fix(internal/syncwriter): stop corrupting test output on sink write errors

internal/syncwriter/syncwriter.go

@@ -24,7 +24,9 @@ func New(w io.Writer) *Writer {
 		w: w,
 
 		errorf: func(f string, v ...interface{}) {
-			println(fmt.Sprintf(f, v...))
+			// Avoid the builtin println, which writes to fd 2
+			// unsynchronized and can corrupt concurrent go test output.
+			fmt.Fprintf(os.Stderr, f+"\n", v...)
 		},
 	}
 }

internal/syncwriter/syncwriter_test.go

@@ -3,6 +3,7 @@ package syncwriter
 import (
 	"io"
 	"os"
+	"strings"
 	"testing"
 
 	"cdr.dev/slog/v3/internal/assert"
@@ -74,6 +75,34 @@ func TestWriter_Sync(t *testing.T) {
 	})
 }
 
+// The default handler must report through the os.Stderr variable, not the
+// builtin println that bypasses it. Reverting to println fails this test.
+//
+// Not parallel: it swaps the process-wide os.Stderr.
+func TestWriter_defaultErrorReportsThroughStderr(t *testing.T) {
+	r, w, err := os.Pipe()
+	assert.Success(t, "pipe", err)
+
+	tw := New(syncWriter{
+		wf: func([]byte) (int, error) { return 0, io.EOF },
+		sf: func() error { return nil },
+	})
+
+	orig := os.Stderr
+	os.Stderr = w
+	tw.Write("sinkname", []byte("entry"))
+	os.Stderr = orig
+	assert.Success(t, "close", w.Close())
+
+	out, err := io.ReadAll(r)
+	assert.Success(t, "read", err)
+
+	got := string(out)
+	assert.True(t, "reported via stderr", strings.Contains(got, "sinkname: failed to write entry"))
+	assert.True(t, "single trailing newline", strings.HasSuffix(got, "\n"))
+	assert.Equal(t, "newline count", 1, strings.Count(got, "\n"))
+}
+
 func Test_errorsIsAny(t *testing.T) {
 	t.Parallel()