list: fix ext sort error sometimes being ignored at the end of a listing

Sorter.Send selects on both the output and error channels of the ext
sort. If the ext sort failed it closes both, so the select could pick
the closed output channel and return without the error. This was seen
as a flaky TestSorterExtTempFileError under the race detector.

fs/list/sorter.go

@@ -305,6 +305,11 @@ func (ls *Sorter) Send() (err error) {
 				}
 			}
 		}
+		// The select may see outputChan closed before reading the
+		// error the sorter sent on errChan
+		if err := <-ls.errChan; err != nil {
+			return err
+		}
 		err = list.Flush()
 		if err != nil {
 			return err

fs/list/sorter_test.go

@@ -3,6 +3,7 @@ package list
 import (
 	"cmp"
 	"context"
+	"errors"
 	"fmt"
 	"os"
 	"path/filepath"
@@ -285,6 +286,31 @@ func TestSorterExtTempFileError(t *testing.T) {
 	require.Error(t, err)
 }
 
+// Test that Send returns the error from the ext sort when the ext
+// sort closes its output and error channels before Send reads them.
+func TestSorterExtSendError(t *testing.T) {
+	ctx := context.Background()
+	callback := func(entries fs.DirEntries) error {
+		return nil
+	}
+	// select picks randomly between ready channels so try many times
+	for range 100 {
+		ls, err := NewSorter(ctx, nil, callback, nil)
+		require.NoError(t, err)
+		outputChan := make(chan string)
+		close(outputChan)
+		errChan := make(chan error, 1)
+		errChan <- errors.New("sort failed")
+		close(errChan)
+		ls.extSort = true
+		ls.inputChan = make(chan string)
+		ls.outputChan = outputChan
+		ls.errChan = errChan
+		require.EqualError(t, ls.Send(), "sort failed")
+		ls.CleanUp()
+	}
+}
+
 // benchFs implements enough of the fs.Fs interface for Sorter
 type benchFs struct{}