cmd: Add very basic parse cmd coverage

There was an issue a little while back where we had a panic in the
command but there is zero test coverage exercising it... so we never
noticed.

This at least will get us some bare-bones test coverage to ensure
it isn't crashing on some basic use-cases.

Signed-off-by: Patrick East <east.patrick@gmail.com>

cmd/parse.go

@@ -7,6 +7,7 @@ package cmd
 import (
 	"encoding/json"
 	"fmt"
+	"io"
 	"os"
 
 	"github.com/spf13/cobra"
@@ -39,11 +40,11 @@ var parseCommand = &cobra.Command{
 		return nil
 	},
 	Run: func(cmd *cobra.Command, args []string) {
-		os.Exit(parse(args))
+		os.Exit(parse(args, os.Stdout, os.Stderr))
 	},
 }
 
-func parse(args []string) int {
+func parse(args []string, stdout io.Writer, stderr io.Writer) int {
 	if len(args) == 0 {
 		return 0
 	}
@@ -53,22 +54,22 @@ func parse(args []string) int {
 	switch parseParams.format.String() {
 	case parseFormatJSON:
 		if err != nil {
-			pr.JSON(os.Stderr, pr.Output{Errors: pr.NewOutputErrors(err)})
+			pr.JSON(stderr, pr.Output{Errors: pr.NewOutputErrors(err)})
 			return 1
 		}
 
 		bs, err := json.MarshalIndent(result.Parsed, "", "  ")
 		if err != nil {
-			fmt.Fprintln(os.Stderr, err)
+			fmt.Fprintln(stderr, err)
 			return 1
 		}
 		fmt.Println(string(bs))
 	default:
 		if err != nil {
-			fmt.Fprintln(os.Stderr, err)
+			fmt.Fprintln(stderr, err)
 			return 1
 		}
-		ast.Pretty(os.Stdout, result.Parsed)
+		ast.Pretty(stdout, result.Parsed)
 	}
 
 	return 0

cmd/parse_test.go

@@ -0,0 +1,59 @@
+package cmd
+
+import (
+	"bytes"
+	"path/filepath"
+	"testing"
+
+	"github.com/open-policy-agent/opa/util/test"
+)
+
+func TestParseExit0(t *testing.T) {
+
+	files := map[string]string{
+		"x.rego": `package x
+		
+		p = 1
+		`,
+	}
+	errc, _, stderr := testParse(t, files)
+	if errc != 0 {
+		t.Fatalf("Expected exit code 0, got %v", errc)
+	}
+	if len(stderr) > 0 {
+		t.Fatalf("Expected no stderr output, got:\n%s\n", string(stderr))
+	}
+}
+
+func TestParseExit1(t *testing.T) {
+
+	files := map[string]string{
+		"x.rego": `???`,
+	}
+	errc, _, stderr := testParse(t, files)
+	if errc != 1 {
+		t.Fatalf("Expected exit code 1, got %v", errc)
+	}
+	if len(stderr) == 0 {
+		t.Fatalf("Expected output in stderr")
+	}
+}
+
+// Runs parse and returns the exit code, stdout, and stderr contents
+func testParse(t *testing.T, files map[string]string) (int, []byte, []byte) {
+	t.Helper()
+
+	stdout := new(bytes.Buffer)
+	stderr := new(bytes.Buffer)
+	var errc int
+
+	test.WithTempFS(files, func(path string) {
+		var args []string
+		for file := range files {
+			args = append(args, filepath.Join(path, file))
+		}
+		errc = parse(args, stdout, stderr)
+	})
+
+	return errc, stdout.Bytes(), stderr.Bytes()
+}