Fix LOADED_FEATURES issue:

This fixes an issue where we were defeating ruby's ability to detect
that a feature's name-as-required has already been loaded in situations
where the $LOAD_PATH changed in such a way as to make the feature expand
to a new absolute path.

lib/bootsnap/load_path_cache.rb

@@ -21,11 +21,13 @@ module LoadPathCache
     CACHED_EXTENSIONS = DLEXT2 ? [DOT_RB, DLEXT, DLEXT2] : [DOT_RB, DLEXT]
 
     class << self
-      attr_reader :load_path_cache, :autoload_paths_cache
+      attr_reader :load_path_cache, :autoload_paths_cache, :loaded_features_index
 
       def setup(cache_path:, development_mode:, active_support: true)
         store = Store.new(cache_path)
 
+        @loaded_features_index = LoadedFeaturesIndex.new
+
         @load_path_cache = Cache.new(store, $LOAD_PATH, development_mode: development_mode)
         require_relative 'load_path_cache/core_ext/kernel_require'
 
@@ -50,3 +52,4 @@ def setup(cache_path:, development_mode:, active_support: true)
 require_relative 'load_path_cache/cache'
 require_relative 'load_path_cache/store'
 require_relative 'load_path_cache/change_observer'
+require_relative 'load_path_cache/loaded_features_index'

lib/bootsnap/load_path_cache/core_ext/kernel_require.rb

@@ -11,78 +11,104 @@ def self.make_load_error(path)
 end
 
 module Kernel
-  alias_method :require_without_cache, :require
+  alias_method :require_without_bootsnap, :require
+
+  # Note that require registers to $LOADED_FEATURES while load does not.
+  def require_with_bootsnap_lfi(path, resolved = nil)
+    Bootsnap::LoadPathCache.loaded_features_index.register(path, resolved) do
+      require_without_bootsnap(resolved || path)
+    end
+  end
+
   def require(path)
+    return false if Bootsnap::LoadPathCache.loaded_features_index.key?(path)
+
     if resolved = Bootsnap::LoadPathCache.load_path_cache.find(path)
-      require_without_cache(resolved)
-    else
-      raise Bootsnap::LoadPathCache::CoreExt.make_load_error(path)
+      return require_with_bootsnap_lfi(path, resolved)
     end
+
+    raise Bootsnap::LoadPathCache::CoreExt.make_load_error(path)
   rescue Bootsnap::LoadPathCache::ReturnFalse
     return false
   rescue Bootsnap::LoadPathCache::FallbackScan
-    require_without_cache(path)
+    require_with_bootsnap_lfi(path)
   end
 
-  alias_method :load_without_cache, :load
+  alias_method :load_without_bootsnap, :load
   def load(path, wrap = false)
     if resolved = Bootsnap::LoadPathCache.load_path_cache.find(path)
-      load_without_cache(resolved, wrap)
-    else
-      # load also allows relative paths from pwd even when not in $:
-      relative = File.expand_path(path)
-      if File.exist?(File.expand_path(path))
-        return load_without_cache(relative, wrap)
-      end
-      raise Bootsnap::LoadPathCache::CoreExt.make_load_error(path)
+      return load_without_bootsnap(resolved, wrap)
     end
+
+    # load also allows relative paths from pwd even when not in $:
+    if File.exist?(relative = File.expand_path(path))
+      return load_without_bootsnap(relative, wrap)
+    end
+
+    raise Bootsnap::LoadPathCache::CoreExt.make_load_error(path)
   rescue Bootsnap::LoadPathCache::ReturnFalse
     return false
   rescue Bootsnap::LoadPathCache::FallbackScan
-    load_without_cache(path, wrap)
+    load_without_bootsnap(path, wrap)
   end
 end
 
 class << Kernel
-  alias_method :require_without_cache, :require
+  alias_method :require_without_bootsnap, :require
+
+  def require_with_bootsnap_lfi(path, resolved = nil)
+    Bootsnap::LoadPathCache.loaded_features_index.register(path, resolved) do
+      require_without_bootsnap(resolved || path)
+    end
+  end
+
   def require(path)
+    return false if Bootsnap::LoadPathCache.loaded_features_index.key?(path)
+
     if resolved = Bootsnap::LoadPathCache.load_path_cache.find(path)
-      require_without_cache(resolved)
-    else
-      raise Bootsnap::LoadPathCache::CoreExt.make_load_error(path)
+      return require_with_bootsnap_lfi(path, resolved)
     end
+
+    raise Bootsnap::LoadPathCache::CoreExt.make_load_error(path)
   rescue Bootsnap::LoadPathCache::ReturnFalse
     return false
   rescue Bootsnap::LoadPathCache::FallbackScan
-    require_without_cache(path)
+    require_with_bootsnap_lfi(path)
   end
 
