main.go: remove file if on Flatten(..) failure

When a Flatten() fails, the main program used to leave an incomplete
file (usually zero-length). Now it will clean itself up on failure.

main.go

@@ -53,15 +53,15 @@ type command struct {
 	Stdout    io.Writer
 }
 
-func (c *command) execute(infile string, outfile string) (err error) {
+func (c *command) execute(infile string, outfile string) (_err error) {
 	rd, err := os.Open(infile)
 	if err != nil {
 		return err
 	}
 	defer func() {
-		err1 := rd.Close()
-		if err == nil {
-			err = err1
+		err := rd.Close()
+		if _err == nil {
+			_err = err
 		}
 	}()
 
@@ -74,9 +74,11 @@ func (c *command) execute(infile string, outfile string) (err error) {
 			return fmt.Errorf("create: %w", err)
 		}
 		defer func() {
-			err1 := outf.Close()
-			if err == nil {
-				err = err1
+			err := outf.Close()
+			if _err != nil {
+				os.Remove(outfile)
+			} else {
+				_err = err
 			}
 		}()
 		out = outf

main_test.go

@@ -2,8 +2,10 @@ package main
 
 import (
 	"bytes"
+	"errors"
 	"io"
 	"io/ioutil"
+	"os"
 	"path/filepath"
 	"regexp"
 	"testing"
@@ -13,11 +15,13 @@ func TestExecute(t *testing.T) {
 	var _foo = []byte("foo foo")
 
 	tests := []struct {
-		name    string
-		fixture func(*testing.T, string)
-		infile  string
-		outfile string
-		wantErr string
+		name      string
+		fixture   func(*testing.T, string)
+		flattener func(io.ReadSeeker, io.Writer) error
+		infile    string
+		outfile   string
+		wantErr   string
+		assertion func(*testing.T, string)
 	}{
 		{
 			name:   "ok passthrough via stdout",
@@ -41,6 +45,31 @@ func TestExecute(t *testing.T) {
 			},
 			outfile: "t20-out.txt",
 		},
+		{
+			name:   "bad flattener should remove the file",
+			infile: "t30-in.txt",
+			fixture: func(t *testing.T, dir string) {
+				fname := filepath.Join(dir, "t30-in.txt")
+				if err := ioutil.WriteFile(fname, _foo, 0644); err != nil {
+					t.Fatalf("unexpected error: %v", err)
+				}
+			},
+			flattener: flattenBad,
+			outfile:   "t30-out.txt",
+			wantErr:   "some error",
+			assertion: func(t *testing.T, dir string) {
+				d, err := os.ReadDir(dir)
+				if err != nil {
+					t.Fatalf("unexpected error: %v", err)
+				}
+				if len(d) != 1 {
+					t.Fatalf("expected 1 entry, got %d", len(d))
+				}
+				if d[0].Name() != "t30-in.txt" {
+					t.Fatalf("expected to find only t30-in.txt, got %s", d[0].Name())
+				}
+			},
+		},
 		{
 			name:    "infile does not exist",
 			infile:  "t3-does-not-exist.txt",
@@ -57,7 +86,10 @@ func TestExecute(t *testing.T) {
 		t.Run(tt.name, func(t *testing.T) {
 			dir := t.TempDir()
 			var stdout bytes.Buffer
-			c := &command{Stdout: &stdout}
+			if tt.flattener == nil {
+				tt.flattener = flattenPassthrough
+			}
+
 			if tt.fixture != nil {
 				tt.fixture(t, dir)
 			}
@@ -65,9 +97,14 @@ func TestExecute(t *testing.T) {
 				tt.outfile = filepath.Join(dir, tt.outfile)
 			}
 			inf := filepath.Join(dir, tt.infile)
-			c.flattener = flattenPassthrough
 
+			c := &command{Stdout: &stdout, flattener: tt.flattener}
 			err := c.execute(inf, tt.outfile)
+
+			if tt.assertion != nil {
+				tt.assertion(t, dir)
+			}
+
 			if tt.wantErr != "" {
 				if err == nil {
 					t.Fatal("expected error, got nil")
@@ -78,10 +115,10 @@ func TestExecute(t *testing.T) {
 				}
 				return
 			}
-			var out []byte
 			if err != nil {
 				t.Fatalf("unexpected error: %v", err)
 			}
+			var out []byte
 			if tt.outfile == "-" {
 				out = stdout.Bytes()
 			} else {
@@ -91,8 +128,9 @@ func TestExecute(t *testing.T) {
 				}
 			}
 			if !bytes.Equal([]byte("foo foo"), out) {
-				t.Errorf("out != foo foo: %s", string(out))
+				t.Errorf("out != foo foo: %q", string(out))
 			}
+
 		})
 	}
 }
@@ -101,3 +139,7 @@ func flattenPassthrough(r io.ReadSeeker, w io.Writer) error {
 	_, err := io.Copy(w, r)
 	return err
 }
+
+func flattenBad(_ io.ReadSeeker, _ io.Writer) error {
+	return errors.New("some error")
+}