LoadedFeaturesIndex#register: Ignore absolute paths entirely

Fix: #383
Ref: fxn/zeitwerk#198

The loaded feature index goal is to prevent requiring the same "feature"
twice if the `$LOAD_PATH` was mutated. e.g:

```ruby
require "bundler" # ~/gems/bundler/lib/bundler.rb
$LOAD_PATH.unshift("some/path")
require "bundler" # some/path/bundler.rb
```

In such scenario Ruby remember that you require "bundler" already
and even though it now maps to a new path, it won't load it again.

This was causing issues with Zeitwerk if it is loaded before bootsnap (see refs),
because the cache wouldn't be cleared.

But ultimately Zeitwerk always require absolute paths, and the concern
described above simply doesn't apply to absolute paths. So we can simply
bail out early in these cases. That fixes the bug, and also means less work
and a smaller index, so win-win.

lib/bootsnap.rb

@@ -123,4 +123,14 @@ def self.default_setup
       end
     end
   end
+
+  if RbConfig::CONFIG['host_os'] =~ /mswin|mingw|cygwin/
+    def self.absolute_path?(path)
+      path[1] == ':'
+    end
+  else
+    def self.absolute_path?(path)
+      path.start_with?('/')
+    end
+  end
 end

lib/bootsnap/load_path_cache/cache.rb

@@ -48,7 +48,7 @@ def find(feature, try_extensions: true)
         reinitialize if (@has_relative_paths && dir_changed?) || stale?
         feature = feature.to_s.freeze
 
-        return feature if absolute_path?(feature)
+        return feature if Bootsnap.absolute_path?(feature)
 
         if feature.start_with?('./', '../')
           return try_extensions ? expand_path(feature) : File.expand_path(feature).freeze
@@ -98,16 +98,6 @@ def find(feature, try_extensions: true)
         raise(LoadPathCache::FallbackScan, '', []) if @development_mode
       end
 
-      if RbConfig::CONFIG['host_os'] =~ /mswin|mingw|cygwin/
-        def absolute_path?(path)
-          path[1] == ':'
-        end
-      else
-        def absolute_path?(path)
-          path.start_with?(SLASH)
-        end
-      end
-
       def unshift_paths(sender, *paths)
         return unless sender == @path_obj
         @mutex.synchronize { unshift_paths_locked(*paths) }

lib/bootsnap/load_path_cache/loaded_features_index.rb

@@ -83,6 +83,11 @@ def key?(feature)
       # 2. Inspect $LOADED_FEATURES upon return from yield to find the matching
       #    entry.
       def register(short, long = nil)
+        # Absolute paths are not a concern.
+        if Bootsnap.absolute_path?(short.to_s)
+          return yield
+        end
+
         if long.nil?
           len = $LOADED_FEATURES.size
           ret = yield

test/load_path_cache/loaded_features_index_test.rb

@@ -136,11 +136,23 @@ def test_derives_initial_state_from_loaded_features
       end
 
       def test_works_with_pathname
-        path = '/tmp/bundler.rb'
+        path = 'bundler.rb'
         pathname = Pathname.new(path)
         @index.register(pathname, path) { true }
         assert(@index.key?(pathname))
       end
+
+      def test_ignores_absolute_paths
+        path = '/tmp/bundler.rb'
+        @index.register(path) { true }
+        refute(@index.key?(path))
+
+        path = '/tmp/bundler.rb'
+        pathname = Pathname.new(path)
+        @index.register(pathname, path) { true }
+        refute(@index.key?(path))
+        refute(@index.key?(pathname))
+      end
     end
   end
 end