Handle long filenames on Windows in a few more places. (#2781)

C.f. #1382

go/tools/builders/pack.go

@@ -347,6 +347,8 @@ func simpleName(name string, names map[string]struct{}) (string, error) {
 }
 
 func appendFiles(goenv *env, archive string, files []string) error {
+	archive = abs(archive) // required for long filenames on Windows.
+
 	// Create an empty archive if one doesn't already exist.
 	// In Go 1.16, 'go tool pack r' reports an error if the archive doesn't exist.
 	// 'go tool pack c' copies export data in addition to creating the archive,

go/tools/builders/protoc.go

@@ -25,6 +25,7 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
+	"runtime"
 	"strings"
 )
 
@@ -77,6 +78,11 @@ func run(args []string) error {
 	for _, m := range imports {
 		options = append(options, fmt.Sprintf("M%v", m))
 	}
+	if runtime.GOOS == "windows" {
+		// Turn the plugin path into raw form, since we're handing it off to a non-go binary.
+		// This is required to work with long paths on Windows.
+		*plugin = "\\\\?\\" + abs(*plugin)
+	}
 	protoc_args := []string{
 		fmt.Sprintf("--%v_out=%v:%v", pluginName, strings.Join(options, ","), tmpDir),
 		"--plugin", fmt.Sprintf("%v=%v", strings.TrimSuffix(pluginBase, ".exe"), *plugin),