carla-simulator/carla

port(streaming): ServerSession::CloseNow idempotency fix to ue5-dev

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#9,759 opened on May 27, 2026

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Description

Context

PR #9740 fixed a double-close race in ServerSession::CloseNow() on ue4-dev when two async paths (deadline timer + a failing write) both triggered a close for the same session, _on_closed() -> DisconnectSession() was called twice, silently evicting a still-alive subscriber from the world-snapshot stream.

The fix adds std::atomic_bool _is_closed{false} to ServerSession and guards CloseNow() with _is_closed.exchange(true) so the close path runs exactly once.

Task

Port the fix from ue4-dev to ue5-dev.

Files to update:

  • LibCarla/source/carla/streaming/detail/tcp/ServerSession.h
  • LibCarla/source/carla/streaming/detail/tcp/ServerSession.cpp

References

  • Fix: #9740

Contributor guide

Tech stack
cpp
Domain
backend
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
under 1 hour
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
C++atomic operationsCARLA streaming subsystem
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
85
Research direction
Review PR #9740 on ue4 dev to understand the double close race fix. Then apply the same changes to ue5 dev: add `std::atomic bool is closed{false}` to ServerSession.h and guard `CloseNow()` with ` is closed.exchange(true)` in ServerSession.cpp. Verify no branch specific differences affect the fix.