├── 08-overlays ├── overlay.json ├── div.go ├── mutations │ └── div.go ├── div_test.go └── main.go ├── slides.pdf ├── go.mod ├── gopher_vs_mutant.jpeg ├── prepare.sh ├── 01-coverage ├── div_test.go └── div.go ├── go.sum ├── README.md ├── 02-fixed ├── div.go └── div_test.go ├── 07-tests ├── div.go ├── mutations │ └── div.go ├── div_test.go └── main.go ├── 03-example1 ├── div.go └── div_test.go ├── 04-example2 ├── div.go └── div_test.go ├── 06-mutations ├── div.go ├── mutations │ └── div.go ├── div_test.go └── main.go ├── 05-ast ├── div.go ├── mutations │ └── div.go ├── main.go └── div_test.go ├── .gitignore └── LICENSE /08-overlays/overlay.json: -------------------------------------------------------------------------------- 1 | {"Replace":{"div.go":"mutations/div.go"}} -------------------------------------------------------------------------------- /slides.pdf: -------------------------------------------------------------------------------- https://raw.githubusercontent.com/danicat/gophercon2024/HEAD/slides.pdf -------------------------------------------------------------------------------- /go.mod: -------------------------------------------------------------------------------- 1 | module gophercon2024 2 | 3 | go 1.22.5 4 | 5 | require golang.org/x/tools v0.22.0 6 | -------------------------------------------------------------------------------- /gopher_vs_mutant.jpeg: -------------------------------------------------------------------------------- https://raw.githubusercontent.com/danicat/gophercon2024/HEAD/gopher_vs_mutant.jpeg -------------------------------------------------------------------------------- /prepare.sh: -------------------------------------------------------------------------------- 1 | rm 04-ast/mutations/* 2 | rm 05-mutations/mutations/* 3 | rm 06-tests/mutations/* 4 | rm 07-overlays/mutations/* 5 | -------------------------------------------------------------------------------- /01-coverage/div_test.go: -------------------------------------------------------------------------------- 1 | package main 2 | 3 | import "testing" 4 | 5 | func TestDivide(t *testing.T) { 6 | _, _ = divide(1, 1) 7 | _, _ = divide(1, 0) 8 | } 9 | -------------------------------------------------------------------------------- /go.sum: -------------------------------------------------------------------------------- 1 | golang.org/x/tools v0.22.0 h1:gqSGLZqv+AI9lIQzniJ0nZDRG5GBPsSi+DRNHWNz6yA= 2 | golang.org/x/tools v0.22.0/go.mod h1:aCwcsjqvq7Yqt6TNyX7QMU2enbQ/Gt0bo6krSeEri+c= 3 | -------------------------------------------------------------------------------- /README.md: -------------------------------------------------------------------------------- 1 | # GopherCon 2024 2 | 3 | Supporting material for my talk "Who Tests the Tests?" at Gophercon 2024. 4 | 5 | ![image](gopher_vs_mutant.jpeg) 6 | 7 | Presentation slides can be found [here](slides.pdf) 8 | 9 | -------------------------------------------------------------------------------- /02-fixed/div.go: -------------------------------------------------------------------------------- 1 | package main 2 | 3 | import "errors" 4 | 5 | var ErrDivideByZero = errors.New("cannot divide by zero") 6 | 7 | func divide(dividend, divisor int) (int, error) { 8 | if divisor == 0 { 9 | return 0, ErrDivideByZero 10 | } 11 | 12 | return dividend / divisor, nil 13 | } 14 | -------------------------------------------------------------------------------- /07-tests/div.go: -------------------------------------------------------------------------------- 1 | package main 2 | 3 | import "errors" 4 | 5 | var ErrDivideByZero = errors.New("cannot divide by zero") 6 | 7 | func divide(dividend, divisor int) (int, error) { 8 | if divisor == 0 { 9 | return 0, ErrDivideByZero 10 | } 11 | 12 | return dividend / divisor, nil 13 | } 14 | -------------------------------------------------------------------------------- /01-coverage/div.go: -------------------------------------------------------------------------------- 1 | package main 2 | 3 | import "errors" 4 | 5 | var ErrDivideByZero = errors.New("cannot divide by zero") 6 | 7 | func divide(dividend, divisor int) (int, error) { 8 | if divisor == 0 { 9 | return 0, ErrDivideByZero 10 | } 11 | 12 | return dividend / divisor, nil 13 | } 14 | -------------------------------------------------------------------------------- /03-example1/div.go: -------------------------------------------------------------------------------- 1 | package main 2 | 3 | import "errors" 4 | 5 | var ErrDivideByZero = errors.New("cannot divide by zero") 6 | 7 | func divide(dividend, divisor int) (int, error) { 8 | if !(divisor == 0) { 9 | return 0, ErrDivideByZero 10 | } 11 | 12 | return dividend / divisor, nil 13 | } 14 | -------------------------------------------------------------------------------- /04-example2/div.go: -------------------------------------------------------------------------------- 1 | package main 2 | 3 | import "errors" 4 | 5 | var ErrDivideByZero = errors.New("cannot divide by zero") 6 | 7 | func divide(dividend, divisor int) (int, error) { 8 | if divisor == 0 { 9 | return 0, ErrDivideByZero 10 | } 11 | 12 | return dividend * divisor, nil 13 | } 14 | -------------------------------------------------------------------------------- /06-mutations/div.go: -------------------------------------------------------------------------------- 1 | package main 2 | 3 | import "errors" 4 | 5 | var ErrDivideByZero = errors.New("cannot divide by zero") 6 | 7 | func divide(dividend, divisor int) (int, error) { 8 | if divisor == 0 { 9 | return 0, ErrDivideByZero 10 | } 11 | 12 | return dividend / divisor, nil 13 | } 14 | -------------------------------------------------------------------------------- /08-overlays/div.go: -------------------------------------------------------------------------------- 1 | package main 2 | 3 | import "errors" 4 | 5 | var ErrDivideByZero = errors.New("cannot divide by zero") 6 | 7 | func divide(dividend, divisor int) (int, error) { 8 | if divisor == 0 { 9 | return 0, ErrDivideByZero 10 | } 11 | 12 | return dividend / divisor, nil 13 | } 14 | -------------------------------------------------------------------------------- /07-tests/mutations/div.go: -------------------------------------------------------------------------------- 1 | package main 2 | 3 | import "errors" 4 | 5 | var ErrDivideByZero = errors.New("cannot divide by zero") 6 | 7 | func divide(dividend, divisor int) (int, error) { 8 | if divisor == 0 { 9 | return 0, ErrDivideByZero 10 | } 11 | 12 | return dividend * divisor, nil 13 | } 14 | -------------------------------------------------------------------------------- /06-mutations/mutations/div.go: -------------------------------------------------------------------------------- 1 | package main 2 | 3 | import "errors" 4 | 5 | var ErrDivideByZero = errors.New("cannot divide by zero") 6 | 7 | func divide(dividend, divisor int) (int, error) { 8 | if divisor == 0 { 9 | return 0, ErrDivideByZero 10 | } 11 | 12 | return dividend * divisor, nil 13 | } 14 | -------------------------------------------------------------------------------- /08-overlays/mutations/div.go: -------------------------------------------------------------------------------- 1 | package main 2 | 3 | import "errors" 4 | 5 | var ErrDivideByZero = errors.New("cannot divide by zero") 6 | 7 | func divide(dividend, divisor int) (int, error) { 8 | if divisor == 0 { 9 | return 0, ErrDivideByZero 10 | } 11 | 12 | return dividend * divisor, nil 13 | } 14 | -------------------------------------------------------------------------------- /05-ast/div.go: -------------------------------------------------------------------------------- 