export: fix bag reindexing (#141)

### Changelog

- Fixed: previously ROS 1 bag files downloaded using the foxglove CLI
would fail with an "unindexed bag" error. These files should now
download correctly.

### Docs


### Description

Since #124, the code to reindex exported bag files has been broken - it
first tries to use a seeking reader to read the unindexed file (which
will not work), then it fails to close the reindexing writer in the
happy path. This PR fixes that and adds a test.

foxglove/cmd/export.go

@@ -203,7 +203,7 @@ func reindexBagFile(w io.Writer, r io.Reader) error {
 	if err != nil {
 		return fmt.Errorf("failed to construct bag reader: %w", err)
 	}
-	it, err := reader.Messages()
+	it, err := reader.Messages(rosbag.ScanLinear(true))
 	if err != nil {
 		return fmt.Errorf("failed to construct bag iterator: %w", err)
 	}
@@ -233,7 +233,7 @@ func reindexBagFile(w io.Writer, r io.Reader) error {
 			return err
 		}
 	}
-	return nil
+	return writer.Close()
 }
 
 // reindexMCAPFile rewrites an MCAP file to a new output location, and properly

foxglove/cmd/export_test.go

@@ -8,6 +8,7 @@ import (
 	"fmt"
 	"io"
 	"os"
+	"path/filepath"
 	"testing"
 	"time"
 
@@ -16,6 +17,7 @@ import (
 	"github.com/foxglove/go-rosbag"
 	"github.com/foxglove/mcap/go/mcap"
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 func withStdoutRedirected(output io.Writer, f func()) error {
@@ -407,3 +409,39 @@ func TestExportCommand(t *testing.T) {
 		assert.Equal(t, 30445, count)
 	})
 }
+
+func copyTo(t *testing.T, from, to string) {
+	infile, err := os.Open(from)
+	require.NoError(t, err)
+	defer func() {
+		require.NoError(t, infile.Close())
+	}()
+	outfile, err := os.Create(to)
+	require.NoError(t, err)
+	defer func() {
+		require.NoError(t, outfile.Close())
+	}()
+	_, err = io.Copy(outfile, infile)
+	require.NoError(t, err)
+}
+
+func TestReindexBag(t *testing.T) {
+	workingPath := filepath.Join(t.TempDir(), "gps.bag.active")
+	copyTo(t, "../testdata/gps.bag.active", workingPath)
+	didReindex, info, err := reindex(t.TempDir(), workingPath, "bag1")
+	require.NoError(t, err)
+	require.True(t, didReindex)
+	require.Equal(t, 30445, int(info.messageCount))
+
+	// check that the new bag is indexed by retrieving Info() from it
+	fd, err := os.Open(workingPath)
+	require.NoError(t, err)
+	defer func() {
+		require.NoError(t, fd.Close())
+	}()
+	reader, err := rosbag.NewReader(fd)
+	require.NoError(t, err)
+	bagInfo, err := reader.Info()
+	require.NoError(t, err)
+	require.Equal(t, 30445, int(bagInfo.MessageCount))
+}