-  alias_method :load_without_cache, :load
+  alias_method :load_without_bootsnap, :load
   def load(path, wrap = false)
     if resolved = Bootsnap::LoadPathCache.load_path_cache.find(path)
-      load_without_cache(resolved, wrap)
-    else
-      # load also allows relative paths from pwd even when not in $:
-      relative = File.expand_path(path)
-      if File.exist?(relative)
-        return load_without_cache(relative, wrap)
-      end
-      raise Bootsnap::LoadPathCache::CoreExt.make_load_error(path)
+      return load_without_bootsnap(resolved, wrap)
     end
+
+    # load also allows relative paths from pwd even when not in $:
+    if File.exist?(relative = File.expand_path(path))
+      return load_without_bootsnap(relative, wrap)
+    end
+
+    raise Bootsnap::LoadPathCache::CoreExt.make_load_error(path)
   rescue Bootsnap::LoadPathCache::ReturnFalse
     return false
   rescue Bootsnap::LoadPathCache::FallbackScan
-    load_without_cache(path, wrap)
+    load_without_bootsnap(path, wrap)
   end
 end
 
 class Module
-  alias_method :autoload_without_cache, :autoload
+  alias_method :autoload_without_bootsnap, :autoload
   def autoload(const, path)
-    autoload_without_cache(const, Bootsnap::LoadPathCache.load_path_cache.find(path) || path)
+    # NOTE: This may defeat LoadedFeaturesIndex, but it's not immediately
+    # obvious how to make it work. This feels like a pretty niche case, unclear
+    # if it will ever burn anyone.
+    #
+    # The challenge is that we don't control the point at which the entry gets
+    # added to $LOADED_FEATURES and won't be able to hook that modification
+    # since it's done in C-land.
+    autoload_without_bootsnap(const, Bootsnap::LoadPathCache.load_path_cache.find(path) || path)
   rescue Bootsnap::LoadPathCache::ReturnFalse
     return false
   rescue Bootsnap::LoadPathCache::FallbackScan
-    autoload_without_cache(const, path)
+    autoload_without_bootsnap(const, path)
   end
 end

