rewrite private name map storage to support build caching

We now store how we obfuscated unexported names in the object file
itself, not a separate file. This means that the data can survive in the
build cache, whereas the separate file was being lost. Luckily, we can
just add an extra header to the archive, and other programs like the Go
linker will just ignore it.

import_obfuscation.go

@@ -7,10 +7,8 @@ import (
 	"bufio"
 	"bytes"
 	"encoding/json"
-	"errors"
 	"fmt"
 	"os"
-	"path/filepath"
 	"sort"
 	"strings"
 
@@ -47,21 +45,21 @@ type privateImports struct {
 	privateNames []string
 }
 
-func appendPrivateNameMap(nameMap map[string]string, packageDirectory string) error {
-	file, err := os.Open(filepath.Join(packageDirectory, garbleMapFile))
-	if errors.Is(err, os.ErrNotExist) {
-		return nil
-	}
-	if err != nil {
-		return err
+func appendPrivateNameMap(pkg *goobj2.Package, nameMap map[string]string) error {
+	for _, member := range pkg.ArchiveMembers {
+		if member.ArchiveHeader.Name != garbleMapHeaderName {
+			continue
 		}
-	defer file.Close()
 
-	if err := json.NewDecoder(file).Decode(&nameMap); err != nil {
+		serializedMap := member.ArchiveHeader.Data
+		serializedMap = serializedMap[:bytes.IndexByte(serializedMap, 0x00)]
+		if err := json.Unmarshal(serializedMap, &nameMap); err != nil {
 			return err
 		}
 		return nil
 	}
+	return nil
+}
 
 func obfuscateImports(objPath, importCfgPath string) (garbledImports, privateNameMap map[string]string, err error) {
 	importCfg, err := goobj2.ParseImportCfg(importCfgPath)
@@ -87,8 +85,7 @@ func obfuscateImports(objPath, importCfgPath string) (garbledImports, privateNam
 
 			pkgs = append(pkgs, pkgInfo{pkg, info.Path, private})
 
-			packageDir := filepath.Dir(info.Path)
-			if err := appendPrivateNameMap(privateNameMap, packageDir); err != nil {
+			if err := appendPrivateNameMap(pkg, privateNameMap); err != nil {
 				return nil, nil, fmt.Errorf("error parsing name map %s at %s: %v", pkgPath, info.Path, err)
 			}
 		}

main.go

@@ -31,6 +31,7 @@ import (
 	"time"
 	"unicode"
 
+	"github.com/Binject/debug/goobj2"
 	"golang.org/x/mod/module"
 	"golang.org/x/mod/semver"
 	"golang.org/x/tools/go/ast/astutil"
@@ -115,7 +116,7 @@ var (
 	seed []byte
 )
 
-const garbleMapFile = "garble.map"
+const garbleMapHeaderName = "garble/nameMap"
 
 func saveListedPackages(w io.Writer, flags, patterns []string) error {
 	args := []string{"list", "-json", "-deps", "-export"}
@@ -664,17 +665,36 @@ func transformCompile(args []string) ([]string, error) {
 	}
 
 	if len(privateNameMap) > 0 {
-		outputDirectory := filepath.Dir(flagValue(flags, "-o"))
+		objPath := flagValue(flags, "-o")
+		importcfg := flagValue(flags, "-importcfg")
+		deferred = append(deferred, func() error {
+			importCfg, err := goobj2.ParseImportCfg(importcfg)
+			if err != nil {
+				return err
+			}
 
-		file, err := os.Create(filepath.Join(outputDirectory, garbleMapFile))
+			pkg, err := goobj2.Parse(objPath, pkgPath, importCfg)
 			if err != nil {
-			return nil, err
+				return err
 			}
-		defer file.Close()
 
-		if err := json.NewEncoder(file).Encode(privateNameMap); err != nil {
-			return nil, err
+			data, err := json.Marshal(privateNameMap)
+			if err != nil {
+				return err
 			}
+
+			// Adding an extra archive header is safe,
+			// and shouldn't break other tools like the linker since our header name is unique
+			pkg.ArchiveMembers = append(pkg.ArchiveMembers, goobj2.ArchiveMember{
+				ArchiveHeader: goobj2.ArchiveHeader{
+					Name: garbleMapHeaderName,
+					Size: int64(len(data)),
+					Data: data,
+				},
+			})
+
+			return pkg.Write(objPath)
+		})
 	}
 
 	return append(flags, newPaths...), nil