Fix crash in HttpSM::tunnel_handler on unhandled VC events

[bryancall]

Summary

Fix a fatal crash in HttpSM::tunnel_handler caused by an overly narrow assertion.

The assertion on line 3117 acts as a whitelist of expected events — it passes silently when the event matches, and crashes the process when it doesn't:

ink_assert(event == HTTP_TUNNEL_EVENT_DONE || event == VC_EVENT_INACTIVITY_TIMEOUT);

The problem is that VC_EVENT_ACTIVE_TIMEOUT, VC_EVENT_ERROR, and VC_EVENT_EOS are all legitimate events that can arrive here (particularly via HTTP/2 code paths), but they weren't in the whitelist. When one of these events arrived, the assertion evaluated to false and aborted the process.

The code immediately after the assertion already handles all events correctly — it sets terminate_sm = true and returns. So the behavior was already right; the assertion was just too narrow and killed the process before the correct code could run.

The fix adds these three events to the assertion whitelist so they pass through to the existing (correct) termination logic.

Root Cause

This was exposed by #5824 ("Reactivate active timeout enforcement"), which changed the InactivityCop to dispatch specific VC_EVENT_ACTIVE_TIMEOUT events instead of the generic EVENT_IMMEDIATE that was used before. That PR also added active timeout checking to the InactivityCop for the first time — before it, VC_EVENT_ACTIVE_TIMEOUT was never dispatched from the cop, so handlers like tunnel_handler never saw it. The assertion was never updated to account for the new event types.

Background

Observed as a fatal crash on controller.trafficserver.org (ATS 10.2.0, ASAN build) through an HTTP/2 code path:

Fatal: HttpSM.cc:3083: failed assertion `event == HTTP_TUNNEL_EVENT_DONE || event == VC_EVENT_INACTIVITY_TIMEOUT`

HttpSM::tunnel_handler
HttpSM::main_handler
Continuation::handleEvent
Http2Stream::main_event_handler

Every other VC handler in HttpSM already accepts these events — tunnel_handler was the only one missing them.

Fixes #12958

Test plan

• Verify existing CI tests pass
• Confirm the assertion no longer fires under HTTP/2 timeout/error conditions

[Copilot]

Pull request overview

Fixes an assertion-triggered crash in HttpSM::tunnel_handler by allowing additional VC event types that can legitimately be dispatched to the tunnel handler (notably via HTTP/2 paths), while preserving the existing “terminate the state machine” behavior.

Changes:

• Expands the accepted event list in the tunnel_handler assertion to include VC_EVENT_ACTIVE_TIMEOUT, VC_EVENT_ERROR, and VC_EVENT_EOS.

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