gem(native): assert on rubygems native darwin platforms

related to #2079

test/test_gem_platform.rb

@@ -0,0 +1,69 @@
+require "helper"
+
+class TestGemPlatform < Nokogiri::TestCase
+  # These tests were written to help me better understand the behavior
+  # of Rubygems's platform-matching behavior. They don't exercise code
+  # in Nokogiri, but do express the expectations we have of Rubygems
+  # and `gem install` behavior for our native darwin gem.
+  #
+  # More specifically, the grpc gem ships a native darwin gem with
+  # platform string `universal-darwin` which I *think* refers to the
+  # fact that it supports i386-and-x86_64 architectures (and not that
+  # it supports x86_64-and-arm64 architectures).
+  #
+  # With ARM support imminent [1] I'm a little worried about using
+  # `universal` until I understand better whether ARM support will be
+  # implied by `universal` or `universal2` or what. From
+  # https://en.wikipedia.org/wiki/Universal_binary:
+  #
+  # > The new Universal 2 binary format was introduced at the 2020
+  # > Worldwide Developers Conference. Universal 2 allows applications
+  # > to run on both Intel x86-64-based and ARM64-based Macintosh
+  # > computers, for the Mac transition to Apple Silicon.
+  #
+  # [1]: https://arstechnica.com/gadgets/2020/06/this-is-apples-roadmap-for-moving-the-first-macs-away-from-intel/
+  #
+  # There seems to be a lot of uncertainty right now, so I'm writing
+  # some tests to express my expectations, and we're gonig to
+  # conservatively use `x86_64-darwin` as the platform name for the
+  # native gem.
+  let(:darwin_gem_platform) { Gem::Platform.new("x86_64-darwin") }
+
+  describe "darwin" do
+    it "builds a gem that works on all darwin x86+64 platforms" do
+      assert_match darwin_gem_platform,
+                   Gem::Platform.new(["universal", "darwin", "19"]),
+                   "gem should match system-installed ruby on catalina"
+      assert_match darwin_gem_platform,
+                   Gem::Platform.new(["x86_64", "darwin", "19"]),
+                   "gem should match user-installed ruby on catalina"
+      assert_match darwin_gem_platform,
+                   Gem::Platform.new(["x86_64", "darwin", "18"]),
+                   "gem should match user-installed ruby on mojave"
+
+      # The intention here is to test that the x86_64 platform gems
+      # won't match a future ruby that is compiled on arm64/aarch64.
+      #
+      # I don't know what the future platform will call itself, but at
+      # least one of these should be right and none of them should
+      # match, so here are all the tests I can reasonably imagine.
+      #
+      # Feel free to delete assertions for clarity once we know more.
+      assert_no_match darwin_gem_platform,
+                      Gem::Platform.new(["arm64", "darwin", "19"]),
+                      "gem should not match an arm64 ruby"
+      assert_no_match darwin_gem_platform,
+                      Gem::Platform.new(["arm", "darwin", "19"]),
+                      "gem should not match an arm ruby"
+      assert_no_match darwin_gem_platform,
+                      Gem::Platform.new(["aarch64", "darwin", "19"]),
+                      "gem should not match an aarch64 ruby"
+      assert_no_match darwin_gem_platform,
+                      Gem::Platform.new(["universal2", "darwin", "19"]),
+                      "gem should not match an aarch64 ruby"
+      assert_no_match darwin_gem_platform,
+                      Gem::Platform.new(["universal_2", "darwin", "19"]),
+                      "gem should not match an aarch64 ruby"
+    end
+  end
+end