[iOS] EXC_BAD_ACCESS crash during printer discovery (TM-m30, LAN mode)

[zameschua]

I'll be honest, I'm a React Native developer and not an iOS developer, the issue was solved with Claude Code. I've tested the fix and it works flawlessly on my devices.

--- Issue description below generated by Claude Code ---

Summary

On iOS, the app crashes with EXC_BAD_ACCESS during printer discovery when using Epson TM-m30 printers over LAN. The crash is caused by a race condition in the onDiscovery: callback in EscPosPrinterDiscovery.mm where the mutable _printerList array is passed directly to the React Native bridge, which may serialize it asynchronously while another callback mutates it.

This issue is iOS-specific. We have not observed this crash on Android.

Root Cause Analysis

After extensive debugging, we identified the root cause in the react-native-esc-pos-printer integration code (ios/EscPosPrinterDiscovery.mm), not the Epson ePOS SDK itself:

1. The Epson ePOS SDK fires onDiscovery: delegate callbacks from background threads (this is expected SDK
   behavior)
2. The current integration code passes _printerList (an NSMutableArray) directly to the React Native
   bridge via sendEventWithName:body:
3. The React Native bridge may serialize this array asynchronously after the method returns
4. While serialization is in progress, another callback can mutate _printerList, causing concurrent
   modification and EXC_BAD_ACCESS

Note: The Epson ePOS SDK is a closed-source binary. The fix is in the Objective-C++ wrapper code that bridges SDK callbacks to React Native, ensuring thread-safe handling of the mutable array.

Evidence: Adding extensive NSLog statements (which block the thread during I/O) prevented the crash, confirming the timing-sensitive nature of the race condition.

Proposed Fix

Changes to ios/EscPosPrinterDiscovery.mm:

- (void) onDiscovery:(Epos2DeviceInfo *)deviceInfo
{
    // Copy all values from deviceInfo IMMEDIATELY on the callback thread.
    // Use [copy] to create our own string objects in case SDK reuses internal buffers.
    NSString *target = [[deviceInfo getTarget] copy] ?: @"";
    NSString *deviceName = [[deviceInfo getDeviceName] copy] ?: @"";
    NSString *ipAddress = [[deviceInfo getIpAddress] copy] ?: @"";
    NSString *macAddress = [[deviceInfo getMacAddress] copy] ?: @"";
    NSString *bdAddress = [[deviceInfo getBdAddress] copy] ?: @"";
    int deviceType = [deviceInfo getDeviceType];

    NSDictionary *deviceDict = @{
        @"target": target,
        @"deviceName": deviceName,
        @"ipAddress": ipAddress,
        @"macAddress": macAddress,
        @"bdAddress": bdAddress,
        @"deviceType": @(deviceType)
    };

    // Static lock to serialize access to _printerList
    static NSLock *discoveryLock;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        discoveryLock = [[NSLock alloc] init];
    });

    // Lock, mutate array, take immutable snapshot, unlock
    [discoveryLock lock];
    [_printerList addObject:deviceDict];
    // CRITICAL: Create immutable copy for the bridge.
    // This prevents crashes from concurrent modification if the bridge
    // serializes asynchronously while another callback mutates _printerList.
    NSArray *printerListSnapshot = [_printerList copy];
    [discoveryLock unlock];

    // Emit event with the immutable snapshot on main queue.
    dispatch_async(dispatch_get_main_queue(), ^{
        #if RCT_NEW_ARCH_ENABLED
        [self emitOnDiscovery:printerListSnapshot];
        #else
        [self sendEventWithName:@"onDiscovery" body:printerListSnapshot];
        #endif
    });
}

Key changes:

1. Immediate string copying - Use [copy] on all strings from deviceInfo in case the SDK reuses internal
   buffers
2. NSLock for thread safety - Serialize access to the mutable _printerList
3. Immutable snapshot - Pass [_printerList copy] to the bridge instead of the mutable array
4. Async dispatch to main queue - Safe because the snapshot is immutable

Related Issues

This may be related to #129 which reports similar EXC_BAD_ACCESS crashes during discovery.

Workaround

For anyone experiencing this issue, you can use pnpm patch (or patch-package for npm/yarn) to apply the fix
above to ios/EscPosPrinterDiscovery.mm.

Steps to reproduce

1. Build and run the app on an iOS device
2. Have an Epson TM-m30III printer connected via LAN
3. Start printer discovery using startDiscovery()
4. The app crashes intermittently with EXC_BAD_ACCESS

Note: The crash can occur with just one printer on the network, but the frequency increases with multiple printers.

react-native-esc-pos-printer version

4.5.0

React Native version

0.81.5

Platforms

iOS

Workflow

Expo Dev Client

Architecture

Fabric (New Architecture)

Build type

Release app & production bundle

Device

Real device

Device model

iPad (A16), iPadOS 26.2.1

Acknowledgements

Yes