[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, 19 insertions, 0 deletions

diff --git a/spec/bundler/install/gems/compact_index_spec.rb b/spec/bundler/install/gems/compact_index_spec.rb
index 72ad40e24f..5c25b1092a 100644
--- a/spec/bundler/install/gems/compact_index_spec.rb
+++ b/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