14 files changed, 18 insertions, 482 deletions

diff --git a/src/cmd/cgo/internal/testso/overlaydir_test.go b/src/cmd/cgo/internal/cgotest/overlaydir.go
index 09a1d512f1..c6b161545d 100644
--- a/src/cmd/cgo/internal/testso/overlaydir_test.go
+++ b/src/cmd/cgo/internal/cgotest/overlaydir.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-package so_test
+package cgotest
 
 import (
 	"io"
@@ -11,11 +11,8 @@ import (
 	"strings"
 )
 
-// overlayDir makes a minimal-overhead copy of srcRoot in which new files may be added.
-//
-// TODO: Once we no longer need to support the misc module in GOPATH mode,
-// factor this function out into a package to reduce duplication.
-func overlayDir(dstRoot, srcRoot string) error {
+// OverlayDir makes a minimal-overhead copy of srcRoot in which new files may be added.
+func OverlayDir(dstRoot, srcRoot string) error {
 	dstRoot = filepath.Clean(dstRoot)
 	if err := os.MkdirAll(dstRoot, 0777); err != nil {
 		return err
diff --git a/src/cmd/cgo/internal/testcarchive/carchive_test.go b/src/cmd/cgo/internal/testcarchive/carchive_test.go
index cdb90fd130..ef59101b1c 100644
--- a/src/cmd/cgo/internal/testcarchive/carchive_test.go
+++ b/src/cmd/cgo/internal/testcarchive/carchive_test.go
@@ -12,6 +12,7 @@ package carchive_test
 import (
 	"bufio"
 	"bytes"
+	"cmd/cgo/internal/cgotest"
 	"debug/elf"
 	"flag"
 	"fmt"
@@ -82,7 +83,7 @@ func testMain(m *testing.M) int {
 	// Copy testdata into GOPATH/src/testarchive, along with a go.mod file
 	// declaring the same path.
 	modRoot := filepath.Join(GOPATH, "src", "testcarchive")
-	if err := overlayDir(modRoot, "testdata"); err != nil {
+	if err := cgotest.OverlayDir(modRoot, "testdata"); err != nil {
 		log.Panic(err)
 	}
 	if err := os.Chdir(modRoot); err != nil {
diff --git a/src/cmd/cgo/internal/testcarchive/overlaydir_test.go b/src/cmd/cgo/internal/testcarchive/overlaydir_test.go
deleted file mode 100644
index 67974c5ed8..0000000000
--- a/src/cmd/cgo/internal/testcarchive/overlaydir_test.go
+++ /dev/null
@@ -1,78 +0,0 @@
-// Copyright 2019 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-package carchive_test
-
-import (
-	"io"
-	"os"
-	"path/filepath"
-	"strings"
-)
-
-// overlayDir makes a minimal-overhead copy of srcRoot in which new files may be added.
-//
-// TODO: Once we no longer need to support the misc module in GOPATH mode,
-// factor this function out into a package to reduce duplication.
-func overlayDir(dstRoot, srcRoot string) error {
-	dstRoot = filepath.Clean(dstRoot)
-	if err := os.MkdirAll(dstRoot, 0777); err != nil {
-		return err
-	}
-
-	srcRoot, err := filepath.Abs(srcRoot)
-	if err != nil {
-		return err
-	}
-
-	return filepath.Walk(srcRoot, func(srcPath string, info os.FileInfo, err error) error {
-		if err != nil || srcPath == srcRoot {
-			return err
-		}
-
-		suffix := strings.TrimPrefix(srcPath, srcRoot)
-		for len(suffix) > 0 && suffix[0] == filepath.Separator {
-			suffix = suffix[1:]
-		}
-		dstPath := filepath.Join(dstRoot, suffix)
-
-		perm := info.Mode() & os.ModePerm
-		if info.Mode()&os.ModeSymlink != 0 {
-			info, err = os.Stat(srcPath)
-			if err != nil {
-				return err
-			}
-			perm = info.Mode() & os.ModePerm
-		}
-
-		// Always copy directories (don't symlink them).
-		// If we add a file in the overlay, we don't want to add it in the original.
