Testing: add a DiffJSON api

[xhd2015]

It's tempting to embed this function in xgo, though it's just a wrapper around git, but it feels powerful when combining with reflect.DeepEqual():

Usage:

package dosomething

import (
	"reflect"
	"testing"
)

func TestDoSomething(t *testing.T) {
	type args struct {
             ...
	}
	tests := []struct {
		name    string
		args    args
		want    ...
		wantErr bool
	}{
            ...
	}
	for _, tt := range tests {
		tt := tt
		t.Run(tt.name, func(t *testing.T) {
			got, err := DoSomething(tt.args)
			if (err != nil) != tt.wantErr {
				t.Errorf("DoSomething() error = %v, wantErr %v", err, tt.wantErr)
				return
			}
			if !reflect.DeepEqual(got, tt.want) {
				t.Errorf("DoSomething() = %v, want %v, diff: %s", got, tt.want, DiffJSON(tt.want, got))
			}
		})
	}
}

Assert message:

dosomething_test.go:109: DoSomething() = map[a.go:map[2:4-0:0:0xc0003cee40]], want map[a.go:map[2:4-0:0:0xc0003ced00]], diff: diff --git a/expected b/actual
    index 77721f7..9e95f61 100755
    --- a/expected
    +++ b/actual
    @@ -1,7 +1,7 @@
     {
       "a.go": {
         "2:4-0:0": {
    -      "package": "github.com/xhd2015/xgo",
    +      "package": "github.com/xhd2015/xgo/.",
           "name": "Test",
           "file": "a.go",
           "line": 2,

Implementation:

package assert

import (
	"encoding/json"
	"io/ioutil"
	"os"
	"os/exec"
	"path/filepath"

	"github.com/xhd2015/xgo/support/cmd"
)

func DiffJSON(expected interface{}, actual interface{}) string {
	diff, err := diffJSON(expected, actual)
	if err != nil {
		sep := ""
		if diff != "" {
			sep = ", "
		}
		diff += sep + "err: " + err.Error()
	}
	return diff
}
func diffJSON(expected interface{}, actual interface{}) (string, error) {
	tmpDir, err := os.MkdirTemp("", "diff")
	if err != nil {
		return "", err
	}
	defer os.RemoveAll(tmpDir)

	jsonExpected, err := json.MarshalIndent(expected, "", "  ")
	if err != nil {
		return "", err
	}
	jsonAcutal, err := json.MarshalIndent(actual, "", "  ")
	if err != nil {
		return "", err
	}

	err = ioutil.WriteFile(filepath.Join(tmpDir, "expected"), jsonExpected, 0755)
	if err != nil {
		return "", err
	}
	err = ioutil.WriteFile(filepath.Join(tmpDir, "actual"), jsonAcutal, 0755)
	if err != nil {
		return "", err
	}

	diff, err := cmd.Dir(tmpDir).Output("git", "diff", "--no-index", "--", "expected", "actual")
	if err != nil {
		if exitErr, ok := err.(*exec.ExitError); ok && exitErr.ExitCode() != 0 {
			err = nil
		}
	}
	return diff, err
}

[xhd2015]

This has been added in #249

Source: https://github.com/xhd2015/xgo/blob/master/support/assert/diff.go