[rubygems/rubygems] Fix misleading error if compact index cannot be copied

Previously if `~/.bundle/cache/compact_index/rubygems.org.*/version`
were owned by root with read-only access, `bundle install` would fail
with a misleading error message. For example:

```
There was an error while trying to write to `/tmp/bundler-compact-index-20220711-1823-npllre/versions`. It is
likely that you need to grant write permissions for that path.
```

This happened because the EACCESS error was caught by
`SharedHelpers.filesystem_access`, which makes it look like the target
directory is at fault instead of the source.

We can't simply drop this guard because that causes the opposite
problem: the permission error appears to come from the source instead of
the target, since `CompactIndexClient::Cache#lines` also wraps read
access errors.

Instead, bring a minimal implementation of `FileUtils.cp` and nest calls
to `SharedHelpers.filesystem_access` properly.

https://github.com/rubygems/rubygems/commit/320822c070

Co-authored-by: Stan Hu <stanhu@gmail.com>

lib/bundler/compact_index_client/updater.rb

@@ -31,9 +31,8 @@ module Bundler
 
           # first try to fetch any new bytes on the existing file
           if retrying.nil? && local_path.file?
-            SharedHelpers.filesystem_access(local_temp_path) do
-              FileUtils.cp local_path, local_temp_path
-            end
+            copy_file local_path, local_temp_path
+
             headers["If-None-Match"] = etag_for(local_temp_path)
             headers["Range"] =
               if local_temp_path.size.nonzero?
@@ -98,6 +97,20 @@ module Bundler
           SharedHelpers.digest(:MD5).hexdigest(File.read(path))
         end
       end
+
+      private
+
+      def copy_file(source, dest)
+        SharedHelpers.filesystem_access(source, :read) do
+          File.open(source, "r") do |s|
+            SharedHelpers.filesystem_access(dest, :write) do
+              File.open(dest, "wb", s.stat.mode) do |f|
+                IO.copy_stream(s, f)
+              end
+            end
+          end
+        end
+      end
     end
   end
 end

spec/bundler/install/gems/compact_index_spec.rb

@@ -163,6 +163,25 @@ The checksum of /versions does not match the checksum provided by the server! So
     expect(the_bundle).to include_gems "rack 1.0.0"
   end
 
+  it "shows proper path when permission errors happen", :permissions do
+    gemfile <<-G
+      source "#{source_uri}"
+      gem "rack"
+    G
+
+    versions = File.join(Bundler.rubygems.user_home, ".bundle", "cache", "compact_index",
+      "localgemserver.test.80.dd34752a738ee965a2a4298dc16db6c5", "versions")
+    FileUtils.mkdir_p(File.dirname(versions))
+    FileUtils.touch(versions)
+    FileUtils.chmod("-r", versions)
+
+    bundle :install, :artifice => "compact_index", :raise_on_error => false
+
+    expect(err).to include(
+      "There was an error while trying to read from `#{versions}`. It is likely that you need to grant read permissions for that path."
+    )
+  end
+
   it "falls back when the user's home directory does not exist or is not writable" do
     ENV["HOME"] = tmp("missing_home").to_s