mac: SIGSEGV in didStopWithError when logging Error (still on 1.17.0)

[bigBearyd]

Summary

macOS OpenDisplay 1.17.0 (141) still crashes with EXC_BAD_ACCESS / SIGSEGV in stream(_:didStopWithError:) when ScreenCaptureKit stops the capture stream. The same stack appeared on 1.16.1; upgrading to 1.17.0 did not fix it.

This looks distinct from #231: the process dies while formatting/logging error, before the new SCStreamError.userStopped / recovery logic can run. App logs never contain stream stopped with error: for these crashes.

Environment

• Mac app: 1.17.0 (141) (also reproduced on 1.16.1 / 137)
• macOS: 14.4.1 (23E224)
• Hardware: MacBook Pro (MacBookPro18,3), Apple M1 Pro, 16 GB
• Mode: WiFi extend to iPhone
• Screen Recording + Accessibility: granted

Steps

1. Connect iPhone over WiFi and start extend.
2. Leave the session running (capture FPS often collapses to ~0–2 with stalls on this network).
3. After some time, ScreenCaptureKit stops the stream (didStopWithError).
4. OpenDisplay quits immediately (segmentation fault).

Crash signature

• Exception: EXC_BAD_ACCESS / SIGSEGV — KERN_INVALID_ADDRESS at 0x0
• Triggered from: -[SCStreamManager stream:didStopWithError:] → OpenDisplay → Swift _print_unlocked / swift_getDynamicType / SwiftError::isPureNSError

Faulting frames (1.17.0 build, OpenDisplay-2026-08-21-111218.ips):

libswiftCore.dylib +4216972
libswiftCore.dylib +3792708
libswiftCore.dylib +887964
OpenDisplay +94644
OpenDisplay +96052
ScreenCaptureKit +138400
ReplayKit +103884
ReplayKit +155640
CoreFoundation +386100
CoreFoundation +385716
ReplayKit +143500
Foundation +8901188

Suspected cause

In Mac/MacSender.swift (v1.17.0):

func stream(_ stream: SCStream, didStopWithError error: Error) {
    guard stream === self.stream else { return }
    Log.info("stream stopped with error: \(error)")  // ← crashes here
    ...
}

Crash stack goes through Swift error printing (_print_unlocked), which suggests the bridged Error from ScreenCaptureKit/ReplayKit is sometimes invalid/null, and string interpolation forces a dynamic type/print that segfaults.

Suggested fix

Avoid interpolating the raw Error until it is known-safe, e.g.:

• log String(describing: error) inside a defensive path, or only log NSError domain/code/userInfo keys
• or wrap logging so a bad error cannot take down the process
• then continue with the existing fix: Honor the system capture stop and survive poisoned display identities #231 recovery / userStopped handling

Notes

• Reproduced multiple times on 2026-08-21 after updating to 1.17.0 (…-111136.ips, …-111218.ips), and earlier on 1.16.1 the same day.
• WiFi session shows very low capFps/fps and rising stalls beforehand; that likely increases how often the stream stops and hits this path.
• Happy to attach full .ips + ~/Library/Logs/OpenDisplay/opendisplay.log if useful.