Fix partsReader.WriteTo to close consumed parts on success

[Copilot]

partsReader.WriteTo advanced r.index without closing readers on the successful copy path, which could leave part readers unclosed and let a later Close() skip them. This update aligns WriteTo behavior with the type contract by closing each part as it is fully consumed.

• Behavioral fix

  • In pkg/iobuf/part_reader.go, WriteTo now closes each part reader after io.Copy, regardless of whether the copy ended with nil or io.EOF.
  • r.index is advanced per consumed part before handling error branches, so reader lifecycle and index progression stay consistent.

• Targeted test coverage

  • In pkg/iobuf/part_reader_test.go, TestPartsReader_WriteTo_AllSuccess now asserts both readers are closed exactly once after WriteTo.
  • The same test also verifies that a subsequent outer Close() does not double-close already consumed readers.

nn, err = io.Copy(w, reader)
n += nn

r.index++
if closeErr := reader.Close(); closeErr != nil {
	return n, closeErr
}

if err != nil {
	if err != io.EOF {
		return n, err
	}
	return
}

[Copilot]

Pull request overview

Fixes partsReader.WriteTo so that it consistently closes each part reader after it has been copied, preventing already-consumed parts from remaining unclosed (and later being skipped by Close() due to r.index having advanced).

Changes:

• Update partsReader.WriteTo to advance r.index and Close() each consumed part on the success path (and before error branching), aligning lifecycle management with Read().
• Extend TestPartsReader_WriteTo_AllSuccess to assert each underlying reader is closed exactly once and that a subsequent outer Close() does not double-close.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
pkg/iobuf/part_reader.go	Ensures each part is closed after io.Copy completes, even on the successful copy path.
pkg/iobuf/part_reader_test.go	Adds assertions verifying per-part closure on success and no double-close on outer Close().

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.