-		if info.IsDir() {
-			return os.MkdirAll(dstPath, perm|0200)
-		}
-
-		// If the OS supports symlinks, use them instead of copying bytes.
-		if err := os.Symlink(srcPath, dstPath); err == nil {
-			return nil
-		}
-
-		// Otherwise, copy the bytes.
-		src, err := os.Open(srcPath)
-		if err != nil {
-			return err
-		}
-		defer src.Close()
-
-		dst, err := os.OpenFile(dstPath, os.O_WRONLY|os.O_CREATE|os.O_EXCL, perm)
-		if err != nil {
-			return err
-		}
-
-		_, err = io.Copy(dst, src)
-		if closeErr := dst.Close(); err == nil {
-			err = closeErr
-		}
-		return err
-	})
-}
diff --git a/src/cmd/cgo/internal/testcshared/cshared_test.go b/src/cmd/cgo/internal/testcshared/cshared_test.go
index d1d79b72b0..0f98f2c1dc 100644
--- a/src/cmd/cgo/internal/testcshared/cshared_test.go
+++ b/src/cmd/cgo/internal/testcshared/cshared_test.go
@@ -7,6 +7,7 @@ package cshared_test
 import (
 	"bufio"
 	"bytes"
+	"cmd/cgo/internal/cgotest"
 	"debug/elf"
 	"debug/pe"
 	"encoding/binary"
@@ -137,7 +138,7 @@ func testMain(m *testing.M) int {
 	os.Setenv("GOPATH", GOPATH)
 
 	modRoot := filepath.Join(GOPATH, "src", "testcshared")
-	if err := overlayDir(modRoot, "testdata"); err != nil {
+	if err := cgotest.OverlayDir(modRoot, "testdata"); err != nil {
 		log.Panic(err)
 	}
 	if err := os.Chdir(modRoot); err != nil {
diff --git a/src/cmd/cgo/internal/testcshared/overlaydir_test.go b/src/cmd/cgo/internal/testcshared/overlaydir_test.go
deleted file mode 100644
index 85d6b44eef..0000000000
--- a/src/cmd/cgo/internal/testcshared/overlaydir_test.go
+++ /dev/null
@@ -1,78 +0,0 @@
-// Copyright 2019 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-package cshared_test
-
-import (
-	"io"
-	"os"
-	"path/filepath"
-	"strings"
-)
-
-// overlayDir makes a minimal-overhead copy of srcRoot in which new files may be added.
-//
-// TODO: Once we no longer need to support the misc module in GOPATH mode,
-// factor this function out into a package to reduce duplication.
-func overlayDir(dstRoot, srcRoot string) error {
-	dstRoot = filepath.Clean(dstRoot)
-	if err := os.MkdirAll(dstRoot, 0777); err != nil {
-		return err
-	}
-
-	srcRoot, err := filepath.Abs(srcRoot)
-	if err != nil {
-		return err
-	}
-
-	return filepath.Walk(srcRoot, func(srcPath string, info os.FileInfo, err error) error {
-		if err != nil || srcPath == srcRoot {
-			return err
-		}
-
-		suffix := strings.TrimPrefix(srcPath, srcRoot)
-		for len(suffix) > 0 && suffix[0] == filepath.Separator {
-			suffix = suffix[1:]
-		}
-		dstPath := filepath.Join(dstRoot, suffix)
-
-		perm := info.Mode() & os.ModePerm
-		if info.Mode()&os.ModeSymlink != 0 {
-			info, err = os.Stat(srcPath)
-			if err != nil {
-				return err
-			}
-			perm = info.Mode() & os.ModePerm
-		}
-
-		// Always copy directories (don't symlink them).
-		// If we add a file in the overlay, we don't want to add it in the original.
-		if info.IsDir() {
-			return os.MkdirAll(dstPath, perm|0200)
-		}
-
-		// If the OS supports symlinks, use them instead of copying bytes.
-		if err := os.Symlink(srcPath, dstPath); err == nil {
-			return nil
-		}
-
-		// Otherwise, copy the bytes.
