Rewrite //go:cgo_ldflag comments to relative paths in stdlib action

go/tools/builders/BUILD.bazel

@@ -70,12 +70,12 @@ filegroup(
         "embedcfg.go",
         "env.go",
         "filter.go",
-        "filter_buildid.go",
         "flags.go",
         "generate_nogo_main.go",
         "generate_test_main.go",
         "importcfg.go",
         "link.go",
+        "make_hermetic.go",
         "read.go",
         "replicate.go",
         "stdlib.go",

go/tools/builders/builder.go

@@ -41,8 +41,8 @@ func main() {
 	switch verb {
 	case "compilepkg":
 		action = compilePkg
-	case "filterbuildid":
-		action = filterBuildID
+	case "make_hermetic":
+		action = makeHermetic
 	case "gentestmain":
 		action = genTestMain
 	case "link":

go/tools/builders/make_hermetic.go

@@ -0,0 +1,111 @@
+// Copyright 2018 The Bazel Authors. All rights reserved.
+//
+// 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 (
+	"bufio"
+	"bytes"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"runtime"
+	"syscall"
+)
+
+// makeHermetic executes the tool on the command line, filtering out any
+// -buildid arguments and rewriting cgo files to not contain absolute paths.
+// It is intended to be used with -toolexec when building the standard library.
+// See stdlib.go for the usage and further comments.
+func makeHermetic(args []string) error {
+	isCgo := filepath.Base(args[0]) == "cgo"
+	var objdir string
+	newArgs := make([]string, 0, len(args))
+	for i := 0; i < len(args); i++ {
+		arg := args[i]
+		if arg == "-buildid" {
+			i++
+			continue
+		} else if isCgo && arg == "-objdir" {
+			objdir = args[i+1]
+		}
+		newArgs = append(newArgs, arg)
+	}
+	if runtime.GOOS == "windows" || isCgo {
+		cmd := exec.Command(newArgs[0], newArgs[1:]...)
+		cmd.Stdout = os.Stdout
+		cmd.Stderr = os.Stderr
+		err := cmd.Run()
+		if err != nil {
+			return err
+		}
+		if isCgo {
+			// Rewrite cgo files to not contain absolute paths in ldflag
+			// comments.
+			return rewriteCgoFiles(objdir)
+		} else {
+			return nil
+		}
+	} else {
+		return syscall.Exec(newArgs[0], newArgs, os.Environ())
+	}
+}
+
+func rewriteCgoFiles(objdir string) error {
+	cgoGoTypes := filepath.Join(objdir, "_cgo_gotypes.go")
+	f, err := os.OpenFile(cgoGoTypes, os.O_RDWR, 0)
+	if err != nil {
+		// If the file doesn't exist for whatever reason, we don't need to
+		// rewrite it.
+		return nil
+	}
+	defer f.Close()
+	fixedData, err := rewriteCgoLdflagComments(f)
+	if err != nil {
+		return err
+	}
+	if err = f.Truncate(0); err != nil {
+		return err
+	}
+	if _, err = f.Seek(0, 0); err != nil {
+		return err
+	}
+	if _, err = f.Write(fixedData); err != nil {
+		return err
+	}
+	return nil
+}
+
+func rewriteCgoLdflagComments(f *os.File) ([]byte, error) {
+	bazelWd := []byte(os.Getenv("BAZEL_WORKING_DIRECTORY") + "/")
+
+	// Replace absolute paths in //go:cgo_ldflag comments with relative paths.
+	// We need to pass absolute paths to the "go install" command for building
+	// the standard library, but we want to strip them from the generated files
+	// to make the build hermetic.
+	s := bufio.NewScanner(f)
+	var buf bytes.Buffer
+	for s.Scan() {
+		line := s.Bytes()
+		if bytes.HasPrefix(line, []byte("//go:cgo_ldflag ")) {
+			line = bytes.ReplaceAll(line, bazelWd, []byte{})
+		}
+		buf.Write(line)
+		buf.WriteByte('\n')
+	}
+	if err := s.Err(); err != nil {
+		return []byte{}, err
+	}
+	return buf.Bytes(), nil
+}

go/tools/builders/stdlib.go

@@ -120,7 +120,14 @@ You may need to use the flags --cpu=x64_windows --compiler=mingw-gcc.`)
 	// creating reproducible builds because the build ids are hashed from
 	// CGO_CFLAGS, which frequently contains absolute paths. As a workaround,
 	// we strip the build ids, since they won't be used after this.
-	installArgs := goenv.goCmd("install", "-toolexec", abs(os.Args[0])+" filterbuildid")
+	// Also rewrite absolute paths in //go:cgo_ldflag comments in files
+	// generated by cgo back to relative paths.
+	bazelWd, err := os.Getwd()
+	if err != nil {
+		return err
+	}
+	os.Setenv("BAZEL_WORKING_DIRECTORY", bazelWd)
+	installArgs := goenv.goCmd("install", "-toolexec", abs(os.Args[0])+" make_hermetic")
 	if len(build.Default.BuildTags) > 0 {
 		installArgs = append(installArgs, "-tags", strings.Join(build.Default.BuildTags, ","))
 	}