do not necessarily support syncing.
+		// E.g. stdout and stderr both do not so we need
+		// to ignore these errors.
 		// See https://github.com/uber-go/zap/issues/370
-		if f == os.Stdout || f == os.Stderr {
+		// See https://github.com/cdr/slog/pull/43
+		if errors.Is(err, syscall.EINVAL) {
 			return nil
 		}
-		return f.Sync()
-	}
-
-	if s, ok := w.w.(syncer); ok {
-		return s.Sync()
 	}
-	return nil
+	return err
 }

Diff for: slog.go

@@ -2,13 +2,11 @@ package slog
 
 import (
 	"context"
-	"errors"
 	"fmt"
 	"os"
 	"runtime"
 	"sync"
 	"sync/atomic"
-	"syscall"
 	"time"
 
 	"go.opencensus.io/trace"
@@ -320,8 +318,7 @@ func (l Logger) log(ctx context.Context, level Level, msg string, fields []Field
 func (l Logger) Sync() {
 	for _, s := range l.sinks {
 		err := s.sink.Sync()
-		// Certain devices do not support sync, maybe because they are synchronous by default.
-		if err != nil && !errors.Is(err, syscall.EINVAL) {
+		if err != nil {
 			fmt.Fprintf(os.Stderr, "slog: sink with name %v and type %T failed to sync: %+v\n", s.name, s.sink, err)
 			continue
 		}