Extract go_merge into its own package

This change is needed to support building gvisor for Fuchsia, which uses
Chromium's GN build system; at the time of writing, Fuchsia's Go support
does not include explicit enumeration of files, assuming instead that Go
binaries are always built from all Go source files in a given package.
Rather than extending Fuchsia's Go support, it is easier simply to
extract a separate package here.

PiperOrigin-RevId: 227133402
Change-Id: I1c64fff286d9c014b4bd1183b76023b35b60c720

2 files changed, 148 insertions, 0 deletions

diff --git a/tools/go_generics/go_merge/BUILD b/tools/go_generics/go_merge/BUILD
new file mode 100644
index 000000000..a60437962
--- /dev/null
+++ b/tools/go_generics/go_merge/BUILD
@@ -0,0 +1,9 @@
+load("@io_bazel_rules_go//go:def.bzl", "go_binary")
+
+package(licenses = ["notice"])  # Apache 2.0
+
+go_binary(
+    name = "go_merge",
+    srcs = ["main.go"],
+    visibility = ["//visibility:public"],
+)
diff --git a/tools/go_generics/go_merge/main.go b/tools/go_generics/go_merge/main.go
new file mode 100644
index 000000000..2f83facf8
--- /dev/null
+++ b/tools/go_generics/go_merge/main.go
@@ -0,0 +1,139 @@
+// Copyright 2018 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package main
+
+import (
+	"bytes"
+	"flag"
+	"fmt"
+	"go/ast"
+	"go/format"
+	"go/parser"
+	"go/token"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"strconv"
+)
+
+var (
+	output = flag.String("o", "", "output `file`")
+)
+
+func fatalf(s string, args ...interface{}) {
+	fmt.Fprintf(os.Stderr, s, args...)
+	os.Exit(1)
+}
+
+func main() {
+	flag.Usage = func() {
+		fmt.Fprintf(os.Stderr, "Usage: %s [options] <input1> [<input2> ...]\n", os.Args[0])
+		flag.PrintDefaults()
+	}
+
+	flag.Parse()
+	if *output == "" || len(flag.Args()) == 0 {
+		flag.Usage()
+		os.Exit(1)
+	}
+
+	// Load all files.
+	files := make(map[string]*ast.File)
+	fset := token.NewFileSet()
+	var name string
+	for _, fname := range flag.Args() {
+		f, err := parser.ParseFile(fset, fname, nil, parser.ParseComments|parser.DeclarationErrors|parser.SpuriousErrors)
+		if err != nil {
+			fatalf("%v\n", err)
+		}
+
+		files[fname] = f
+		if name == "" {
+			name = f.Name.Name
+		} else if name != f.Name.Name {
+			fatalf("Expected '%s' for package name instead of '%s'.\n", name, f.Name.Name)
+		}
+	}
+
+	// Merge all files into one.
+	pkg := &ast.Package{
+		Name:  name,
+		Files: files,
+	}
+	f := ast.MergePackageFiles(pkg, ast.FilterUnassociatedComments|ast.FilterFuncDuplicates|ast.FilterImportDuplicates)
+
+	// Create a new declaration slice with all imports at the top, merging any
+	// redundant imports.
+	imports := make(map[string]*ast.ImportSpec)
+	var anonImports []*ast.ImportSpec
+	for _, d := range f.Decls {
+		if g, ok := d.(*ast.GenDecl); ok && g.Tok == token.IMPORT {
+			for _, s := range g.Specs {
+				i := s.(*ast.ImportSpec)
+				p, _ := strconv.Unquote(i.Path.Value)
+				var n string
+				if i.Name == nil {
+					n = filepath.Base(p)
+				} else {
+					n = i.Name.Name
+				}
+				if n == "_" {
+					anonImports = append(anonImports, i)
+				} else {
+					if i2, ok := imports[n]; ok {
+						if first, second := i.Path.Value, i2.Path.Value; first != second {
+							fatalf("Conflicting paths for import name '%s': '%s' vs. '%s'\n", n, first, second)
+						}
+					} else {
+						imports[n] = i
+					}
+				}
+			}
+		}
+	}
+	newDecls := make([]ast.Decl, 0, len(f.Decls))
+	if l := len(imports) + len(anonImports); l > 0 {
+		// Non-NoPos Lparen is needed for Go to recognize more than one spec in
+		// ast.GenDecl.Specs.
+		d := &ast.GenDecl{
+			Tok:    token.IMPORT,
+			Lparen: token.NoPos + 1,
+			Specs:  make([]ast.Spec, 0, l),
+		}
+		for _, i := range imports {
+			d.Specs = append(d.Specs, i)
+		}
+		for _, i := range anonImports {
+			d.Specs = append(d.Specs, i)
+		}
+		newDecls = append(newDecls, d)
+	}
+	for _, d := range f.Decls {
+		if g, ok := d.(*ast.GenDecl); !ok || g.Tok != token.IMPORT {
+			newDecls = append(newDecls, d)
+		}
+	}
+	f.Decls = newDecls
+
+	// Write the output file.
+	var buf bytes.Buffer
+	if err := format.Node(&buf, fset, f); err != nil {
+		fatalf("%v\n", err)
+	}
+
+	if err := ioutil.WriteFile(*output, buf.Bytes(), 0644); err != nil {
+		fatalf("%v\n", err)
+	}
+}