backup: prevent hang using --stdin-from-command if upload fails (#21829)

changelog/unreleased/issue-5683

@@ -0,0 +1,9 @@
+Bugfix: Prevent `backup --stdin-from-command` from hanging
+
+When using `--stdin-from-command`, the `backup` command could hang until
+manually cancelled if the backup stopped before all subprocess output was
+consumed, for example after a failed upload to the repository. This has been
+fixed.
+
+https://github.com/restic/restic/issues/5683
+https://github.com/restic/restic/pull/21829

internal/fs/fs_reader_command.go

@@ -97,5 +97,8 @@ func (fp *CommandReader) Close() error {
 		return nil
 	}
 
+	// This line can only be reached if the command was not read until the end. This can, for example,
+	// happen if a backup run is interrupted. Kill the command to avoid hanging.
+	_ = fp.cmd.Cancel()
 	return fp.wait()
 }

internal/fs/fs_reader_command_test.go

@@ -6,6 +6,7 @@ import (
 	"io"
 	"strings"
 	"testing"
+	"time"
 
 	"github.com/restic/restic/internal/fs"
 	"github.com/restic/restic/internal/test"
@@ -51,3 +52,14 @@ func TestCommandReaderOutput(t *testing.T) {
 
 	test.Equals(t, "hello world", strings.TrimSpace(buf.String()))
 }
+
+func TestCommandReaderQuickClose(t *testing.T) {
+	ctx, cancel := context.WithTimeout(context.TODO(), 10*time.Second)
+	defer cancel()
+	reader, err := fs.NewCommandReader(ctx, []string{"sleep", "3600"}, func(msg string, args ...interface{}) {})
+	test.OK(t, err)
+
+	// test that close returns before the context expires
+	_ = reader.Close()
+	test.OK(t, ctx.Err())
+}