multi: split multi_runsingle into sub functions

[bagder]

To reduce complexity.

• is_finished() checks if the individual transfer is done

• handle_completed() is the logic that runs for a completed transfer

[Copilot]

Pull request overview

This PR refactors multi_runsingle() in lib/multi.c by extracting completion/error-handling logic into dedicated helper functions to reduce complexity and improve readability.

Changes:

• Extracted end-of-loop completion/error transition logic into is_finished()
• Extracted completed-transfer message/callback handling into handle_completed()
• Simplified multi_runsingle() by replacing large inline blocks with helper calls

---

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

[bagder]

augment review

[augmentcode]

🤖 Augment PR Summary

Summary: This PR refactors the multi interface transfer state machine to reduce complexity in multi_runsingle().

Changes:

• Extracted the “transfer finished?” / abort handling block into is_finished(), which may update state/connection cleanup and returns the (possibly updated) CURLcode.
• Extracted the “completed transfer” bookkeeping and message dispatch into handle_completed().
• Simplified multi_runsingle() by delegating completion/cleanup/message logic to these helpers while preserving the existing state transition flow.

Technical Notes: The refactor keeps the same completion path: errors can force MSTATE_COMPLETED, progress-callback aborts can transition to MSTATE_DONE/MSTATE_COMPLETED, and completed transfers move to MSTATE_MSGSENT with set bookkeeping updates.

_{🤖 Was this summary useful? React with 👍 or 👎}

[augmentcode]

Review completed. No suggestions at this time.

Comment augment review to trigger a new review at any time.

[bagder]

The fuzzer build now reports:

2026-02-12 16:06:59,118 - root - INFO - Base OS version is Ubuntu 24.04, but running in a different OS. Pivoting to Ubuntu 24.04 container.

docker: Error response from daemon: client version 1.41 is too old. Minimum supported API version is 1.44, please upgrade your client to a newer version.