-		src, err := os.Open(srcPath)
-		if err != nil {
-			return err
-		}
-		defer src.Close()
-
-		dst, err := os.OpenFile(dstPath, os.O_WRONLY|os.O_CREATE|os.O_EXCL, perm)
-		if err != nil {
-			return err
-		}
-
-		_, err = io.Copy(dst, src)
-		if closeErr := dst.Close(); err == nil {
-			err = closeErr
-		}
-		return err
-	})
-}
diff --git a/src/cmd/cgo/internal/testlife/life_test.go b/src/cmd/cgo/internal/testlife/life_test.go
index 7beeaa9823..e93d29c4d9 100644
--- a/src/cmd/cgo/internal/testlife/life_test.go
+++ b/src/cmd/cgo/internal/testlife/life_test.go
@@ -6,6 +6,7 @@ package life_test
 
 import (
 	"bytes"
+	"cmd/cgo/internal/cgotest"
 	"internal/testenv"
 	"log"
 	"os"
@@ -30,7 +31,7 @@ func testMain(m *testing.M) int {
 	// Copy testdata into GOPATH/src/cgolife, along with a go.mod file
 	// declaring the same path.
 	modRoot := filepath.Join(GOPATH, "src", "cgolife")
-	if err := overlayDir(modRoot, "testdata"); err != nil {
+	if err := cgotest.OverlayDir(modRoot, "testdata"); err != nil {
 		log.Panic(err)
 	}
 	if err := os.Chdir(modRoot); err != nil {
diff --git a/src/cmd/cgo/internal/testlife/overlaydir_test.go b/src/cmd/cgo/internal/testlife/overlaydir_test.go
deleted file mode 100644
index 034c836248..0000000000
--- a/src/cmd/cgo/internal/testlife/overlaydir_test.go
+++ /dev/null
@@ -1,78 +0,0 @@
-// Copyright 2019 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-package life_test
-
-import (
-	"io"
-	"os"
-	"path/filepath"
-	"strings"
-)
-
-// overlayDir makes a minimal-overhead copy of srcRoot in which new files may be added.
-//
-// TODO: Once we no longer need to support the misc module in GOPATH mode,
-// factor this function out into a package to reduce duplication.
-func overlayDir(dstRoot, srcRoot string) error {
-	dstRoot = filepath.Clean(dstRoot)
-	if err := os.MkdirAll(dstRoot, 0777); err != nil {
-		return err
-	}
-
-	srcRoot, err := filepath.Abs(srcRoot)
-	if err != nil {
-		return err
-	}
-
-	return filepath.Walk(srcRoot, func(srcPath string, info os.FileInfo, err error) error {
-		if err != nil || srcPath == srcRoot {
-			return err
-		}
-
-		suffix := strings.TrimPrefix(srcPath, srcRoot)
-		for len(suffix) > 0 && suffix[0] == filepath.Separator {
-			suffix = suffix[1:]
-		}
-		dstPath := filepath.Join(dstRoot, suffix)
-
-		perm := info.Mode() & os.ModePerm
-		if info.Mode()&os.ModeSymlink != 0 {
-			info, err = os.Stat(srcPath)
-			if err != nil {
-				return err
-			}
-			perm = info.Mode() & os.ModePerm
-		}
-
-		// Always copy directories (don't symlink them).
-		// If we add a file in the overlay, we don't want to add it in the original.
-		if info.IsDir() {
-			return os.MkdirAll(dstPath, perm|0200)
-		}
-
-		// If the OS supports symlinks, use them instead of copying bytes.
-		if err := os.Symlink(srcPath, dstPath); err == nil {
-			return nil
-		}
-
-		// Otherwise, copy the bytes.
-		src, err := os.Open(srcPath)
-		if err != nil {
-			return err
-		}
-		defer src.Close()
-
-		dst, err := os.OpenFile(dstPath, os.O_WRONLY|os.O_CREATE|os.O_EXCL, perm)
-		if err != nil {
-			return err
-		}
-
-		_, err = io.Copy(dst, src)
-		if closeErr := dst.Close(); err == nil {
-			err = closeErr
-		}
-		return err
-	})
-}
diff --git a/src/cmd/cgo/internal/testplugin/overlaydir_test.go b/src/cmd/cgo/internal/testplugin/overlaydir_test.go
deleted file mode 100644
index e2c32d83ce..0000000000
--- a/src/cmd/cgo/internal/testplugin/overlaydir_test.go
+++ /dev/null
@@ -1,78 +0,0 @@
-// Copyright 2019 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-package plugin_test
-
-import (
-	"io"
-	"os"
-	"path/filepath"
-	"strings"
-)
-
-// overlayDir makes a minimal-overhead copy of srcRoot in which new files may be added.
