fix: lock log sink against concurrent write and close (#10668)

fixes #10663

cli/cliutil/sink.go

@@ -0,0 +1,38 @@
+package cliutil
+
+import (
+	"io"
+	"sync"
+)
+
+type discardAfterClose struct {
+	sync.Mutex
+	wc     io.WriteCloser
+	closed bool
+}
+
+// DiscardAfterClose is an io.WriteCloser that discards writes after it is closed without errors.
+// It is useful as a target for a slog.Sink such that an underlying WriteCloser, like a file, can
+// be cleaned up without race conditions from still-active loggers.
+func DiscardAfterClose(wc io.WriteCloser) io.WriteCloser {
+	return &discardAfterClose{wc: wc}
+}
+
+func (d *discardAfterClose) Write(p []byte) (n int, err error) {
+	d.Lock()
+	defer d.Unlock()
+	if d.closed {
+		return len(p), nil
+	}
+	return d.wc.Write(p)
+}
+
+func (d *discardAfterClose) Close() error {
+	d.Lock()
+	defer d.Unlock()
+	if d.closed {
+		return nil
+	}
+	d.closed = true
+	return d.wc.Close()
+}

cli/cliutil/sink_test.go

@@ -0,0 +1,54 @@
+package cliutil_test
+
+import (
+	"errors"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	"github.com/coder/coder/v2/cli/cliutil"
+)
+
+func TestDiscardAfterClose(t *testing.T) {
+	t.Parallel()
+	exErr := errors.New("test")
+	fwc := &fakeWriteCloser{err: exErr}
+	uut := cliutil.DiscardAfterClose(fwc)
+
+	n, err := uut.Write([]byte("one"))
+	require.Equal(t, 3, n)
+	require.NoError(t, err)
+
+	n, err = uut.Write([]byte("two"))
+	require.Equal(t, 3, n)
+	require.NoError(t, err)
+
+	err = uut.Close()
+	require.Equal(t, exErr, err)
+
+	n, err = uut.Write([]byte("three"))
+	require.Equal(t, 5, n)
+	require.NoError(t, err)
+
+	require.Len(t, fwc.writes, 2)
+	require.EqualValues(t, "one", fwc.writes[0])
+	require.EqualValues(t, "two", fwc.writes[1])
+}
+
+type fakeWriteCloser struct {
+	writes [][]byte
+	closed bool
+	err    error
+}
+
+func (f *fakeWriteCloser) Write(p []byte) (n int, err error) {
+	q := make([]byte, len(p))
+	copy(q, p)
+	f.writes = append(f.writes, q)
+	return len(p), nil
+}
+
+func (f *fakeWriteCloser) Close() error {
+	f.closed = true
+	return f.err
+}