fix: restore cached binaries into the gem dir

Gemfile.lock

@@ -1,7 +1,7 @@
 PATH
   remote: .
   specs:
-    prebake (0.3.2)
+    prebake (0.3.3)
 
 GEM
   remote: https://rubygems.org/
@@ -91,7 +91,7 @@ CHECKSUMS
   mocha (3.1.0) sha256=75f42d69ebfb1f10b32489dff8f8431d37a418120ecdfc07afe3bc183d4e1d56
   parallel (1.28.0) sha256=33e6de1484baf2524792d178b0913fc8eb94c628d6cfe45599ad4458c638c970
   parser (3.3.11.1) sha256=d17ace7aabe3e72c3cc94043714be27cc6f852f104d81aa284c2281aecc65d54
-  prebake (0.3.2)
+  prebake (0.3.3)
   prism (1.9.0) sha256=7b530c6a9f92c24300014919c9dcbc055bf4cdf51ec30aed099b06cd6674ef85
   public_suffix (7.0.5) sha256=1a8bb08f1bbea19228d3bed6e5ed908d1cb4f7c2726d18bd9cadf60bc676f623
   racc (1.8.1) sha256=4a7f6929691dbec8b5209a0b373bc2614882b55fc5d2e447a21aaa691303d62f

README.md

@@ -88,7 +88,7 @@ All configuration is done through environment variables. No code changes require
 2. Prebake intercepts `Gem::Ext::Builder#build_extensions` before compilation starts.
 3. A cache key is generated from the gem name, version, platform (for example `aarch64-linux`), and Ruby ABI version (e.g., `4.0`).
 4. The plugin checks the configured backend for a precompiled binary matching that key.
-5. **Cache hit**: the prebuilt `.so`/`.bundle` files are extracted directly, with no compiler needed.
+5. **Cache hit**: the prebuilt `.so`/`.bundle` files are extracted to the same places a source build puts them, with no compiler needed.
 6. **Cache miss**: Bundler compiles from source as usual (no impact, same as without the plugin).
 
 ### Publishing compiled gems (publisher)
@@ -227,7 +227,7 @@ PREBAKE_LOG_LEVEL=debug bundle install  # full diagnostics
 - `Backend initialization failed`: check your backend URL and credentials. The plugin disables itself gracefully and Bundler continues normally.
 - `Checksum mismatch`: the downloaded binary doesn't match the stored SHA-256. The plugin automatically falls back to compiling from source.
 - `LoadError: cannot load such file -- bigdecimal.so` (Ruby 4.0): RubyGems 4 [stopped copying `.so` files into `lib/`](https://github.com/ruby/rubygems/pull/9240), and Bundler 2.5.x doesn't properly resolve the new extension directory layout. This is a Bundler bug, not a prebake issue. Fix it by upgrading Bundler: `gem install bundler && bundle update --bundler`. If you're using `ruby/setup-ruby` with `bundler-cache: true` in CI, also delete the stale Actions cache so the extensions are rebuilt with the new Bundler.
-- `LoadError: Could not open library '.../sassc-2.4.0/lib/sassc/libsass.so'` (Ruby 4.0): same root cause as above — RubyGems 4 [stopped copying `.so` files into `lib/`](https://github.com/ruby/rubygems/pull/9240) — but `sassc` resolves `libsass.so` via FFI's absolute-path `File.expand_path("libsass.#{dl_ext}", __dir__)`, which dlopens the literal path under `gem_dir/lib/sassc/`. Bundler can't make `$LOAD_PATH` rescue this, so a Bundler upgrade alone won't fix it. Reproducible without prebake. Switch to [`sassc-embedded`](https://rubygems.org/gems/sassc-embedded), a drop-in replacement that ships a precompiled `libsass` and avoids the FFI absolute-path dance, or pin Ruby 3.x.
+- `LoadError: Could not open library '.../sassc-2.4.0/lib/sassc/libsass.so'`, or `cannot find 'llhttp-ext' library` with `ffi-compiler` below 1.4: prebake before 0.3.3 put cached binaries only in the gem's extension directory, but `sassc` and `llhttp-ffi` load their library over FFI from a fixed path inside the gem directory. Upgrade prebake, then reinstall the affected gem once with `bundle pristine sassc` (or `llhttp-ffi`). Bundler treats the broken install as complete, so a plain `bundle install` leaves it in place.
 - To disable for a single run: `PREBAKE_ENABLED=false bundle install`.
 
