tstest/integration/integration.go

@@ -73,7 +73,11 @@ type Binaries struct {
 
 // BinaryInfo describes a tailscale or tailscaled binary.
 type BinaryInfo struct {
-	Path string // abs path to tailscale or tailscaled binary
+	// Path is the absolute path to the tailscale or tailscaled binary.
+	// This path may become invalid after the owning test's TempDir is
+	// cleaned up; use FD (or Contents on Windows) to access the binary
+	// contents.
+	Path string
 	Size int64
 
 	// FD and FDmu are set on Unix to efficiently copy the binary to a new
@@ -88,16 +92,24 @@ type BinaryInfo struct {
 	Contents []byte
 }
 
+// CopyTo copies or hardlinks the binary into dir, returning a new BinaryInfo
+// with an updated Path. The source bytes come from FD (or Contents on Windows),
+// not from b.Path, which may have been deleted when its owning test's TempDir
+// was cleaned up.
 func (b BinaryInfo) CopyTo(dir string) (BinaryInfo, error) {
 	ret := b
 	ret.Path = filepath.Join(dir, path.Base(b.Path))
 
 	switch runtime.GOOS {
 	case "linux":
-		// TODO(bradfitz): be fancy and use linkat with AT_EMPTY_PATH to avoid
-		// copying? I couldn't get it to work, though.
-		// For now, just do the same thing as every other Unix and copy
-		// the binary.
+		// Try to hardlink from the open FD via /proc/self/fd, avoiding a
+		// full copy of the binary. We can't use os.Link(b.Path, ret.Path)
+		// because b.Path is in the first test's TempDir, which may be
+		// cleaned up before later tests call CopyTo. The open FD keeps the
+		// inode alive after the path is deleted.
+		if err := tryLinkat(b.FD, ret.Path); err == nil {
+			return ret, nil
+		}
 		fallthrough
 	case "darwin", "freebsd", "openbsd", "netbsd":
 		f, err := os.OpenFile(ret.Path, os.O_RDWR|os.O_CREATE|os.O_EXCL, 0o755)

tstest/integration/integration_linkat_linux.go

@@ -0,0 +1,24 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+package integration
+
+import (
+	"fmt"
+	"os"
+
+	"golang.org/x/sys/unix"
+)
+
+// tryLinkat attempts to hardlink the file referenced by fd to newpath,
+// avoiding a full copy of the binary. It uses /proc/self/fd/<N> with
+// AT_SYMLINK_FOLLOW, which works without elevated privileges (unlike
+// AT_EMPTY_PATH which requires CAP_DAC_READ_SEARCH).
+func tryLinkat(fd *os.File, newpath string) error {
+	procPath := fmt.Sprintf("/proc/self/fd/%d", fd.Fd())
+	err := unix.Linkat(unix.AT_FDCWD, procPath, unix.AT_FDCWD, newpath, unix.AT_SYMLINK_FOLLOW)
+	if err != nil {
+		return fmt.Errorf("linkat via /proc/self/fd: %w", err)
+	}
+	return nil
+}

tstest/integration/integration_linkat_linux_test.go

@@ -0,0 +1,48 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+package integration
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"golang.org/x/sys/unix"
+)
+
+func TestTryLinkat(t *testing.T) {
+	src := filepath.Join(t.TempDir(), "src")
+	if err := os.WriteFile(src, []byte("hello world"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	fd, err := os.Open(src)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer fd.Close()
+
+	dst := filepath.Join(t.TempDir(), "dst")
+	if err := tryLinkat(fd, dst); err != nil {
+		t.Fatal(err)
+	}
+
+	got, err := os.ReadFile(dst)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(got) != "hello world" {
+		t.Fatalf("got %q, want %q", got, "hello world")
+	}
+
+	var stSrc, stDst unix.Stat_t
+	if err := unix.Stat(src, &stSrc); err != nil {
+		t.Fatal(err)
+	}
+	if err := unix.Stat(dst, &stDst); err != nil {
+		t.Fatal(err)
+	}
+	if stSrc.Ino != stDst.Ino {
+		t.Fatalf("inodes differ: src=%d, dst=%d", stSrc.Ino, stDst.Ino)
+	}
+}

tstest/integration/integration_linkat_other.go

@@ -0,0 +1,15 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !linux
+
+package integration
+
+import (
+	"errors"
+	"os"
+)
+
+func tryLinkat(_ *os.File, _ string) error {
+	return errors.New("linkat with AT_EMPTY_PATH not supported on this OS")
+}