[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>

1 files changed, 16 insertions, 3 deletions

diff --git a/lib/bundler/compact_index_client/updater.rb b/lib/bundler/compact_index_client/updater.rb
index d9b9cec0d4..5b430dfbe2 100644
--- a/lib/bundler/compact_index_client/updater.rb
+++ b/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