unpack_strategy/directory: try preserving hardlinks

Try running `cp -al` to preserve hardlinks on both macOS and Linux.

If that fails, fall back on `cp -a` which can preserve hardlinks on
Linux (but not macOS) assuming target filesystem supports them.

Library/Homebrew/test/unpack_strategy/directory_spec.rb

@@ -9,6 +9,7 @@ RSpec.describe UnpackStrategy::Directory do
     mktmpdir.tap do |path|
       FileUtils.touch path/"file"
       FileUtils.ln_s "file", path/"symlink"
+      FileUtils.ln path/"file", path/"hardlink"
       FileUtils.mkdir path/"folder"
       FileUtils.ln_s "folder", path/"folderSymlink"
     end
@@ -26,6 +27,11 @@ RSpec.describe UnpackStrategy::Directory do
     expect(unpack_dir/"folderSymlink").to be_a_symlink
   end
 
+  it "preserves hardlinks" do
+    strategy.extract(to: unpack_dir)
+    expect((unpack_dir/"file").stat.ino).to eq (unpack_dir/"hardlink").stat.ino
+  end
+
   it "preserves permissions of contained files" do
     FileUtils.chmod 0644, path/"file"
 

Library/Homebrew/unpack_strategy/directory.rb

@@ -20,11 +20,29 @@ module UnpackStrategy
 
     sig { override.params(unpack_dir: Pathname, basename: Pathname, verbose: T::Boolean).void }
     def extract_to_dir(unpack_dir, basename:, verbose:)
-      path.children.each do |child|
-        system_command! "cp",
-                        args:    ["-pR", (child.directory? && !child.symlink?) ? "#{child}/." : child,
-                                  unpack_dir/child.basename],
-                        verbose:
+      path_children = path.children
+      return if path_children.empty?
+
+      existing = unpack_dir.children
+
+      # We run a few cp attempts in the following order:
+      #
+      # 1. Start with `-al` to create hardlinks rather than copying files if the source and
+      #    target are on the same filesystem. On macOS, this is the only cp option that can
+      #    preserve hardlinks but it is only available since macOS 12.3 (file_cmds-353.100.22).
+      # 2. Try `-a` as GNU `cp -a` preserves hardlinks. macOS `cp -a` is identical to `cp -pR`.
+      # 3. Fall back on `-pR` to handle the case where GNU `cp -a` failed. This may happen if
+      #    installing into a filesystem that doesn't support hardlinks like an exFAT USB drive.
+      cp_arg_attempts = ["-a", "-pR"]
+      cp_arg_attempts.unshift("-al") if path.stat.dev == unpack_dir.stat.dev
+
+      cp_arg_attempts.each do |arg|
+        args = [arg, *path_children, unpack_dir]
+        must_succeed = print_stderr = (arg == cp_arg_attempts.last)
+        result = system_command("cp", args:, verbose:, must_succeed:, print_stderr:)
+        break if result.success?
+
+        FileUtils.rm_r(unpack_dir.children - existing)
       end
     end
   end