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) // required to work with long paths on Windows
 
 	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/env.go

@@ -41,6 +41,10 @@ type GoEnv struct {
 	ld_flags     multiFlag
 }
 
+// abs returns the absolute representation of path. Some tools/APIs require
+// absolute paths to work correctly. Most notably, golang on Windows cannot
+// handle relative paths to files whose absolute path is > ~250 chars, while
+// it can handle absolute paths. See http://goo.gl/eqeWjm.
 func abs(path string) string {
 	if abs, err := filepath.Abs(path); err != nil {
 		return path

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/filter_test.go

@@ -124,3 +124,8 @@ func runTest(t *testing.T, bctx build.Context, inputs []string, expect []string)
 		t.Errorf("filter %v,%v,%v,%v: expect %v got %v", bctx.GOOS, bctx.GOARCH, bctx.CgoEnabled, bctx.BuildTags, expect, got)
 	}
 }
+
+// abs is a dummy env.go abs to avoid depending on env.go and flags.go.
+func abs(p string) string {
+	return p
+}

go/tools/builders/protoc.go

@@ -57,14 +57,25 @@ 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)        // required to work with long paths on Windows
+	absOutPath := abs(*outPath) // required to work with long paths on Windows
+	defer os.RemoveAll(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 +106,45 @@ 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
 		}
-		info := files[path]
-		if info != nil {
-			info.created = true
+
+		if f.IsDir() {
+			if err := os.Mkdir(filepath.Join(absOutPath, relPath), f.Mode()); !os.IsExist(err) {
+				return err
+			}
 			return nil
 		}
-		info = &genFileInfo{
+
+		if !strings.HasSuffix(path, ".go") {
+			return nil
+		}
+
+		info := &genFileInfo{
 			path:    path,
 			base:    filepath.Base(path),
 			created: true,
 		}
-		files[path] = info
+
+		if foundInfo, ok := files[relPath]; ok {
+			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 +166,7 @@ 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:
@@ -149,6 +177,7 @@ func run(args []string) error {
 			return errors.New(buf.String())
 		}
 	}
+
 	return nil
 }