Delete last rpath when requested (#556)

Deleting the first rpath is a good default because it mirrors the
behaviour of `install_name_tool`. However, it's not useful behaviour
when deleting duplicate rpaths, because it changes the order in which
the runtime paths are searched.

We delete duplicate rpaths in `brew`, so being able to delete the last
instance of the requested rpath instead of the first one would be
useful.

Co-authored-by: William Woodruff <william@yossarian.net>

lib/macho/macho_file.rb

@@ -418,15 +418,24 @@ def add_rpath(path, _options = {})
     # @param options [Hash]
     # @option options [Boolean] :uniq (false) if true, also delete
     #  duplicates of the requested path. If false, delete the first
-    #  instance (by offset) of the requested path.
+    #  instance (by offset) of the requested path, unless :last is true.
+    #  Incompatible with :last.
+    # @option options [Boolean] :last (false) if true, delete the last
+    # instance (by offset) of the requested path. Incompatible with :uniq.
     # @return void
     # @raise [RpathUnknownError] if no such runtime path exists
+    # @raise [ArgumentError] if both :uniq and :last are true
     def delete_rpath(path, options = {})
       uniq = options.fetch(:uniq, false)
-      search_method = uniq ? :select : :find
+      last = options.fetch(:last, false)
+      raise ArgumentError, "Cannot set both :uniq and :last to true" if uniq && last
+
+      search_method = uniq || last ? :select : :find
+      rpath_cmds = command(:LC_RPATH).public_send(search_method) { |r| r.path.to_s == path }
+      rpath_cmds = rpath_cmds.last if last
 
       # Cast rpath_cmds into an Array so we can handle the uniq and non-uniq cases the same way
-      rpath_cmds = Array(command(:LC_RPATH).method(search_method).call { |r| r.path.to_s == path })
+      rpath_cmds = Array(rpath_cmds)
       raise RpathUnknownError, path if rpath_cmds.empty?
 
       # delete the commands in reverse order, offset descending.

test/test_macho.rb

@@ -520,12 +520,57 @@ def test_delete_rpath_uniq
       assert_operator file.ncmds, :<, orig_ncmds
       assert_operator file.sizeofcmds, :<, orig_sizeofcmds
       assert_operator file.rpaths.size, :<, orig_npaths
+      # libdupe rpaths: ["foo", "bar", "foo"]
+      assert_equal file.rpaths, ["bar"] if filename.end_with?("libdupe.dylib")
 
       file.write(actual)
       # ensure we can actually re-load and parse the modified file
       modified = MachO::MachOFile.new(actual)
 
-      assert_empty modified.rpaths
+      assert_empty modified.rpaths unless filename.end_with?("libdupe.dylib")
+      assert_equal file.serialize.bytesize, modified.serialize.bytesize
+      assert_operator modified.ncmds, :<, orig_ncmds
+      assert_operator modified.sizeofcmds, :<, orig_sizeofcmds
+      assert_equal file.rpaths.size, modified.rpaths.size
+      assert_operator modified.rpaths.size, :<, orig_npaths
+    end
+  ensure
+    groups.each do |_, actual|
+      delete_if_exists(actual)
+    end
+  end
+
+  def test_delete_rpath_last
+    groups = SINGLE_ARCHES.map do |arch|
+      ["hello.bin", "hello_actual.bin"].map do |fn|
+        fixture(arch, fn)
+      end
+    end
+
+    groups << ["libdupe.dylib", "libdupe_actual.dylib"].map do |fn|
+      fixture(:x86_64, fn)
+    end
+
+    groups.each do |filename, actual|
+      file = MachO::MachOFile.new(filename)
+
+      refute_empty file.rpaths
+      orig_ncmds = file.ncmds
+      orig_sizeofcmds = file.sizeofcmds
+      orig_npaths = file.rpaths.size
+
+      file.delete_rpath(file.rpaths.first, :last => true)
+      assert_operator file.ncmds, :<, orig_ncmds
+      assert_operator file.sizeofcmds, :<, orig_sizeofcmds
+      assert_operator file.rpaths.size, :<, orig_npaths
+      # libdupe rpaths: ["foo", "bar", "foo"]
+      assert_equal file.rpaths, ["foo", "bar"] if filename.end_with?("libdupe.dylib")
+
+      file.write(actual)
+      # ensure we can actually re-load and parse the modified file
+      modified = MachO::MachOFile.new(actual)
+
+      assert_empty modified.rpaths unless filename.end_with?("libdupe.dylib")
       assert_equal file.serialize.bytesize, modified.serialize.bytesize
       assert_operator modified.ncmds, :<, orig_ncmds
       assert_operator modified.sizeofcmds, :<, orig_sizeofcmds