-//
-// TODO: Once we no longer need to support the misc module in GOPATH mode,
-// factor this function out into a package to reduce duplication.
-func overlayDir(dstRoot, srcRoot string) error {
-	dstRoot = filepath.Clean(dstRoot)
-	if err := os.MkdirAll(dstRoot, 0777); err != nil {
-		return err
-	}
-
-	srcRoot, err := filepath.Abs(srcRoot)
-	if err != nil {
-		return err
-	}
-
-	return filepath.Walk(srcRoot, func(srcPath string, info os.FileInfo, err error) error {
-		if err != nil || srcPath == srcRoot {
-			return err
-		}
-
-		suffix := strings.TrimPrefix(srcPath, srcRoot)
-		for len(suffix) > 0 && suffix[0] == filepath.Separator {
-			suffix = suffix[1:]
-		}
-		dstPath := filepath.Join(dstRoot, suffix)
-
-		perm := info.Mode() & os.ModePerm
-		if info.Mode()&os.ModeSymlink != 0 {
-			info, err = os.Stat(srcPath)
-			if err != nil {
-				return err
-			}
-			perm = info.Mode() & os.ModePerm
-		}
-
-		// Always copy directories (don't symlink them).
-		// If we add a file in the overlay, we don't want to add it in the original.
-		if info.IsDir() {
-			return os.MkdirAll(dstPath, perm|0200)
-		}
-
-		// If the OS supports symlinks, use them instead of copying bytes.
-		if err := os.Symlink(srcPath, dstPath); err == nil {
-			return nil
-		}
-
-		// Otherwise, copy the bytes.
-		src, err := os.Open(srcPath)
-		if err != nil {
-			return err
-		}
-		defer src.Close()
-
-		dst, err := os.OpenFile(dstPath, os.O_WRONLY|os.O_CREATE|os.O_EXCL, perm)
-		if err != nil {
-			return err
-		}
-
-		_, err = io.Copy(dst, src)
-		if closeErr := dst.Close(); err == nil {
-			err = closeErr
-		}
-		return err
-	})
-}
diff --git a/src/cmd/cgo/internal/testplugin/plugin_test.go b/src/cmd/cgo/internal/testplugin/plugin_test.go
index e211a96304..4c8e3d7833 100644
--- a/src/cmd/cgo/internal/testplugin/plugin_test.go
+++ b/src/cmd/cgo/internal/testplugin/plugin_test.go
@@ -6,6 +6,7 @@ package plugin_test
 
 import (
 	"bytes"
+	"cmd/cgo/internal/cgotest"
 	"context"
 	"flag"
 	"fmt"
@@ -80,7 +81,7 @@ func testMain(m *testing.M) int {
 		"testdata":                           modRoot,
 		filepath.Join("altpath", "testdata"): altRoot,
 	} {
-		if err := overlayDir(dstRoot, srcRoot); err != nil {
+		if err := cgotest.OverlayDir(dstRoot, srcRoot); err != nil {
 			log.Panic(err)
 		}
 		prettyPrintf("mkdir -p %s\n", dstRoot)
diff --git a/src/cmd/cgo/internal/testshared/overlaydir_test.go b/src/cmd/cgo/internal/testshared/overlaydir_test.go
deleted file mode 100644
index eb587a2d44..0000000000
--- a/src/cmd/cgo/internal/testshared/overlaydir_test.go
+++ /dev/null
@@ -1,78 +0,0 @@
-// Copyright 2019 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-package shared_test
-
-import (
-	"io"
-	"os"
-	"path/filepath"
-	"strings"
-)
-
-// overlayDir makes a minimal-overhead copy of srcRoot in which new files may be added.
-//
-// TODO: Once we no longer need to support the misc module in GOPATH mode,
-// factor this function out into a package to reduce duplication.
