test: chain export cli command

Modified ChainExportCmd to use io.WriterCloser instead of os.File so
it the file can be mocked in unit tests, without side effects to the FS.

cli/chain.go

@@ -7,6 +7,7 @@ import (
 	"encoding/hex"
 	"encoding/json"
 	"fmt"
+	"io"
 	"os"
 	"os/exec"
 	"path"
@@ -1074,7 +1075,7 @@ var ChainExportCmd = &cli.Command{
 			return fmt.Errorf("\"recent-stateroots\" has to be greater than %d", build.Finality)
 		}
 
-		fi, err := os.Create(cctx.Args().First())
+		fi, err := createExportFile(cctx.App, cctx.Args().First())
 		if err != nil {
 			return err
 		}
@@ -1436,3 +1437,16 @@ var chainEncodeParamsCmd = &cli.Command{
 		return nil
 	},
 }
+
+// createExportFile returns the export file from the app metadata, or creates a new file if it doesn't exist
+func createExportFile(app *cli.App, path string) (io.WriteCloser, error) {
+	if wc, ok := app.Metadata["export-file"]; ok {
+		return wc.(io.WriteCloser), nil
+	}
+
+	fi, err := os.Create(path)
+	if err != nil {
+		return nil, err
+	}
+	return fi, nil
+}

cli/chain_test.go

@@ -472,7 +472,35 @@ func TestChainBisect(t *testing.T) {
 	assert.Contains(t, out, path)
 }
 
-func TestChainExport(t *testing.T) {}
+func TestChainExport(t *testing.T) {
+	app, mockApi, _, done := NewMockAppWithFullAPI(t, WithCategory("chain", ChainExportCmd))
+	defer done()
+
+	mockFile := mockExportFile{new(bytes.Buffer)}
+	app.Metadata["export-file"] = mockFile
+
+	blk := mock.MkBlock(nil, 0, 0)
+	ts := mock.TipSet(blk)
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	export := make(chan []byte, 2)
+	expBytes := []byte("whatever")
+	export <- expBytes
+	export <- []byte{} // empty slice means export is complete
+	close(export)
+
+	gomock.InOrder(
+		mockApi.EXPECT().ChainHead(ctx).Return(ts, nil),
+		mockApi.EXPECT().ChainExport(ctx, abi.ChainEpoch(0), false, ts.Key()).Return(export, nil),
+	)
+
+	err := app.Run([]string{"chain", "export", "whatever.car"})
+	assert.NoError(t, err)
+
+	assert.Equal(t, expBytes, mockFile.Bytes())
+}
 
 func TestSlashConsensusFault(t *testing.T) {}
 
@@ -481,3 +509,11 @@ func TestChainGasPrice(t *testing.T) {}
 func TestChainDecode(t *testing.T) {}
 
 func TestChainEncode(t *testing.T) {}
+
+type mockExportFile struct {
+	*bytes.Buffer
+}
+
+func (mef mockExportFile) Close() error {
+	return nil
+}