1 | package main 2 | 3 | import "errors" 4 | 5 | var ErrDivideByZero = errors.New("cannot divide by zero") 6 | 7 | func divide(dividend, divisor int) (int, error) { 8 | if divisor == 0 { 9 | // I'm a comment 10 | return 0, ErrDivideByZero 11 | } 12 | 13 | return dividend / divisor, nil 14 | } 15 | -------------------------------------------------------------------------------- /05-ast/mutations/div.go: -------------------------------------------------------------------------------- 1 | package main 2 | 3 | import "errors" 4 | 5 | var ErrDivideByZero = errors.New("cannot divide by zero") 6 | 7 | func divide(dividend, divisor int) (int, error) { 8 | if divisor == 0 { 9 | // I'm a comment 10 | return 0, ErrDivideByZero 11 | } 12 | 13 | return dividend / divisor, nil 14 | } 15 | -------------------------------------------------------------------------------- /05-ast/main.go: -------------------------------------------------------------------------------- 1 | package main 2 | 3 | import ( 4 | "go/parser" 5 | "go/printer" 6 | "go/token" 7 | "log" 8 | "os" 9 | ) 10 | 11 | func main() { 12 | filename := "div.go" 13 | fset := token.NewFileSet() 14 | file, err := parser.ParseFile(fset, filename, nil, parser.ParseComments) 15 | if err != nil { 16 | log.Fatalln(err) 17 | } 18 | 19 | out, err := os.Create("mutations/" + filename) 20 | if err != nil { 21 | log.Fatalln(err) 22 | } 23 | 24 | printer.Fprint(out, fset, file) 25 | } 26 | -------------------------------------------------------------------------------- /05-ast/div_test.go: -------------------------------------------------------------------------------- 1 | package main 2 | 3 | import ( 4 | "errors" 5 | "testing" 6 | ) 7 | 8 | func TestDivide(t *testing.T) { 9 | expected := 1 10 | res, err := divide(1, 1) 11 | if err != nil { 12 | t.Fatalf("expected no error, but got %s", err) 13 | } 14 | 15 | if res != expected { 16 | t.Fatalf("expected result %d, but got %d", expected, res) 17 | } 18 | } 19 | 20 | func TestDivideByZero(t *testing.T) { 21 | _, err := divide(1, 0) 22 | if !errors.Is(err, ErrDivideByZero) { 23 | t.Fatalf("expected %s, but got %s", ErrDivideByZero, err) 24 | } 25 | } 26 | -------------------------------------------------------------------------------- /02-fixed/div_test.go: -------------------------------------------------------------------------------- 1 | package main 2 | 3 | import ( 4 | "errors" 5 | "testing" 6 | ) 7 | 8 | func TestDivide(t *testing.T) { 9 | expected := 1 10 | res, err := divide(1, 1) 11 | if err != nil { 12 | t.Fatalf("expected no error, but got %s", err) 13 | } 14 | 15 | if res != expected { 16 | t.Fatalf("expected result %d, but got %d", expected, res) 17 | } 18 | } 19 | 20 | func TestDivideByZero(t *testing.T) { 21 | _, err := divide(1, 0) 22 | if !errors.Is(err, ErrDivideByZero) { 23 | t.Fatalf("expected %s, but got %s", ErrDivideByZero, err) 24 | } 25 | } 26 | -------------------------------------------------------------------------------- /03-example1/div_test.go: -------------------------------------------------------------------------------- 1 | package main 2 | 3 | import ( 4 | "errors" 5 | "testing" 6 | ) 7 | 8 | func TestDivide(t *testing.T) { 9 | expected := 1 10 | res, err := divide(1, 1) 11 | if err != nil { 12 | t.Fatalf("expected no error, but got %s", err) 13 | } 14 | 15 | if res != expected { 16 | t.Fatalf("expected result %d, but got %d", expected, res) 17 | } 18 | } 19 | 20 | func TestDivideByZero(t *testing.T) { 21 | _, err := divide(1, 0) 22 | if !errors.Is(err, ErrDivideByZero) { 23 | t.Fatalf("expected %s, but got %s", ErrDivideByZero, err) 24 | } 25 | } 26 | -------------------------------------------------------------------------------- /04-example2/div_test.go: -------------------------------------------------------------------------------- 1 | package main 2 | 3 | import ( 