lib/bootsnap/load_path_cache/loaded_features_index.rb

@@ -0,0 +1,95 @@
+module Bootsnap
+  module LoadPathCache
+    # LoadedFeaturesIndex partially mirrors an internal structure in ruby that
+    # we can't easily obtain an interface to.
+    #
+    # This works around an issue where, without bootsnap, *ruby* knows that it
+    # has already required a file by its short name (e.g. require 'bundler') if
+    # a new instance of bundler is added to the $LOAD_PATH which resolves to a
+    # different absolute path. This class makes bootsnap smart enough to
+    # realize that it has already loaded 'bundler', and not just
+    # '/path/to/bundler'.
+    #
+    # If you disable LoadedFeaturesIndex, you can see the problem this solves by:
+    #
+    # 1. `require 'a'`
+    # 2. Prepend a new $LOAD_PATH element containing an `a.rb`
+    # 3. `require 'a'`
+    #
+    # Ruby returns false from step 3.
+    # With bootsnap but with no LoadedFeaturesIndex, this loads two different
+    #   `a.rb`s.
+    # With bootsnap and with LoadedFeaturesIndex, this skips the second load,
+    #   returning false like ruby.
+    class LoadedFeaturesIndex
+      def initialize
+        @lfi = {}
+        @mutex = defined?(::Mutex) ? ::Mutex.new : ::Thread::Mutex.new # TODO: Remove once Ruby 2.2 support is dropped.
+
+        # In theory the user could mutate $LOADED_FEATURES and invalidate our
+        # cache. If this ever comes up in practice — or if you, the
+        # enterprising reader, feels inclined to solve this problem — we could
+        # parallel the work done with ChangeObserver on $LOAD_PATH to mirror
+        # updates to our @lfi.
+        $LOADED_FEATURES.each do |feat|
+          $LOAD_PATH.each do |lpe|
+            next unless feat.start_with?(lpe)
+            # /a/b/lib/my/foo.rb
+            #          ^^^^^^^^^
+            short = feat[(lpe.length + 1)..-1]
+            @lfi[short] = true
+            @lfi[strip_extension(short)] = true
+          end
+        end
+      end
+
+      def key?(feature)
+        @mutex.synchronize { @lfi.key?(feature) }
+      end
+
+      # There is a relatively uncommon case where we could miss adding an
+      # entry:
+      #
+      # If the user asked for e.g. `require 'bundler'`, and we went through the
+      # `FallbackScan` pathway in `kernel_require.rb` and therefore did not
+      # pass `long` (the full expanded absolute path), then we did are not able
+      # to confidently add the `bundler.rb` form to @lfi.
+      #
+      # We could either:
+      #
+      # 1. Just add `bundler.rb`, `bundler.so`, and so on, which is close but
+      #    not quite right; or
+      # 2. Inspect $LOADED_FEATURES upon return from yield to find the matching
+      #    entry.
+      def register(short, long = nil)
+        ret = yield
+
+        # do we have 'bundler' or 'bundler.rb'?
+        altname = if File.extname(short) != ''
+          # strip the path from 'bundler.rb' -> 'bundler'
+          strip_extension(short)
+        elsif long && ext = File.extname(long)
+          # get the extension from the expanded path if given
+          # 'bundler' + '.rb'
+          short + ext
+        end
+
+        @mutex.synchronize do
+          @lfi[short] = true
+          (@lfi[altname] = true) if altname
+        end
+
+        ret
+      end
+
+      private
+
+      STRIP_EXTENSION = /\..*?$/
+      private_constant :STRIP_EXTENSION
+
+      def strip_extension(f)
+        f.sub(STRIP_EXTENSION, '')
+      end
+    end
+  end
+end

lib/bootsnap/version.rb

@@ -1,3 +1,3 @@
 module Bootsnap
-  VERSION = "1.1.8"
+  VERSION = "1.2.0.pre"
 end

test/load_path_cache/loaded_features_index_test.rb

@@ -0,0 +1,62 @@
+require 'test_helper'
+
+module Bootsnap
+  module LoadPathCache
+    class LoadedFeaturesIndexTest < MiniTest::Test
+      def setup
+        @index = LoadedFeaturesIndex.new
+        # not really necessary but let's just make it a clean slate
+        @index.instance_variable_set(:@lfi, {})
+      end
+
+      def test_successful_addition
+        refute @index.key?('bundler')
+        refute @index.key?('bundler.rb')
+        refute @index.key?('foo')
+        @index.register('bundler', '/a/b/bundler.rb') {}
+        assert @index.key?('bundler')
+        assert @index.key?('bundler.rb')
+        refute @index.key?('foo')
+      end
+
+      def test_no_add_on_raise
+        refute @index.key?('bundler')
+        refute @index.key?('bundler.rb')
+        refute @index.key?('foo')
+        assert_raises(RuntimeError) do
+          @index.register('bundler', '/a/b/bundler.rb') { raise }
+        end
+        refute @index.key?('bundler')
+        refute @index.key?('bundler.rb')
+        refute @index.key?('foo')
+      end
+
+      def test_infer_base_from_ext
+        refute @index.key?('bundler')
+        refute @index.key?('bundler.rb')
+        refute @index.key?('foo')
+        @index.register('bundler.rb') {}
+        assert @index.key?('bundler')
+        assert @index.key?('bundler.rb')
+        refute @index.key?('foo')
+      end
+
+      def test_cannot_infer_ext_from_base # Current limitation
+        refute @index.key?('bundler')
+        refute @index.key?('bundler.rb')
+        refute @index.key?('foo')
+        @index.register('bundler') {}
+        assert @index.key?('bundler')
+        refute @index.key?('bundler.rb')
+        refute @index.key?('foo')
+      end
+
+      def test_derives_initial_state_from_loaded_features
+        index = LoadedFeaturesIndex.new
+        assert index.key?('minitest/autorun')
+        assert index.key?('minitest/autorun.rb')
+        refute index.key?('minitest/autorun.so')
+      end
+    end
+  end
+end