Debug build crashes on Call Link end: owsFailDebug fires when AVAudioSession.setCategory throws during call teardown #6246

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• I am submitting a bug report for existing functionality that does not work as intended
• This isn't a feature request or a discussion topic

Bug description

Ending a Call Link by tapping the red end-call button reproducibly crashes the app in Debug builds. The crash is an owsFailDebug (fatalError in Debug, no-op in Release) inside CallAudioService.setAudioSession(category:mode:options:), triggered when AVAudioSession.setCategory throws during call teardown.

This affects Debug builds only. Release builds (TestFlight / App Store) catch the same throw and log it without crashing, so end users on production builds do not see this. It does, however, reproducibly crash any contributor running a Debug build the moment they end a Call Link, which makes development of any call-related code on a real device awkward.

Reproduces every time, on both an iPhone 14 Pro (iOS 26.1) and an iPad Pro 12.9" 5th-gen / M1 (iPadOS 26.4).

Steps to reproduce

• Build and run the Signal scheme in Debug on a real iOS device
• Open or create a Call Link and join the call
• Tap the red end-call button

Actual result: App crashes with EXC_BREAKPOINT / SIGTRAP from owsFailDebug inside CallAudioService.setAudioSession.

Expected result: The Call Link ends and the user returns to the chat list without crash. (The audio-session change failure during teardown is benign — the call is ending anyway — and should be logged at warn/error level, not fatal-asserted.)

Stack trace (top frames, faulting thread)

0  Signal  _assertionFailure(_:_:file:line:flags:)
1  Signal  fatalError(_:file:line:)
2  Signal  assertionFailure(_:file:line:)
3  Signal  owsFailDebug(_:logger:file:function:line:)
4  Signal  CallAudioService.setAudioSession(category:mode:options:)
5  Signal  CallAudioService.ensureProperAudioSession(call:)
6  Signal  CallAudioService.groupCallLocalDeviceStateChanged(_:)
7  Signal  protocol witness for GroupCallObserver.groupCallLocalDeviceStateChanged(_:)
8  Signal  closure #1 in GroupCall.groupCall(onLocalDeviceStateChanged:)
9  Signal  Sequence.forEach(_:)
10 Signal  GroupCall.groupCall(onLocalDeviceStateChanged:)
11 Signal  protocol witness for GroupCallDelegate.groupCall(onLocalDeviceStateChanged:)
12 Signal  GroupCall.handleJoinStateChanged(joinState:demuxId:)
13 Signal  closure #1 in CallManager.handleJoinStateChanged(clientId:joinState:demuxId:)

Root cause

Signal/Calls/CallAudioService.swift (around the setAudioSession private method) catches a throw from AVAudioSession.setCategory and reports it via owsFailDebug:

do {
    try avAudioSession.setCategory(category, mode: mode, options: options)
    Logger.info("AVAudioSession changed from [...] to [...]")
} catch {
    let message = "AVAudioSession failed to change from [...] to [...] with error: \(error)"
    owsFailDebug(message)
}

When the user ends a Call Link, RingRTC fires handleJoinStateChanged (the user is leaving the call), which propagates to CallAudioService.groupCallLocalDeviceStateChanged → ensureProperAudioSession → setAudioSession. At that moment iOS is concurrently deactivating the audio session as part of the call ending, so setCategory throws, and owsFailDebug turns the throw into a fatal assertion in Debug.

Suggested handling

The catch site is the right place to address this. A couple of options:

1. Downgrade the catch from owsFailDebug to Logger.warn (or Logger.error) — the call is ending anyway, so the audio session not updating to the new category is not a meaningful failure mode worth a fatal assertion.
2. Detect the call-teardown timing window and skip the setCategory attempt, or special-case the specific NSError domain/code that iOS returns when the session is being deactivated.

(1) is the smaller change.

Screenshots

n/a — backtrace-only crash.

Device info

Reproducible on both devices in Debug builds:

Device: iPhone 14 Pro
iOS version: 26.1
Signal version: 8.9 (locally built, Debug)

Device: iPad Pro 12.9" (5th generation, M1)
iOS version: iPadOS 26.4
Signal version: 8.9 (locally built, Debug)

Link to debug log

n/a — this is a developer-only crash that fires before the in-app "Submit Debug Log" flow can be exercised. Stack trace from the .ips crash report is included above; the full crash report can be supplied via email per the bug-report template instructions if it would help.