-func overlayDir(dstRoot, srcRoot string) error {
-	dstRoot = filepath.Clean(dstRoot)
-	if err := os.MkdirAll(dstRoot, 0777); err != nil {
-		return err
-	}
-
-	srcRoot, err := filepath.Abs(srcRoot)
-	if err != nil {
-		return err
-	}
-
-	return filepath.Walk(srcRoot, func(srcPath string, info os.FileInfo, err error) error {
-		if err != nil || srcPath == srcRoot {
-			return err
-		}
-
-		suffix := strings.TrimPrefix(srcPath, srcRoot)
-		for len(suffix) > 0 && suffix[0] == filepath.Separator {
-			suffix = suffix[1:]
-		}
-		dstPath := filepath.Join(dstRoot, suffix)
-
-		perm := info.Mode() & os.ModePerm
-		if info.Mode()&os.ModeSymlink != 0 {
-			info, err = os.Stat(srcPath)
-			if err != nil {
-				return err
-			}
-			perm = info.Mode() & os.ModePerm
-		}
-
-		// Always copy directories (don't symlink them).
-		// If we add a file in the overlay, we don't want to add it in the original.
-		if info.IsDir() {
-			return os.MkdirAll(dstPath, perm|0200)
-		}
-
-		// If the OS supports symlinks, use them instead of copying bytes.
-		if err := os.Symlink(srcPath, dstPath); err == nil {
-			return nil
-		}
-
-		// Otherwise, copy the bytes.
-		src, err := os.Open(srcPath)
-		if err != nil {
-			return err
-		}
-		defer src.Close()
-
-		dst, err := os.OpenFile(dstPath, os.O_WRONLY|os.O_CREATE|os.O_EXCL, perm)
-		if err != nil {
-			return err
-		}
-
-		_, err = io.Copy(dst, src)
-		if closeErr := dst.Close(); err == nil {
-			err = closeErr
-		}
-		return err
-	})
-}
diff --git a/src/cmd/cgo/internal/testshared/shared_test.go b/src/cmd/cgo/internal/testshared/shared_test.go
index 05e87b9d50..6596d88952 100644
--- a/src/cmd/cgo/internal/testshared/shared_test.go
+++ b/src/cmd/cgo/internal/testshared/shared_test.go
@@ -7,6 +7,7 @@ package shared_test
 import (
 	"bufio"
 	"bytes"
+	"cmd/cgo/internal/cgotest"
 	"debug/elf"
 	"encoding/binary"
 	"flag"
@@ -203,7 +204,7 @@ func TestMain(m *testing.M) {
 // It returns the directory within gopath at which the module root is located.
 func cloneTestdataModule(gopath string) (string, error) {
 	modRoot := filepath.Join(gopath, "src", "testshared")
-	if err := overlayDir(modRoot, "testdata"); err != nil {
+	if err := cgotest.OverlayDir(modRoot, "testdata"); err != nil {
 		return "", err
 	}
 	if err := os.WriteFile(filepath.Join(modRoot, "go.mod"), []byte("module testshared\n"), 0644); err != nil {
@@ -255,7 +256,7 @@ func cloneGOROOTDeps(goroot string) error {
 		if testing.Verbose() {
 			fmt.Fprintf(os.Stderr, "+ cp -r %s %s\n", filepath.Join(oldGOROOT, dir), filepath.Join(goroot, dir))
 		}
-		if err := overlayDir(filepath.Join(goroot, dir), filepath.Join(oldGOROOT, dir)); err != nil {
+		if err := cgotest.OverlayDir(filepath.Join(goroot, dir), filepath.Join(oldGOROOT, dir)); err != nil {
 			return err
 		}
 	}
diff --git a/src/cmd/cgo/internal/testso/so_test.go b/src/cmd/cgo/internal/testso/so_test.go
index e7fa0cadc3..e011167f38 100644
--- a/src/cmd/cgo/internal/testso/so_test.go
+++ b/src/cmd/cgo/internal/testso/so_test.go
@@ -5,6 +5,7 @@
 package so_test
 
 import (
+	"cmd/cgo/internal/cgotest"
 	"internal/testenv"
 	"log"
 	"os"
@@ -38,7 +39,7 @@ func testSO(t *testing.T, dir string) {
 	defer os.RemoveAll(GOPATH)
 
 	modRoot := filepath.Join(GOPATH, "src", "cgosotest")
-	if err := overlayDir(modRoot, filepath.Join("testdata", dir)); err != nil {
+	if err := cgotest.OverlayDir(modRoot, filepath.Join("testdata", dir)); err != nil {
 		log.Panic(err)
 	}
 	if err := os.WriteFile(filepath.Join(modRoot, "go.mod"), []byte("module cgosotest\n"), 0666); err != nil {
diff --git a/src/cmd/cgo/internal/teststdio/overlaydir_test.go b/src/cmd/cgo/internal/teststdio/overlaydir_test.go
deleted file mode 100644
index 027ebf17c3..0000000000
--- a/src/cmd/cgo/internal/teststdio/overlaydir_test.go
+++ /dev/null
@@ -1,78 +0,0 @@
-// Copyright 2019 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-package stdio_test
-
-import (
-	"io"
-	"os"
-	"path/filepath"
-	"strings"
-)
-
-// overlayDir makes a minimal-overhead copy of srcRoot in which new files may be added.
