cmd/tailscaled: handle tailscaled symlink on macOS

When Tailscale is installed via Homebrew, `/usr/local/bin/tailscaled`
is a symlink to the actual binary.

Now when `tailscaled install-system-daemon` runs, it will not attempt
to overwrite that symlink if it already points to the tailscaled binary.
However, if executed binary and the link target differ, the path will
he overwritten - this can happen when a user decides to replace
Homebrew-installed tailscaled with a one compiled from source code.

Fixes #5353

Signed-off-by: Anton Tolchanov <anton@tailscale.com>

cmd/tailscaled/install_darwin.go

@@ -11,6 +11,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"io/fs"
 	"os"
 	"os/exec"
 	"path/filepath"
@@ -83,6 +84,13 @@ func uninstallSystemDaemonDarwin(args []string) (ret error) {
 			ret = err
 		}
 	}
+
+	// Do not delete targetBin if it's a symlink, which happens if it was installed via
+	// Homebrew.
+	if isSymlink(targetBin) {
+		return ret
+	}
+
 	if err := os.Remove(targetBin); err != nil {
 		if os.IsNotExist(err) {
 			err = nil
@@ -107,26 +115,56 @@ func installSystemDaemonDarwin(args []string) (err error) {
 	// Best effort:
 	uninstallSystemDaemonDarwin(nil)
 
-	// Copy ourselves to /usr/local/bin/tailscaled.
-	if err := os.MkdirAll(filepath.Dir(targetBin), 0755); err != nil {
-		return err
-	}
 	exe, err := os.Executable()
 	if err != nil {
 		return fmt.Errorf("failed to find our own executable path: %w", err)
 	}
-	tmpBin := targetBin + ".tmp"
+
+	same, err := sameFile(exe, targetBin)
+	if err != nil {
+		return err
+	}
+
+	// Do not overwrite targetBin with the binary file if it it's already
+	// pointing to it. This is primarily to handle Homebrew that writes
+	// /usr/local/bin/tailscaled is a symlink to the actual binary.
+	if !same {
+		if err := copyBinary(exe, targetBin); err != nil {
+			return err
+		}
+	}
+	if err := os.WriteFile(sysPlist, []byte(darwinLaunchdPlist), 0700); err != nil {
+		return err
+	}
+
+	if out, err := exec.Command("launchctl", "load", sysPlist).CombinedOutput(); err != nil {
+		return fmt.Errorf("error running launchctl load %s: %v, %s", sysPlist, err, out)
+	}
+
+	if out, err := exec.Command("launchctl", "start", service).CombinedOutput(); err != nil {
+		return fmt.Errorf("error running launchctl start %s: %v, %s", service, err, out)
+	}
+
+	return nil
+}
+
+// copyBinary copies binary file `src` into `dst`.
+func copyBinary(src, dst string) error {
+	if err := os.MkdirAll(filepath.Dir(dst), 0755); err != nil {
+		return err
+	}
+	tmpBin := dst + ".tmp"
 	f, err := os.Create(tmpBin)
 	if err != nil {
 		return err
 	}
-	self, err := os.Open(exe)
+	srcf, err := os.Open(src)
 	if err != nil {
 		f.Close()
 		return err
 	}
-	_, err = io.Copy(f, self)
-	self.Close()
+	_, err = io.Copy(f, srcf)
+	srcf.Close()
 	if err != nil {
 		f.Close()
 		return err
@@ -137,21 +175,27 @@ func installSystemDaemonDarwin(args []string) (err error) {
 	if err := os.Chmod(tmpBin, 0755); err != nil {
 		return err
 	}
-	if err := os.Rename(tmpBin, targetBin); err != nil {
+	if err := os.Rename(tmpBin, dst); err != nil {
 		return err
 	}
 
-	if err := os.WriteFile(sysPlist, []byte(darwinLaunchdPlist), 0700); err != nil {
-		return err
-	}
+	return nil
+}
 
-	if out, err := exec.Command("launchctl", "load", sysPlist).CombinedOutput(); err != nil {
-		return fmt.Errorf("error running launchctl load %s: %v, %s", sysPlist, err, out)
-	}
+func isSymlink(path string) bool {
+	fi, err := os.Lstat(path)
+	return err == nil && (fi.Mode()&os.ModeSymlink == os.ModeSymlink)
+}
 
-	if out, err := exec.Command("launchctl", "start", service).CombinedOutput(); err != nil {
-		return fmt.Errorf("error running launchctl start %s: %v, %s", service, err, out)
+// sameFile returns true if both file paths exist and resolve to the same file.
+func sameFile(path1, path2 string) (bool, error) {
+	dst1, err := filepath.EvalSymlinks(path1)
+	if err != nil && !errors.Is(err, fs.ErrNotExist) {
+		return false, fmt.Errorf("EvalSymlinks(%s): %w", path1, err)
 	}
-
-	return nil
+	dst2, err := filepath.EvalSymlinks(path2)
+	if err != nil && !errors.Is(err, fs.ErrNotExist) {
+		return false, fmt.Errorf("EvalSymlinks(%s): %w", path2, err)
+	}
+	return dst1 == dst2, nil
 }