4 | "errors" 5 | "testing" 6 | ) 7 | 8 | func TestDivide(t *testing.T) { 9 | expected := 1 10 | res, err := divide(1, 1) 11 | if err != nil { 12 | t.Fatalf("expected no error, but got %s", err) 13 | } 14 | 15 | if res != expected { 16 | t.Fatalf("expected result %d, but got %d", expected, res) 17 | } 18 | } 19 | 20 | func TestDivideByZero(t *testing.T) { 21 | _, err := divide(1, 0) 22 | if !errors.Is(err, ErrDivideByZero) { 23 | t.Fatalf("expected %s, but got %s", ErrDivideByZero, err) 24 | } 25 | } 26 | -------------------------------------------------------------------------------- /07-tests/div_test.go: -------------------------------------------------------------------------------- 1 | package main 2 | 3 | import ( 4 | "errors" 5 | "testing" 6 | ) 7 | 8 | func TestDivide(t *testing.T) { 9 | expected := 10 10 | res, err := divide(1, 1) 11 | if err != nil { 12 | t.Fatalf("expected no error, but got %s", err) 13 | } 14 | 15 | if res != expected { 16 | t.Fatalf("expected result %d, but got %d", expected, res) 17 | } 18 | } 19 | 20 | func TestDivideByZero(t *testing.T) { 21 | _, err := divide(1, 0) 22 | if !errors.Is(err, ErrDivideByZero) { 23 | t.Fatalf("expected %s, but got %s", ErrDivideByZero, err) 24 | } 25 | } 26 | -------------------------------------------------------------------------------- /08-overlays/div_test.go: -------------------------------------------------------------------------------- 1 | package main 2 | 3 | import ( 4 | "errors" 5 | "testing" 6 | ) 7 | 8 | func TestDivide(t *testing.T) { 9 | expected := 4 10 | res, err := divide(8, 2) 11 | if err != nil { 12 | t.Fatalf("expected no error, but got %s", err) 13 | } 14 | 15 | if res != expected { 16 | t.Fatalf("expected result %d, but got %d", expected, res) 17 | } 18 | } 19 | 20 | func TestDivideByZero(t *testing.T) { 21 | _, err := divide(1, 0) 22 | if !errors.Is(err, ErrDivideByZero) { 23 | t.Fatalf("expected %s, but got %s", ErrDivideByZero, err) 24 | } 25 | } 26 | -------------------------------------------------------------------------------- /06-mutations/div_test.go: -------------------------------------------------------------------------------- 1 | package main 2 | 3 | import ( 4 | "errors" 5 | "testing" 6 | ) 7 | 8 | func TestDivide(t *testing.T) { 9 | expected := 1 10 | res, err := divide(1, 1) 11 | if err != nil { 12 | t.Fatalf("expected no error, but got %s", err) 13 | } 14 | 15 | if res != expected { 16 | t.Fatalf("expected result %d, but got %d", expected, res) 17 | } 18 | } 19 | 20 | func TestDivideByZero(t *testing.T) { 21 | _, err := divide(1, 0) 22 | if !errors.Is(err, ErrDivideByZero) { 23 | t.Fatalf("expected %s, but got %s", ErrDivideByZero, err) 24 | } 25 | } 26 | -------------------------------------------------------------------------------- /.gitignore: -------------------------------------------------------------------------------- 1 | # If you prefer the allow list template instead of the deny list, see community template: 2 | # https://github.com/github/gitignore/blob/main/community/Golang/Go.AllowList.gitignore 3 | # 4 | # Binaries for programs and plugins 5 | *.exe 6 | *.exe~ 7 | *.dll 8 | *.so 9 | *.dylib 10 | 11 | # Test binary, built with `go test -c` 12 | *.test 13 | 14 | # Output of the go coverage tool, specifically when used with LiteIDE 15 | *.out 16 | 17 | # Dependency directories (remove the comment below to include it) 18 | # vendor/ 19 | 20 | # Go workspace file 21 | go.work 22 | go.work.sum 23 | -------------------------------------------------------------------------------- /06-mutations/main.go: -------------------------------------------------------------------------------- 1 | package main 2 | 3 | import ( 4 | "go/ast" 5 | "go/parser" 6 | "go/printer" 7 | "go/token" 8 | "log" 9 | "os" 10 | 11 | "golang.org/x/tools/go/ast/astutil" 12 | ) 13 | 14 | func main() { 15 | filename := "div.go" 16 | fset := token.NewFileSet() 17 | file, err := parser.ParseFile(fset, filename, nil, 0) 18 | if err != nil { 19 | log.Fatalln(err) 20 | } 21 | 22 | astutil.Apply(file, nil, changeBinaryOperator) 23 | 24 | out, err := os.Create("mutations/" + filename) 25 | if err != nil { 26 | log.Fatalln(err) 27 | } 28 | 29 | printer.Fprint(out, fset, file) 30 | } 31 | 32 | func reverseIfCond(c *astutil.Cursor) bool { 33 | n := c.Node() 34 | switch x := n.