cmd: removes hidden dependency on diff command (#6452)

Replace call to local diff command as a hidden dependency
with call to go-diff, which is already included in the repo.

Fixes #6284

Signed-off-by: Colin Lacy <colinjlacy@gmail.com>

cmd/fmt.go

@@ -9,9 +9,9 @@ import (
 	"fmt"
 	"io"
 	"os"
-	"os/exec"
 	"path/filepath"
 
+	"github.com/sergi/go-diff/diffmatchpatch"
 	"github.com/spf13/cobra"
 
 	"github.com/open-policy-agent/opa/format"
@@ -135,14 +135,10 @@ func formatFile(params *fmtCommandParams, out io.Writer, filename string, info o
 
 	if params.diff {
 		if changed {
-			stdout, stderr, err := doDiff(contents, formatted)
-			if err != nil && stdout.Len() == 0 {
-				fmt.Fprintln(os.Stderr, stderr.String())
-				return newError("failed to diff formatting: %v", err)
+			diffString := doDiff(contents, formatted)
+			if _, err := fmt.Fprintln(out, diffString); err != nil {
+				return newError("failed to print contents: %v", err)
 			}
-
-			fmt.Fprintln(out, stdout.String())
-
 			if params.fail {
 				return newError("unexpected diff")
 			}
@@ -183,34 +179,10 @@ func formatStdin(params *fmtCommandParams, r io.Reader, w io.Writer) error {
 	return err
 }
 
-func doDiff(old, new []byte) (stdout, stderr bytes.Buffer, err error) {
-	o, err := os.CreateTemp("", ".opafmt")
-	if err != nil {
-		return stdout, stderr, err
-	}
-	defer os.Remove(o.Name())
-	defer o.Close()
-
-	n, err := os.CreateTemp("", ".opafmt")
-	if err != nil {
-		return stdout, stderr, err
-	}
-	defer os.Remove(n.Name())
-	defer n.Close()
-
-	_, err = o.Write(old)
-	if err != nil {
-		return stdout, stderr, err
-	}
-	_, err = n.Write(new)
-	if err != nil {
-		return stdout, stderr, err
-	}
-
-	cmd := exec.Command("diff", "-u", o.Name(), n.Name())
-	cmd.Stdout = &stdout
-	cmd.Stderr = &stderr
-	return stdout, stderr, cmd.Run()
+func doDiff(old, new []byte) (diffString string) {
+	dmp := diffmatchpatch.New()
+	diffs := dmp.DiffMain(string(old), string(new), false)
+	return dmp.DiffPrettyText(diffs)
 }
 
 type fmtError struct {

cmd/fmt_test.go

@@ -54,6 +54,14 @@ p {
 }
 `
 
+type errorWriter struct {
+	ErrMsg string
+}
+
+func (ew errorWriter) Write(p []byte) (n int, err error) {
+	return 0, fmt.Errorf(ew.ErrMsg)
+}
+
 func TestFmtFormatFile(t *testing.T) {
 	params := fmtCommandParams{}
 	var stdout bytes.Buffer
@@ -77,6 +85,36 @@ func TestFmtFormatFile(t *testing.T) {
 	})
 }
 
+func TestFmtFormatFileFailToReadFile(t *testing.T) {
+
+	params := fmtCommandParams{
+		diff: true,
+	}
+
+	var stdout = bytes.Buffer{}
+
+	files := map[string]string{
+		"policy.rego": unformatted,
+	}
+
+	notThere := "notThere.rego"
+
+	test.WithTempFS(files, func(path string) {
+		policyFile := filepath.Join(path, "policy.rego")
+		info, err := os.Stat(policyFile)
+		err = formatFile(&params, &stdout, notThere, info, err)
+		if err == nil {
+			t.Fatalf("Expected error, found none")
+		}
+
+		actual := err.Error()
+
+		if !strings.Contains(actual, notThere) {
+			t.Fatalf("Expected error message to include %s, got:\n%s\n\n", notThere, actual)
+		}
+	})
+}
+
 func TestFmtFormatFileNoChanges(t *testing.T) {
 	params := fmtCommandParams{}
 	var stdout bytes.Buffer
@@ -152,6 +190,35 @@ func TestFmtFormatFileDiff(t *testing.T) {
 	})
 }
 
+func TestFmtFormatFileFailToPrintDiff(t *testing.T) {
+
+	params := fmtCommandParams{
+		diff: true,
+	}
+
+	errMsg := "io.Write error"
+	var stdout = errorWriter{ErrMsg: errMsg}
+
+	files := map[string]string{
+		"policy.rego": unformatted,
+	}
+
+	test.WithTempFS(files, func(path string) {
+		policyFile := filepath.Join(path, "policy.rego")
+		info, err := os.Stat(policyFile)
+		err = formatFile(&params, &stdout, policyFile, info, err)
+		if err == nil {
+			t.Fatalf("Expected error, found none")
+		}
+
+		actual := err.Error()
+
+		if !strings.Contains(actual, errMsg) {
+			t.Fatalf("Expected error message to include %s, got:\n%s\n\n", errMsg, actual)
+		}
+	})
+}
+
 func TestFmtFormatFileList(t *testing.T) {
 	params := fmtCommandParams{
 		list: true,