Add a filesystem access hook to Gem::CompactIndexClient

Bundler's client wraps every cache read and write in
SharedHelpers.filesystem_access so permission problems surface as
friendly Bundler errors instead of bare Errno exceptions. Funnel the
same operations through an injectable hook, defaulting to a plain
yield, so Bundler can install its helper when it switches to this
client.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: hsbt

lib/rubygems/compact_index_client.rb

@@ -32,6 +32,21 @@ def self.debug
 
   class Error < StandardError; end
 
+  # Filesystem reads and writes funnel through this hook so that a host
+  # (e.g. Bundler) can translate low-level Errno exceptions into its own
+  # friendlier errors. The default just yields the path.
+  def self.filesystem_access(path, action = :write, &block)
+    if @filesystem_access
+      @filesystem_access.call(path, action, &block)
+    else
+      yield path
+    end
+  end
+
+  def self.filesystem_access=(hook)
+    @filesystem_access = hook
+  end
+
   require_relative "compact_index_client/cache"
   require_relative "compact_index_client/cache_file"
   require_relative "compact_index_client/http_fetcher"

lib/rubygems/compact_index_client/cache.rb

@@ -86,12 +86,16 @@ def validate_name!(name)
 
     def checksum_for_file(path)
       return unless path.file?
-      Digest::MD5.file(path).hexdigest
+      Gem::CompactIndexClient.filesystem_access(path, :read) do
+        Digest::MD5.file(path).hexdigest
+      end
     end
 
     def mkdir(name)
       directory.join(name).tap do |dir|
-        FileUtils.mkdir_p(dir)
+        Gem::CompactIndexClient.filesystem_access(dir) do
+          FileUtils.mkdir_p(dir)
+        end
       end
     end
 
@@ -112,7 +116,7 @@ def already_fetched?(remote_path)
 
     def read(path)
       return unless path.file?
-      path.read
+      Gem::CompactIndexClient.filesystem_access(path, :read, &:read)
     end
   end
 end

lib/rubygems/compact_index_client/cache_file.rb

@@ -25,8 +25,10 @@ def self.copy(path, &block)
       new(path) do |file|
         file.initialize_digests
 
-        path.open("rb") do |s|
-          file.open {|f| IO.copy_stream(s, f) }
+        Gem::CompactIndexClient.filesystem_access(path, :read) do
+          path.open("rb") do |s|
+            file.open {|f| IO.copy_stream(s, f) }
+          end
         end
 
         yield file
@@ -91,8 +93,10 @@ def digests?
     # Open the temp file for writing, reusing original permissions, yielding the IO object.
     def open(write_mode = "wb", perm = @perm, &block)
       raise ClosedError, "Cannot reopen closed file" if @closed
-      path.open(write_mode, perm) do |f|
-        yield digests? ? Gem::Package::DigestIO.new(f, @digests) : f
+      Gem::CompactIndexClient.filesystem_access(path, :write) do
+        path.open(write_mode, perm) do |f|
+          yield digests? ? Gem::Package::DigestIO.new(f, @digests) : f
+        end
       end
     end
 
@@ -126,7 +130,9 @@ def verify
     # The file is permanently closed.
     def commit
       raise ClosedError, "Cannot commit closed file" if @closed
-      FileUtils.mv(path, original_path)
+      Gem::CompactIndexClient.filesystem_access(original_path, :write) do
+        FileUtils.mv(path, original_path)
+      end
       @closed = true
     end
 

test/rubygems/test_gem_compact_index_client.rb

@@ -109,6 +109,23 @@ def test_reset_refetches_versions
     assert_equal %w[versions versions], @fetcher.requests
   end
 
+  def test_filesystem_access_hook_wraps_reads_and_writes
+    accesses = []
+    Gem::CompactIndexClient.filesystem_access = lambda do |path, action, &block|
+      accesses << [path.basename.to_s, action]
+      block.call(path)
+    end
+
+    client = Gem::CompactIndexClient.new(File.join(@tempdir, "hooked_index"), @fetcher)
+    client.versions
+
+    assert_includes accesses, ["info", :write]
+    assert_includes accesses, ["versions", :write]
+    assert_includes accesses, ["versions", :read]
+  ensure
+    Gem::CompactIndexClient.filesystem_access = nil
+  end
+
   def test_info_uses_local_cache_when_checksum_matches
     @client.versions # prime info checksums and write cache
     @client.info("a")