(type) { 35 | case *ast.IfStmt: 36 | x.Cond = &ast.UnaryExpr{ 37 | Op: token.NOT, 38 | X: x.Cond, 39 | } 40 | return false 41 | } 42 | return true 43 | } 44 | 45 | func changeBinaryOperator(c *astutil.Cursor) bool { 46 | n := c.Node() 47 | switch x := n.(type) { 48 | case *ast.BinaryExpr: 49 | if x.Op == token.QUO { 50 | x.Op = token.MUL 51 | return false 52 | } 53 | } 54 | return true 55 | } 56 | -------------------------------------------------------------------------------- /LICENSE: -------------------------------------------------------------------------------- 1 | MIT License 2 | 3 | Copyright (c) 2024 Daniela Petruzalek 4 | 5 | Permission is hereby granted, free of charge, to any person obtaining a copy 6 | of this software and associated documentation files (the "Software"), to deal 7 | in the Software without restriction, including without limitation the rights 8 | to use, copy, modify, merge, publish, distribute, sublicense, and/or sell 9 | copies of the Software, and to permit persons to whom the Software is 10 | furnished to do so, subject to the following conditions: 11 | 12 | The above copyright notice and this permission notice shall be included in all 13 | copies or substantial portions of the Software. 14 | 15 | THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR 16 | IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, 17 | FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE 18 | AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER 19 | LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, 20 | OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE 21 | SOFTWARE. 22 | -------------------------------------------------------------------------------- /07-tests/main.go: -------------------------------------------------------------------------------- 1 | package main 2 | 3 | import ( 4 | "encoding/json" 5 | "fmt" 6 | "go/ast" 7 | "go/parser" 8 | "go/printer" 9 | "go/token" 10 | "log" 11 | "os" 12 | "os/exec" 13 | "strings" 14 | "time" 15 | 16 | "golang.org/x/tools/go/ast/astutil" 17 | ) 18 | 19 | func main() { 20 | filename := "div.go" 21 | fset := token.NewFileSet() 22 | file, err := parser.ParseFile(fset, filename, nil, 0) 23 | if err != nil { 24 | log.Fatalln(err) 25 | } 26 | 27 | astutil.Apply(file, nil, changeBinaryOperator) 28 | 29 | out, err := os.Create("mutations/" + filename) 30 | if err != nil { 31 | log.Fatalln(err) 32 | } 33 | 34 | printer.Fprint(out, fset, file) 35 | 36 | results, err := runTests(".") 37 | if err != nil { 38 | log.Fatalln(err) 39 | } 40 | 41 | for _, test := range results { 42 | if test.Test != "" && (test.Action == "pass" || test.Action == "fail") { 43 | fmt.Printf("%20v: %v\n", test.Test, test.Action) 44 | } 45 | } 46 | } 47 | 48 | func reverseIfCond(c *astutil.Cursor) bool { 49 | n := c.Node() 50 | switch x := n.(type) { 51 | case *ast.IfStmt: 52 | x.Cond = &ast.UnaryExpr{ 53 | Op: token.NOT, 54 | X: x.Cond, 55 | } 56 | return false 57 | } 58 | return true 59 | } 60 | 61 | func changeBinaryOperator(c *astutil.Cursor) bool { 62 | n := c.Node() 63 | switch x := n.