-//
-// TODO: Once we no longer need to support the misc module in GOPATH mode,
-// factor this function out into a package to reduce duplication.
-func overlayDir(dstRoot, srcRoot string) error {
-	dstRoot = filepath.Clean(dstRoot)
-	if err := os.MkdirAll(dstRoot, 0777); err != nil {
-		return err
-	}
-
-	srcRoot, err := filepath.Abs(srcRoot)
-	if err != nil {
-		return err
-	}
-
-	return filepath.Walk(srcRoot, func(srcPath string, info os.FileInfo, err error) error {
-		if err != nil || srcPath == srcRoot {
-			return err
-		}
-
-		suffix := strings.TrimPrefix(srcPath, srcRoot)
-		for len(suffix) > 0 && suffix[0] == filepath.Separator {
-			suffix = suffix[1:]
-		}
-		dstPath := filepath.Join(dstRoot, suffix)
-
-		perm := info.Mode() & os.ModePerm
-		if info.Mode()&os.ModeSymlink != 0 {
-			info, err = os.Stat(srcPath)
-			if err != nil {
-				return err
-			}
-			perm = info.Mode() & os.ModePerm
-		}
-
-		// Always copy directories (don't symlink them).
-		// If we add a file in the overlay, we don't want to add it in the original.
-		if info.IsDir() {
-			return os.MkdirAll(dstPath, perm|0200)
-		}
-
-		// If the OS supports symlinks, use them instead of copying bytes.
-		if err := os.Symlink(srcPath, dstPath); err == nil {
-			return nil
-		}
-
-		// Otherwise, copy the bytes.
-		src, err := os.Open(srcPath)
-		if err != nil {
-			return err
-		}
-		defer src.Close()
-
-		dst, err := os.OpenFile(dstPath, os.O_WRONLY|os.O_CREATE|os.O_EXCL, perm)
-		if err != nil {
-			return err
-		}
-
-		_, err = io.Copy(dst, src)
-		if closeErr := dst.Close(); err == nil {
-			err = closeErr
-		}
-		return err
-	})
-}
diff --git a/src/cmd/cgo/internal/teststdio/stdio_test.go b/src/cmd/cgo/internal/teststdio/stdio_test.go
index f191c50a42..3883422d6f 100644
--- a/src/cmd/cgo/internal/teststdio/stdio_test.go
+++ b/src/cmd/cgo/internal/teststdio/stdio_test.go
@@ -6,6 +6,7 @@ package stdio_test
 
 import (
 	"bytes"
+	"cmd/cgo/internal/cgotest"
 	"internal/testenv"
 	"log"
 	"os"
@@ -31,7 +32,7 @@ func testMain(m *testing.M) int {
 	// Copy testdata into GOPATH/src/cgostdio, along with a go.mod file
 	// declaring the same path.
 	modRoot := filepath.Join(GOPATH, "src", "cgostdio")
-	if err := overlayDir(modRoot, "testdata"); err != nil {
+	if err := cgotest.OverlayDir(modRoot, "testdata"); err != nil {
 		log.Panic(err)
 	}
 	if err := os.Chdir(modRoot); err != nil {