 ## Compatibility

lib/prebake/extractor.rb

@@ -22,6 +22,7 @@ def self.install(gem_path, spec)
           dest = File.join(spec.extension_dir, normalized_path(binary, tmpdir))
           FileUtils.mkdir_p(File.dirname(dest))
           FileUtils.cp(binary, dest)
+          restore_in_gem_dir(binary, tmpdir, spec)
           extracted_count += 1
         end
       end
@@ -60,5 +61,19 @@ def self.normalized_path(binary, tmpdir)
 
       relative.sub(%r{\Aextensions?/[^/]+/[^/]+/}, "")
     end
+
+    # A source build leaves binaries in the gem dir too: ffi-compiler builds in
+    # place under ext/, and RubyGems copies extconf output into lib/. FFI gems
+    # dlopen those exact paths (llhttp-ffi, sassc). Only ext/ and lib/ are
+    # restored, because self-hosted cached gems put extension_dir binaries at
+    # the gem root, where no build would leave them.
+    def self.restore_in_gem_dir(binary, tmpdir, spec)
+      relative = binary.delete_prefix("#{tmpdir}/")
+      return unless relative.start_with?("ext/", "lib/")
+
+      dest = File.join(spec.full_gem_path, relative)
+      FileUtils.mkdir_p(File.dirname(dest))
+      FileUtils.cp(binary, dest)
+    end
   end
 end

prebake.gemspec

@@ -2,7 +2,7 @@
 
 Gem::Specification.new do |spec|
   spec.name          = "prebake"
-  spec.version       = "0.3.2"
+  spec.version       = "0.3.3"
   spec.authors       = ["Thejus Paul"]
   spec.email         = ["thejuspaul@pm.me"]
 

test/extractor_test.rb

@@ -9,7 +9,9 @@ class ExtractorTest < Minitest::Test
   def setup
     @tmpdir = Dir.mktmpdir
     @extension_dir = File.join(@tmpdir, "extensions")
+    @gem_dir = File.join(@tmpdir, "gem")
     FileUtils.mkdir_p(@extension_dir)
+    FileUtils.mkdir_p(@gem_dir)
   end
 
   def teardown
@@ -73,6 +75,7 @@ def test_strips_ext_prefix_from_legacy_cached_gems
 
     spec = mock("spec")
     spec.stubs(:extension_dir).returns(@extension_dir)
+    spec.stubs(:full_gem_path).returns(@gem_dir)
 
     Prebake::Extractor.install(gem_path, spec)
 
@@ -137,6 +140,7 @@ def test_strips_lib_prefix_from_legacy_cached_gems
 
     spec = mock("spec")
     spec.stubs(:extension_dir).returns(@extension_dir)
+    spec.stubs(:full_gem_path).returns(@gem_dir)
 
     Prebake::Extractor.install(gem_path, spec)
 
@@ -145,6 +149,51 @@ def test_strips_lib_prefix_from_legacy_cached_gems
     assert_equal "legacy-binary", File.read(extracted)
   end
 
+  # ffi-compiler builds in place under ext/ and RubyGems copies extconf output
+  # into lib/. FFI gems such as llhttp-ffi and sassc dlopen those paths.
+  def test_restores_gem_dir_binaries_to_their_packaged_paths
+    gem_path = build_fake_platform_gem(
+      "testgem", "1.0.0",
+      files: {
+        "ext/x86_64-linux/libtestgem-ext.so" => "in-place-build-output",
+        "lib/testgem/libtestgem.so" => "lib-copy"
+      }
+    )
+
+    spec = mock("spec")
+    spec.stubs(:extension_dir).returns(@extension_dir)
+    spec.stubs(:full_gem_path).returns(@gem_dir)
+
+    Prebake::Extractor.install(gem_path, spec)
+
+    in_place = File.join(@gem_dir, "ext/x86_64-linux/libtestgem-ext.so")
+    lib_copy = File.join(@gem_dir, "lib/testgem/libtestgem.so")
+    assert File.exist?(in_place), "Expected #{in_place} to exist"
+    assert File.exist?(lib_copy), "Expected #{lib_copy} to exist"
+    assert_equal "in-place-build-output", File.read(in_place)
+    assert_equal "lib-copy", File.read(lib_copy)
+  end
+
+  # Self-hosted cached gems carry extension_dir binaries at the gem root, and
+  # extension/ is the hosted copy of extension_dir. Neither came from gem_dir.
+  def test_does_not_write_extension_dir_artifacts_into_gem_dir
+    gem_path = build_fake_platform_gem(
+      "testgem", "1.0.0",
+      files: {
+        "testgem.so" => "self-hosted-root-level",
+        "extension/x86_64-linux/4.0.0/testgem/testgem.so" => "extension-dir-copy"
+      }
+    )
+
+    spec = mock("spec")
+    spec.stubs(:extension_dir).returns(@extension_dir)
+    spec.stubs(:full_gem_path).returns(@gem_dir)
+
+    Prebake::Extractor.install(gem_path, spec)
+
+    assert_empty Dir.children(@gem_dir)
+  end
+
   def test_writes_prebake_marker_on_successful_extraction
     gem_path = build_fake_platform_gem(
       "testgem", "1.0.0",