Proposed fix for crash in calculateFirmwareRanges() due to division by zero

[dinesharjani]

If the maxSize UInt32 is zero, the internal division inside calculateFirmwareRanges() will crash both the framework and the caller app.

[dinesharjani]

Attached crash report sourced from nRF Connect for iOS 2.4.12:

Thread 7 name:
Thread 7 Crashed:
0 iOSDFULibrary 0x00000001012eea20 Swift runtime failure: Division by zero + 0 (SecureDFUExecutor.swift:0)
1 iOSDFULibrary 0x00000001012eea20 SecureDFUExecutor.calculateFirmwareRanges(:) + 772 (SecureDFUExecutor.swift:411)
2 iOSDFULibrary 0x00000001012ee864 SecureDFUExecutor.calculateFirmwareRanges(:) + 328 (SecureDFUExecutor.swift:0)
3 iOSDFULibrary 0x00000001012edb78 SecureDFUExecutor.peripheralDidSendDataObjectInfo(maxLen:offset:crc:) + 72 (SecureDFUExecutor.swift:289)
4 iOSDFULibrary 0x00000001012f5500 closure #1 in SecureDFUPeripheral.readDataObjectInfo() + 84 (SecureDFUPeripheral.swift:165)
5 iOSDFULibrary 0x00000001012ebaec specialized SecureDFUControlPoint.peripheral(:didUpdateValueFor:error:) + 1444 (SecureDFUControlPoint.swift:568)
6 iOSDFULibrary 0x00000001012ea220 @objc SecureDFUControlPoint.peripheral(:didUpdateNotificationStateFor:error:) + 108
7 CoreBluetooth 0x00000001b5b98c44 -[CBPeripheral handleAttributeEvent:args:attributeSelector:delegateSelector:delegateFlag:] + 228 (CBPeripheral.m:793)
8 CoreBluetooth 0x00000001b5b98db0 -[CBPeripheral handleCharacteristicEvent:characteristicSelector:delegateSelector:delegateFlag:] + 124 (CBPeripheral.m:815)
9 CoreBluetooth 0x00000001b5b9536c -[CBPeripheral handleMsg:args:] + 364 (CBPeripheral.m:229)
10 CoreBluetooth 0x00000001b5b8a150 -[CBCentralManager handleMsg:args:] + 196 (CBCentralManager.m:1234)
11 CoreBluetooth 0x00000001b5bb1f78 -[CBManager xpcConnectionDidReceiveMsg:args:] + 208 (CBManager.m:273)
12 CoreBluetooth 0x00000001b5ba5874 __30-[CBXpcConnection _handleMsg:]_block_invoke + 68 (CBXpcConnection.m:234)
13 libdispatch.dylib 0x000000019bf45a84 _dispatch_call_block_and_release + 32 (init.c:1466)
14 libdispatch.dylib 0x000000019bf4781c _dispatch_client_callout + 20 (object.m:559)
15 libdispatch.dylib 0x000000019bf4f004 _dispatch_lane_serial_drain + 620 (inline_internal.h:2557)
16 libdispatch.dylib 0x000000019bf4fc34 _dispatch_lane_invoke + 456 (queue.c:3862)
17 libdispatch.dylib 0x000000019bf4eecc _dispatch_lane_serial_drain + 308 (inline_internal.h:2598)
18 libdispatch.dylib 0x000000019bf4fc34 _dispatch_lane_invoke + 456 (queue.c:3862)
19 libdispatch.dylib 0x000000019bf4eecc _dispatch_lane_serial_drain + 308 (inline_internal.h:2598)
20 libdispatch.dylib 0x000000019bf4fc00 _dispatch_lane_invoke + 404 (queue.c:3862)
21 libdispatch.dylib 0x000000019bf5a4bc _dispatch_workloop_worker_thread + 764 (queue.c:6589)
22 libsystem_pthread.dylib 0x00000001e7ebe7a4 0x1e7ebb000 + 14244
23 libsystem_pthread.dylib 0x00000001e7ec574c 0x1e7ebb000 + 42828

[dinesharjani]

I thought about fixing this crash in multiple ways, like changing the specific division and inserting a min(maxLen, 1) into it to ensure this doesn't happen again. However, if maxLength returned from the Device is zero, it means no Data is supported in return, so I don't consider the DFU Process can continue and it's worse to keep the code going as usual.

I'm open to fix this in any other way that @philips77 or anyone else considers to be better overall.

[philips77]

The maxSize parameter is max size of an object. The message may say: Received invalid max size = 0.
Although, I think the issue comes from a fact that the device disconnects, or the value is somehow overwritten and the value gets 0ed. Either way, reporting an error here seems the right thing to do.