(type) { 64 | case *ast.BinaryExpr: 65 | if x.Op == token.QUO { 66 | x.Op = token.MUL 67 | return false 68 | } 69 | } 70 | return true 71 | } 72 | 73 | func runTests(pkgDir string) ([]testEvent, error) { 74 | res, err := exec.Command("go", "test", "--json", pkgDir).CombinedOutput() 75 | if err != nil { 76 | log.Println(err) 77 | } 78 | 79 | return parseTestOutput(res) 80 | } 81 | 82 | type testEvent struct { 83 | Time time.Time // encodes as an RFC3339-format string 84 | Action string 85 | Package string 86 | Test string 87 | Elapsed float64 // seconds 88 | Output string 89 | } 90 | 91 | func parseTestOutput(output []byte) ([]testEvent, error) { 92 | var result []testEvent 93 | list := "[" + strings.ReplaceAll(string(output[:len(output)-1]), "\n", ",") + "]" 94 | err := json.Unmarshal([]byte(list), &result) 95 | if err != nil { 96 | return nil, err 97 | } 98 | return result, nil 99 | } 100 | -------------------------------------------------------------------------------- /08-overlays/main.go: -------------------------------------------------------------------------------- 1 | package main 2 | 3 | import ( 4 | "encoding/json" 5 | "fmt" 6 | "go/ast" 7 | "go/parser" 8 | "go/printer" 9 | "go/token" 10 | "log" 11 | "os" 12 | "os/exec" 13 | "strings" 14 | "time" 15 | 16 | "golang.org/x/tools/go/ast/astutil" 17 | ) 18 | 19 | var translate = map[string]string{ 20 | "pass": "SURVIVED", 21 | "fail": "KILLED", 22 | } 23 | 24 | func main() { 25 | filename := "div.go" 26 | fset := token.NewFileSet() 27 | file, err := parser.ParseFile(fset, filename, nil, 0) 28 | if err != nil { 29 | log.Fatalln(err) 30 | } 31 | 32 | astutil.Apply(file, nil, changeBinaryOperator) 33 | 34 | out, err := os.Create("mutations/" + filename) 35 | if err != nil { 36 | log.Fatalln(err) 37 | } 38 | 39 | printer.Fprint(out, fset, file) 40 | 41 | ov := overlay{ 42 | Replace: map[string]string{ 43 | filename: "mutations/" + filename, 44 | }, 45 | } 46 | 47 | ovFile, err := ov.writeTo(".") 48 | if err != nil { 49 | log.Fatalln(err) 50 | } 51 | 52 | results, err := runTests(".", ovFile) 53 | if err != nil { 54 | log.Fatalln(err) 55 | } 56 | 57 | for _, test := range results { 58 | if test.Test != "" && (test.Action == "pass" || test.Action == "fail") { 59 | fmt.Printf("%20v: %v\n", test.Test, translate[test.Action]) 60 | } 61 | } 62 | } 63 | 64 | func reverseIfCond(c *astutil.Cursor) bool { 65 | n := c.Node() 66 | switch x := n.(type) { 67 | case *ast.IfStmt: 68 | x.Cond = &ast.UnaryExpr{ 69 | Op: token.NOT, 70 | X: x.Cond, 71 | } 72 | return false 73 | } 74 | return true 75 | } 76 | 77 | func changeBinaryOperator(c *astutil.Cursor) bool { 78 | n := c.Node() 79 | switch x := n.(type) { 80 | case *ast.BinaryExpr: 81 | if x.Op == token.QUO { 82 | x.Op = token.MUL 83 | return false 84 | } 85 | } 86 | return true 87 | } 88 | 89 | func runTests(pkgDir, overlay string) ([]testEvent, error) { 90 | res, err := exec.Command("go", "test", "--json", "--overlay", overlay, pkgDir).CombinedOutput() 91 | if err != nil { 92 | log.Println(err) 93 | } 94 | 95 | return parseTestOutput(res) 96 | } 97 | 98 | type testEvent struct { 99 | Time time.Time // encodes as an RFC3339-format string 100 | Action string 101 | Package string 102 | Test string 103 | Elapsed float64 // seconds 104 | Output string 105 | } 106 | 107 | func parseTestOutput(output []byte) ([]testEvent, error) { 108 | var result []testEvent 109 | list := "[" + strings.ReplaceAll(string(output[:len(output)-1]), "\n", ",") + "]" 110 | err := json.Unmarshal([]byte(list), &result) 111 | if err != nil { 112 | return nil, err 113 | } 114 | return result, nil 115 | } 116 | 117 | type overlay struct { 118 | Replace map[string]string 119 | } 120 | 121 | func (o *overlay) writeTo(dir string) (string, error) { 122 | bytes, err := json.Marshal(o) 123 | if err != nil { 124 | return "", err 125 | } 126 | 127 | file := dir + "/overlay.json" 128 | f, err := os.Create(file) 129 | if err != nil { 130 | return "", err 131 | } 132 | fmt.Fprintf(f, "%s", bytes) 133 | f.Close() 134 | 135 | return file, nil 136 | 137 | } 138 | --------------------------------------------------------------------------------