avoid patching our reflect code into _cgo_gotypes.go

When obfuscating a main package whose Go files all import "C",
the first Go file in CompiledGoFiles ends up being _cgo_gotypes.go.
We cannot add our code from reflect_abi_code.go there,
as it leads to the following error about its _trieNode type:

    typecheck error: $WORK/b001/_cgo_gotypes.go:185:10: cannot define new methods on non-local type _trieNode

Avoid patching any _cgo_*.go file with our reflect code,
as all of those files are special glue code for cgo.

While here, tweak reflectMainPrePatch to return a string
for consistency with abiNamePatch.

Fixes #916.

main.go

@@ -922,12 +922,14 @@ func parseFiles(lpkg *listedPackage, dir string, paths []string) (files []*ast.F
 
 		var src any
 
-		if lpkg.ImportPath == "internal/abi" && filepath.Base(path) == "type.go" {
+		base := filepath.Base(path)
+		if lpkg.ImportPath == "internal/abi" && base == "type.go" {
 			src, err = abiNamePatch(path)
 			if err != nil {
 				return nil, err
 			}
-		} else if mainPackage && reflectPatchFile == "" {
+		} else if mainPackage && reflectPatchFile == "" && !strings.HasPrefix(base, "_cgo_") {
+			// Note that we cannot add our code to e.g. _cgo_gotypes.go.
 			src, err = reflectMainPrePatch(path)
 			if err != nil {
 				return nil, err
@@ -941,12 +943,15 @@ func parseFiles(lpkg *listedPackage, dir string, paths []string) (files []*ast.F
 			return nil, err
 		}
 
-		if mainPackage && src != nil {
+		if mainPackage && src != "" {
 			astutil.AddNamedImport(fset, file, "_", "unsafe")
 		}
 
 		files = append(files, file)
 	}
+	if mainPackage && reflectPatchFile == "" {
+		return nil, fmt.Errorf("main packages must get reflect code patched in")
+	}
 	return files, nil
 }
 

reflect_abi_patch.go

@@ -43,21 +43,21 @@ var reflectPatchFile = ""
 // We split this into pre/post steps so that all variable names in the generated code
 // can be properly obfuscated - if we added the filled map directly, the obfuscated names
 // would appear as plain strings in the binary.
-func reflectMainPrePatch(path string) ([]byte, error) {
+func reflectMainPrePatch(path string) (string, error) {
 	if reflectPatchFile != "" {
 		// already patched another file in main
-		return nil, nil
+		return "", nil
 	}
 
 	content, err := os.ReadFile(path)
 	if err != nil {
-		return nil, err
+		return "", err
 	}
 	_, code, _ := strings.Cut(reflectAbiCode, "// Injected code below this line.")
 	code = strings.ReplaceAll(code, "//disabledgo:", "//go:")
 	// This constant is declared in our hash.go file.
 	code = strings.ReplaceAll(code, "minHashLength", strconv.Itoa(minHashLength))
-	return append(content, []byte(code)...), nil
+	return string(content) + code, nil
 }
 
 // reflectMainPostPatch populates the name mapping with the final obfuscated->real name

testdata/script/cgo.txtar

@@ -1,8 +1,6 @@
 [!cgo] skip 'this test requires cgo to be enabled'
 
-! exec garble build
-stderr 'cannot define new methods on non-local type _trieNode'
-stop
+exec garble build
 ! stderr 'warning' # check that the C toolchain is happy
 exec ./main
 cmp stdout main.stdout