tstest/integration: use linkat to hardlink test binaries on Linux

Use linkat via /proc/self/fd with AT_SYMLINK_FOLLOW to create a
hardlink of the test binary instead of copying it. This avoids
copying ~50MB+ binaries into each test's temp directory, making
test setup faster and reducing disk I/O.

The simpler os.Link(b.Path, ret.Path) can't be used here because
the source binary lives 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, but os.Link needs a valid
path. (See also b9f468240f which tried os.Link but is racy for
this reason.)

The /proc/self/fd approach works without elevated privileges,
unlike AT_EMPTY_PATH which requires CAP_DAC_READ_SEARCH. If the
linkat fails for any reason (e.g. cross-filesystem temp dirs), it
falls back to the existing full-copy path.

Fixes #19397

Change-Id: I4b1f97f7e63a9ae9e09dce36dfbdd1f6cff92320
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

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")
+}