Handle Windows long file names in a few more places.

Go can handle long file names on Windows if they are not relative.
This change makes some file paths absolute so that go can handle them.
See https://github.com/golang/go/blob/b86e76681366447798c94abb959bb60875bcc856/src/os/path_windows.go#L133.

The protoc binary cannot handle long filenames either, so this change
generates go protos in the temporary folder and then moves the contents
into place.

go/tools/builders/BUILD.bazel

@@ -121,6 +121,7 @@ go_tool_binary(
 go_tool_binary(
     name = "go-protoc",
     srcs = [
+        "env.go",
         "flags.go",
         "protoc.go",
     ],

go/tools/builders/compile.go

@@ -49,6 +49,7 @@ func run(args []string) error {
 	if err := goenv.update(); err != nil {
 		return err
 	}
+	absOutput := abs(*output)
 
 	var matcher func(f *goMetadata) bool
 	switch *testfilter {
@@ -80,7 +81,7 @@ func run(args []string) error {
 		}
 	}
 	if len(files) <= 0 {
-		return ioutil.WriteFile(*output, []byte(""), 0644)
+		return ioutil.WriteFile(absOutput, []byte(""), 0644)
 	}
 
 	goargs := []string{"tool", "compile"}
@@ -102,7 +103,7 @@ func run(args []string) error {
 		goargs = append(goargs, "-importmap", mapping)
 		strictdeps = append(strictdeps, source)
 	}
-	goargs = append(goargs, "-pack", "-o", *output)
+	goargs = append(goargs, "-pack", "-o", absOutput)
 	goargs = append(goargs, flags.Args()...)
 	for _, f := range files {
 		goargs = append(goargs, f.filename)

go/tools/builders/filter.go

@@ -37,7 +37,7 @@ type goMetadata struct {
 func readFiles(bctx build.Context, inputs []string) ([]*goMetadata, error) {
 	outputs := []*goMetadata{}
 	for _, input := range inputs {
-		if m, err := readGoMetadata(bctx, input, true); err != nil {
+		if m, err := readGoMetadata(bctx, abs(input), true); err != nil {
 			return nil, err
 		} else if m.matched {
 			outputs = append(outputs, m)

go/tools/builders/protoc.go

@@ -57,14 +57,23 @@ func run(args []string) error {
 	if err := flags.Parse(args); err != nil {
 		return err
 	}
+
+	// Output to a temporary folder and then move the contents into place below.
+	// This is to work around long file paths on Windows.
+	tmpDir, err := ioutil.TempDir("", "go_proto")
+	if err != nil {
+		return err
+	}
+	tmpDir = abs(tmpDir)
+
 	pluginBase := filepath.Base(*plugin)
 	pluginName := strings.TrimSuffix(
 		strings.TrimPrefix(filepath.Base(*plugin), "protoc-gen-"), ".exe")
 	for _, m := range imports {
 		options = append(options, fmt.Sprintf("M%v", m))
 	}
 	protoc_args := []string{
-		fmt.Sprintf("--%v_out=%v:%v", pluginName, strings.Join(options, ","), *outPath),
+		fmt.Sprintf("--%v_out=%v:%v", pluginName, strings.Join(options, ","), tmpDir),
 		"--plugin", fmt.Sprintf("%v=%v", strings.TrimSuffix(pluginBase, ".exe"), *plugin),
 		"--descriptor_set_in", strings.Join(descriptors, string(os.PathListSeparator)),
 	}
@@ -95,28 +104,46 @@ func run(args []string) error {
 		}
 	}
 	// Walk the generated files
-	filepath.Walk(*outPath, func(path string, f os.FileInfo, err error) error {
-		if !strings.HasSuffix(path, ".go") {
+	filepath.Walk(tmpDir, func(path string, f os.FileInfo, err error) error {
+		relPath, err := filepath.Rel(tmpDir, path)
+		if err != nil {
+			return err
+		}
+		if relPath == "." {
+			return nil
+		}
+
+		if f.IsDir() {
+			if err := os.Mkdir(abs(filepath.Join(*outPath, relPath)), f.Mode()); !os.IsExist(err) {
+				return err
+			}
 			return nil
 		}
-		info := files[path]
-		if info != nil {
-			info.created = true
+
+		if !strings.HasSuffix(path, ".go") {
 			return nil
 		}
-		info = &genFileInfo{
+
+		info := &genFileInfo{
 			path:    path,
 			base:    filepath.Base(path),
 			created: true,
 		}
-		files[path] = info
+
+		foundInfo := files[relPath]
+		if foundInfo != nil {
+			foundInfo.created = true
+			foundInfo.from = info
+			return nil
+		}
+		files[relPath] = info
 		copyTo := byBase[info.base]
 		switch {
 		case copyTo == nil:
 			// Unwanted output
 		case !copyTo.unique:
 			// not unique, no copy allowed
-		case info.from != nil:
+		case copyTo.from != nil:
 			copyTo.ambiguious = true
 			info.ambiguious = true
 		default:
@@ -138,7 +165,7 @@ func run(args []string) error {
 			if err != nil {
 				return err
 			}
-			if err := ioutil.WriteFile(f.path, data, 0644); err != nil {
+			if err := ioutil.WriteFile(abs(f.path), data, 0644); err != nil {
 				return err
 			}
 		case !f.expected: