internal/png: update for Go 1.18

internal/png/gen.go

@@ -25,37 +25,48 @@ import (
 	"go/token"
 	"io/ioutil"
 	"os"
+	"os/exec"
 	"path/filepath"
-	"runtime"
 	"strings"
 
 	"golang.org/x/tools/go/ast/astutil"
 )
 
-func run() error {
-	// TODO: Use go/packages with specifying build tags so that stdlibfuzz can be avoided.
-	dir := filepath.Join(runtime.GOROOT(), "src", "image", "png")
+func pngDir() (string, error) {
+	dir, err := exec.Command("go", "list", "-f", "{{.Dir}}", "image/png").Output()
+	if err != nil {
+		return "", err
+	}
+	return strings.TrimSpace(string(dir)), nil
+}
 
-	files, err := ioutil.ReadDir(dir)
+func pngFiles() ([]string, error) {
+	files, err := exec.Command("go", "list", "-f", `{{join .GoFiles ","}}`, "image/png").Output()
+	if err != nil {
+		return nil, err
+	}
+	return strings.Split(strings.TrimSpace(string(files)), ","), nil
+}
+
+func run() error {
+	dir, err := pngDir()
+	if err != nil {
+		return err
+	}
+
+	files, err := pngFiles()
 	if err != nil {
 		return err
 	}
 
 	for _, f := range files {
-		if f.IsDir() {
-			continue
-		}
-		if strings.HasSuffix(f.Name(), "_test.go") {
-			continue
-		}
-
-		in, err := os.Open(filepath.Join(dir, f.Name()))
+		in, err := os.Open(filepath.Join(dir, f))
 		if err != nil {
 			return err
 		}
 		defer in.Close()
 
-		out, err := os.Create("stdlib" + f.Name())
+		out, err := os.Create("stdlib" + f)
 		if err != nil {
 			return err
 		}

internal/png/stdlibfuzz.go

@@ -1,55 +0,0 @@
-// Code generated by gen.go. DO NOT EDIT.
-
-// Copyright 2019 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-//go:build gofuzz
-// +build gofuzz
-
-package png
-
-import (
-	"bytes"
-	"fmt"
-)
-
-func Fuzz(data []byte) int {
-	cfg, err := DecodeConfig(bytes.NewReader(data))
-	if err != nil {
-		return 0
-	}
-	if cfg.Width*cfg.Height > 1e6 {
-		return 0
-	}
-	img, err := Decode(bytes.NewReader(data))
-	if err != nil {
-		return 0
-	}
-	levels := []CompressionLevel{
-		DefaultCompression,
-		NoCompression,
-		BestSpeed,
-		BestCompression,
-	}
-	for _, l := range levels {
-		var w bytes.Buffer
-		e := &Encoder{CompressionLevel: l}
-		err = e.Encode(&w, img)
-		if err != nil {
-			panic(err)
-		}
-		img1, err := Decode(&w)
-		if err != nil {
-			panic(err)
-		}
-		got := img1.Bounds()
-		want := img.Bounds()
-		if !got.Eq(want) {
-			fmt.Printf("bounds0: %#v\n", want)
-			fmt.Printf("bounds1: %#v\n", got)
-			panic("bounds have changed")
-		}
-	}
-	return 1
-}

internal/png/stdlibreader.go

@@ -823,9 +823,17 @@ func (d *decoder) mergePassInto(dst image.Image, src image.Image, pass int) {
 		dstPix, stride, rect = target.Pix, target.Stride, target.Rect
 		bytesPerPixel = 8
 	case *image.Paletted:
-		srcPix = src.(*image.Paletted).Pix
+		source := src.(*image.Paletted)
+		srcPix = source.Pix
 		dstPix, stride, rect = target.Pix, target.Stride, target.Rect
 		bytesPerPixel = 1
+		if len(target.Palette) < len(source.Palette) {
+			// readImagePass can return a paletted image whose implicit palette
+			// length (one more than the maximum Pix value) is larger than the
+			// explicit palette length (what's in the PLTE chunk). Make the
+			// same adjustment here.
+			target.Palette = source.Palette
+		}
 	case *image.RGBA:
 		srcPix = src.(*image.RGBA).Pix
 		dstPix, stride, rect = target.Pix, target.Stride, target.Rect